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“Husband’s Income, Wife’s Income, and Number of Biological Children in the U.S.”, Hopcroft 2022

2022-hopcroft.pdf: “Husband’s income, wife’s income, and number of biological children in the U.S.”⁠, Rosemary L. Hopcroft (2022-02-21; ; similar):

[previous: Hopcroft 2021] Previous studies have found that the positive relationship between personal income and fertility for men in the United States is primarily due to childlessness among low-income men. Yet because of the opposite effects of income on fertility for men and women, it is important to examine the effects of income net of spouse’s income.

An analysis of income from all sources and biological fertility data for husbands and wives from the Survey of Income and Program Participation (2014) shows that for men their own income is positively associated with the number of their biological children, while their spouse’s income is negatively associated with total children ever fathered. The reverse is true for women.

These results are not because of childlessness among low-income men and high-income women, but also hold true among all those with children. For men and women aged 45–65, who likely have completed fertility, these results hold regardless of whether or not education is controlled.

These findings suggest that if status is measured as personal income for men and husband’s income for women, the positive relationship between status and fertility persists in a post-demographic transition society.

…When this data was collected, the United States was a society characterized by low fertility, high educational homogamy (Schwartz & Mare 2005) and where more than half of all married couple families had 2 breadwinners (U.S. Bureau of Labor Statistics 2017). Yet these results suggest that men with high incomes with a spouse with a low income have the most biological children in the United States, while women with low incomes with a spouse who earns a high income have the most biological children. For women there is no doubt reverse causation given tradeoffs between childbearing and raising and income earning. Nevertheless, the results support theorizing from behavioral ecology about the positive effect of an important dimension of social status—personal income—on reproductive success for men in the United States. For women, the results suggest an age-old form of female status—marriage to a high status man—is positively associated with reproductive success for women in the contemporary United States. Thus the central problem of modern sociobiology (Vining 1986) may be more a problem of appropriately measuring social status for men and women in modern societies, rather than a change to behavior that is no longer adaptive.

“Reduced Reproductive Success Is Associated With Selective Constraint on Human Genes”, Gardner et al 2022

“Reduced reproductive success is associated with selective constraint on human genes”⁠, Eugene J. Gardner, Matthew D. C. Neville, Kaitlin E. Samocha, Kieron Barclay, Martin Kolk, Mari E. K. Niemi et al (2022-02-03; ⁠, ⁠, ; similar):

Genome-wide sequencing of human populations has revealed substantial variation among genes in the intensity of purifying selection acting on damaging genetic variants1. While genes under the strongest selective constraint are highly enriched for associations with Mendelian disorders, most of these genes are not associated with disease and therefore the nature of the selection acting on them is not known2.

Here we show that genetic variants that damage these genes are associated with markedly reduced reproductive success, primarily due to increased childlessness, with a stronger effect in males than in females. We present evidence that increased childlessness is likely mediated by genetically associated cognitive and behavioural traits, which may mean male carriers are less likely to find reproductive partners.

This reduction in reproductive success may account for 20% of purifying selection against heterozygous variants that ablate protein-coding genes. While this genetic association could only account for a very minor fraction of the overall likelihood of being childless (less than 1%), especially when compared to more influential sociodemographic factors, it may influence how genes evolve over time.

“Contemporary Selection Pressures in Modern Societies? Which Factors Best Explain Variance in Human Reproduction and Mating?”, Fieder & Huber 2022

2022-fieder.pdf: “Contemporary selection pressures in modern societies? Which factors best explain variance in human reproduction and mating?”⁠, Martin Fieder, Susanne Huber (2022; ; similar):

Phenotypic traits in humans are under selection pressure and are still evolving, but the relative importance of these traits remains to be investigated.

We therefore analyzed jointly phenotypic traits associated with number of children and having ever been married. This provides insights into the relative contribution of each trait and indicates the potential selection pressure induced by a specific trait relative to others. To shed light on potential selection on the genome level, all analyses include a multivariate polygenic risk score of general cognitive ability.

We used the data from the Wisconsin Longitudinal Study (WLS), a dataset consisting of 4,991 men and 5,326 women almost all whites, educated at least at A-level. The focus was on the association between age, education level, wages, religious intensity, fathers’ age at child’s birth, ratings of facial attractiveness, number of siblings of the respondent, as well as the polygenic risk score of general cognitive ability on the following dependent variables: (1) number of children, (2) ever being married, and (3) age at first birth. For each factor we additionally examined the relative contribution to the overall variance explained of the dependent variable.

Having been married and, thus, mate selection, is the most important determinant for the number of children for both men and women. Wages explain most of the total variance for “ever married”, yet in different directions for men and women, as is also the case for the association between wages and number of children. In both women and men, education explains most of the variance in age at first birth, and the effect is postponing. Furthermore, although the phenotype education is negatively associated with the number of children in both sexes, this holds true for the polygenic risk score for cognitive ability only in men. In addition, in men, the polygenic risk score for cognitive ability also has a positive effect on reproduction due to its positive interaction with wages. Anyhow, except for having ever been married, all other variables explain only a small proportion of the variation in fertility outcomes.

Although our results are consistent with the hypothesis that there is selection pressure for rather recently arising traits as education and income, on the basis of our results we are not able to draw any conclusion on selection.

[Keywords: variance mating and reproduction, income, polygenic risk score, religiousness, education, selection]

“A Selection Pressure Landscape for 870 Human Polygenic Traits”, Song et al 2021

2021-song.pdf: “A selection pressure landscape for 870 human polygenic traits”⁠, Weichen Song, Yueqi Shi, Weidi Wang, Weihao Pan, Wei Qian, Shunying Yu, Min Zhao, Guan Ning Lin (2021-11-15; similar):

Characterizing the natural selection of complex traits is important for understanding human evolution and both biological and pathological mechanisms.

We leveraged genome-wide summary statistics for 870 polygenic traits and attempted to quantify signals of selection on traits of different forms in European ancestry across 4 periods in human history and evolution.

We found that 88% of these traits underwent polygenic change in the past 2,000–3,000 years. Recent selection was associated with ancient selection signals in the same trait. Traits related to pigmentation, body measurement and nutritional intake exhibited strong selection signals across different time scales. Our findings are limited by our use of exclusively European data and the use of genome-wide association study data, which identify associations between genetic variants and phenotypes that may not be causal.

In sum, we provide an overview of signals of selection on human polygenic traits and their characteristics across human evolution, based on a European subset of human genetic diversity. These findings could serve as a foundation for further population and medical genetic studies.

…As shown in Figure 1, we focus on 2 primary goals. First, we describe the selection pressure on each trait at 4 different time scales (Figures 2–5). This is achieved using various metrics derived from different statistical models (Mendelian randomization (MR), singleton density score⁠, ancient genome analysis and so on), each fitting a specific timeframe or form of selection. Second, we integrate these metrics to explore the association among selection pressures, trait characteristics and functional genomic patterns (Figures 6–8), using linear regression and unsupervised clustering.

Body measurements and contemporary reproductive success: Our analysis started by exploring natural selection pressure at the present time. We hypothesized that the current natural selection of a trait is relevant to whether it could causally impact human reproductive success (that is, number of offspring) and mating success (for this, we used the proxy of number of overall sexual partners). To quantify these causal effects, we applied MR on GWAS summary statistics between tested traits and reproductive success, as well as between tested traits and mating success. At the statistical-significance cutoff of |zMR| > 4 (Methods), we found that 7.4% of traits with valid MR results (that is, traits passing sensitivity analysis) (40 out of 539) had a causal effect on the number of offspring of males, whereas 5.9% (32 out of 542) of traits with valid MR results impacted the number of offspring of females (Supplementary Table 2). Separating the traits into 15 categories (Figure 2A, Figure 2B), we observed that 52% (23⁄44) of anthropometric body measurement traits such as height (zMR = 8.09, p = 3.33 × 10−16 in males; zMR = 4.91, p = 4.55 × 10−7 in females) were causally related to the number of offspring of males. By contrast, only 30% (14⁄47) of body measurement traits were causally related to the number of offspring of females. In addition, the effect of another type of body measurement (dermatology traits such as skin colour) on reproductive success also exhibited sex specificity: 38% (5⁄13) of dermatology traits influenced the number of offspring of males, but none affected the number of offspring of females. However, when testing for 112 complex conditions such as schizophrenia11 and stroke15, polygenic risks showed no statistically-significant causal effect on the numbers of offspring for either males or females (nominal p > 0.05/​112). The distribution of effect direction was also similar between disease and non-disease traits (Fisher p = 0.40 for males, p = 0.71 for females).

For mating success (Supplementary Figure 2), body measurement traits also had an impact: 44% of body measurement traits impacted the number of sexual partners of males, compared with 12% affecting the number of sexual partners of females. Interestingly, among all 112 tested polygenic disease traits, schizophrenia (zMR = 7.37, p = 8.53 × 10−14) and attention-deficit hyperactivity disorder (zMR = 4.62, p = 1.92 × 10−6) increased the number of sexual partners of males, in line with previous findings that increased genetic liability for schizophrenia does not confer a fitness advantage but does increase mating success16. For males, the impact on reproductive success of a trait was positively correlated with its impact on mating success (Supplementary Figure 2; Pearson correlation coefficient (PCC) 0.47, 95% CI 0.39 to 0.55, p = 9.30 × 10−31). However, this was not true for females, for whom the impact on reproductive success of a trait was negatively correlated with its impact on mating success (Supplementary Figure 2; PCC −0.10, 95% CI −0.20 to 0, p = 0.02). This discrepancy is consistent with the evolutionary psychology theory that males and females adopt distinct sexual strategies that shape assortative selection17.

Next, we investigated whether the trait impact on reproductive success and mating would differ between the sexes. In general, trait impact on human reproductive success was similar for males and females (Figure 2c; PCC 0.38, 95% CI 0.32 to 0.44, p = 6.85 × 10−31). Trait impacts of mating success were also similar between the sexes (Supplementary Figure 2; PCC 0.64, 95% CI 0.58 to 0.70, p = 9.18 × 10−106). Notably, high intelligence trait statistically-significantly reduced the number of offspring in both females and males (zMR = −7.55, p = 2.18 × 10−14 in females, zMR = −5.13, p = 1.45 × 10−7 in males), and increased the expected number of sexual partners for females (zMR = 7.05, p = 8.97 × 10−13) (Supplementary Figure 1).

In addition, we applied causal analysis using summary effect estimates18 to all MR results to analyse the role of genetic correlation⁠. We found that most of the results were explained mainly by causal effects instead of genetic correlation⁠. Using another GWAS19 dataset and applying MR bias estimation20, we again showed that our results were not explained by GWAS sample overlap (‘MR analysis details’ in Supplementary Information).

Figure 2: Selection pressure in the present day and in recent history. A, B: Proportion of traits showing MR causal effects on the number of offspring of males (a) and females (b) for each category. c, Comparison of MR z scores between males (x-axis) and females (y-axis). Dashed lines indicate the statistical-significance threshold (|z| > 4). The text indicates selected traits with results of special interest. DER, dermatology; NUT, nutrition; REP, reproduction; GI, gastrointestinal; PSY, psychiatry; RES, respiratory; MED, medication; COG, social cognition; MUSC, musculoskeletal; MET, metabolism; CIRC, circulation; NEU, neurology.

Widespread polygenic adaptation in the past 2,000–3,000 years: At the statistical-significance threshold of p < (0.05/​870 = 5.7 × 10−5), we found that 88% (761⁄870) of polygenic traits had a statistically-significant correlation between the GWAS p-value and tSDS (ρSDS; Supplementary Table 3). Previous analysis has found that population stratification of UK Biobank might bias the estimated polygenic adaptation22. Thus, to exclude this potential confound in our analyses, we applied another method with a different statistical model, which involves reconstructing the history of polygenic scores (RHPS)23, based on RELATE24 (RHPS-RELATE, Methods). We set the reference panel as all European participants of 1000 Genomes to avoid population stratification. As shown in Supplementary Table 3, the polygenic risk score (PRS) alteration in the past 100 generations (roughly equivalent to 2,800 years (ref. 24)) was mostly in accordance with ρSDS (PCC 0.25, 95% CI 0.18 to 0.32, p = 3.96 × 10−13). Among 755 traits with statistically-significant non-zero ρSDS, 13.8% (104⁄755) showed a consistent statistically-significant alteration of PRS (p for ‘Tx test’ from RHPS < 0.05/​870, Methods), and 26.1% (197⁄755) showed a nominally statistically-significant alteration (p for Tx test < 0.05). Notably, our RHPS-RELATE results also highlighted those traits with the highest ρSDS, such as ease of skin tanning (p for ρSDS <10−100; p for Tx test <10−100) and raw vegetable intake (p for ρSDS <10−100; p for Tx test 2.69 × 10−51) (Supplementary Table 3). In general, the results of RHPS-RELATE were consistent with the ρSDS analysis, albeit at lower statistical power⁠. Thus, we conclude that the ρSDS results are credible and can truly reflect recent adaptation prevalence…by utilizing simulations of genetic drift and demographic isolation strategies, our results suggest that population stratification did not drive a systematic bias on ρSDS. We consequently propose that the observed bias on height might not represent the majority of traits.

…When analysing all traits, we observed that dermatology traits generally showed the most statistically-significant selection signals (median |ρSDS| = 0.69, Figure 3A, B), followed by nutrition intake (median |ρSDS| = 0.48; Supplementary Figure 4) and reproduction-related traits (median |ρSDS| = 0.30; Supplementary Figure 4). Ease of skin tanning was the trait with the most statistically-significant adaptation (ρSDS = 0.96, p < 10−100; Figure 3c). Ever been drinkers (ρSDS = −0.82, p < 10−100) and sitting height (ρSDS = 0.84, p < 10−100) were also among traits with an extreme adaptation signal (|ρSDS| > 0.8), which made up 3.3% of all traits (Supplementary Figure 4). Neurological traits such as brain structures exhibited the least polygenic adaptation (median |ρ| = 0.05).

In contrast to non-disease traits, the adaptive pressure on polygenic disease traits was generally negative (median ρSDS = −0.08; permutation p = 3.22 × 10−6), especially for early-onset conditions such as autism spectrum disorder (median ρSDS = −0.12; Supplementary Figure 5). The greatest evidence of negative adaptation was found for high cholesterol (ρSDS = −0.66, p < 10−100; Supplementary Figure 5). Still, we found evidence of positive adaptation for a few diseases such as skin cancer and inflammatory bowel diseaseSDS > 0.2, p <10−100; Supplementary Figure 5), and even some early-onset conditions such as attention deficit hyperactivity disorder (ρSDS = 0.20, p <2.16 × 10−24) and anorexia nervosaSDS = 0.16, p = 1.24 × 10−19) (Supplementary Table 3). This result suggested that some of the disease traits might be by-products of other positive selection events.

…As shown in Figure 4a and Supplementary Table 5, after controlling for covariances (for example, latitude, longitude and genotyping coverage) and multiple tests, the polygenic burden of 78 traits was statistically-significantly associated with the percentage of hunter-gatherer ancestry (HG%). By contrast, another 6 traits, such as denture usage, were associated with time in at least one of 3 datasets. 7 of 13 dermatology traits were most predominantly associated with HG% (Figure 4a), with ‘ease of skin tanning’ as the most statistically-significant example (regression tHG = 20.3, p = 1.74 × 10−38; Figure 4b). In the Near East dataset, we observed that signals of selection on skin tanning varied by latitude (Figure 4c), with signals of positive selection observed in regions of low latitude (latitude < 50°; t = 4.12, p = 1.91 × 10−5), but signals of negative selection observed at high latitudes (t = 4.95, p = 3.80 × 10−7). After controlling for the impact of latitude, we observed a general ascending trend for ‘ease of skin tanning’ for the Near East dataset, suggesting overall positive selection (regression tNear East = 5.81, p = 2.29 × 10−8; Figure 4c). We also found a nominally statistically-significant increment for ease of skin tanning in the pre-Neolithic period (regression tpre-Neolithic = 4.25, p = 1.11 × 10−5), but not in the Neolithic period (regression tNeolithic = 0.92, p = 0.18; Supplementary Figure 7).

Figure 3: Selection pressure in recent history. a, Distribution of absolute Spearman correlation (|ρSDS|) between the tSDS and GWAS p-value for each category. The upper and lower margins of the box indicate the first and third quartiles of ρSDS, and the thickened line its median. b, ρSDS for all dermatology traits. The diagonal of the rhombus indicates ρSDS, and the width its 95% CI, Scatter plot showing the correlation between tSDS and GWAS p-value bin for the trait ‘ease of skin tanning’. Each point represents a bin of 1,000 SNPs ranked by their GWAS p-value. The y-axis indicates the bin median tSDS. Abbreviations as in Figure 2.
Figure 7: Population-average polygenic risk score trajectory for 765 traits. Trajectories are grouped into 4 clusters according to their time-series similarity by hierarchical clustering. The y-axis shows the z scores of PRS. Colour marks different traits with overlapping trajectories, and dashed line marks median trajectory of each cluster.

…although all the above possibilities explained a proportion of disease heritability, there is still room for another ‘trivial explanation’: natural selection was indeed eliminating the risk alleles but simply not fast enough, due to the small effect of each allele and the small effective population size at the risk loci8⁠,46

“High Income Men Have High Value As Long-term Mates in the U.S.: Personal Income and the Probability of Marriage, Divorce, and Childbearing in the U.S.”, Hopcroft 2021

2021-hopcroft.pdf: “High income men have high value as long-term mates in the U.S.: personal income and the probability of marriage, divorce, and childbearing in the U.S.”⁠, Rosemary L. Hopcroft (2021-09-01; ⁠, ; backlinks; similar):

Using data from the first Census data set that includes complete measures of male biological fertility for a large-scale probability sample of the U.S. population (the 2014 wave of the Study of Income and Program ParticipationN = 55,281), this study shows that:

high income men are more likely to marry, are less likely to divorce, if divorced are more likely to remarry, and are less likely to be childless than low income men. Men who remarry marry relatively younger women than other men, on average, although this does not vary by personal income. For men who divorce who have children, high income is not associated with an increased probability of having children with new partners. Income is not associated with the probability of marriage for women and is positively associated with the probability of divorce.

High income women are less likely to remarry after divorce and more likely to be childless than low income women. For women who divorce who have children, high income is associated with a lower chance of having children with new partners, although the relationship is curvilinear.

These results are behavioral evidence that women are more likely than men to prioritize earning capabilities in a long-term mate and suggest that high income men have high value as long-term mates in the U.S.

[Keywords: evolutionary psychology, fertility, marriage, childlessness, divorce, sex differences]

Figure 1: Probability of ever having married by income. (Model prediction with age, education, proportions Black and Hispanic set to their means.)
Figure 2: Probability ever having divorced by income. (Model prediction with age, education, proportions Black and Hispanic set to their means.)
Figure 4: Probability ever have children by income. (Model prediction with age, education, proportions Black and Hispanic set to their means.)

“The Children of the Missed Pill”, Rau et al 2021

2021-rau.pdf: “The children of the missed pill”⁠, Tomás Rau, Miguel Sarzosa, Sergio Urzúa (2021-09-01; ⁠, ; similar):

We assess the impact of exogenous variation in oral contraceptives prices—a year-long decline followed by a sharp increase due to a documented collusion case—on fertility decisions and newborns’ outcomes. Our empirical strategy follows an interrupted time-series design, which is implemented using multiple sources of administrative information. As prices skyrocketed (45% within a few weeks), the Pill’s consumption plunged, and weekly conceptions increased (3.2% after a few months).

We show large effects on the number of children born to unmarried mothers, to mothers in their early twenties, and to primiparae women. The incidence of low birth weight and fetal/​infant deaths increased (declined) as the cost of birth control pills rose (fell). In addition, we document a disproportional increase in the weekly miscarriage and stillbirth rates. As children reached school age, we find lower school enrollment rates and higher participation in special education programs.

Our evidence suggests these “extra” conceptions were more likely to face adverse conditions during critical periods of development.

…This paper quantifies the Pill’s role in fertility and child outcomes using a sequence of events in which unexpected shocks affected the access to oral contraceptives. In particular, we exploit a well-established case of anticompetitive behavior in the pharmaceutical market, which—after a year-long price war between the 3 largest pharmaceutical retailers in Chile—triggered a sharp and unexpected increase in the prices of birth control pills.

The price war took place during 2007, and it effectively reduced the prices of medicines across the board. In particular, prices of oral contraceptives fell by 24% during that year. By the end of 2007, the 3 largest pharmacies agreed to end the price war and engaged in a collusion scheme in which they strategically increased the prices of 222 medicines. Oral contraceptives were included in this group, experiencing price increases ranging from 30 to 100% in just a few weeks (45% on average in the first 3 weeks). We use daily information on prices and quantities sold in the country by the 3 companies from almost 40 million transactions to determine the date when the price changes for birth control pills took place. Using these data, we implement an interrupted time-series analysis (Bloom, 2003; Cauley & Iksoon 1988), which takes into account the seasonality of births, the general trends of fertility, as well as dynamics that arise because it takes time for the menstrual cycle to be fully regulated after discontinuing the Pill’s intake. We complement the pharmacies’ transaction data with administrative information from birth and death certificates collected between 2005 and 2008 and administrative records on school enrollment from 2013 to 2016. Our empirical strategy considers 2 different treatments: one stemming from a sustained and steady decline in prices (2007) and another one from a massive and sudden increase (first weeks of 2008).

“The Secular Trend of Intelligence Test Scores in the Present Century: The Danish Experience”, Hegelund et al 2021

2021-hegelund.pdf: “The secular trend of intelligence test scores in the present century: The Danish experience”⁠, Emilie R. Hegelund, Gunhild T. Okholm, Thomas W. Teasdale (2021-03-01; ; similar):

  • Changes in mean intelligence test scores were minimal in Denmark in 2006–2019 [2006–2010: 111.5, 111.1, 110.8, 110.7, 110.6, | 2011–2019: 109.1, 109.2, 109.0, 109.1, 109.3, 109.2, 109.1, 108.7, 108.8].
  • A change in the format of the intelligence test resulted in a sudden drop in scores.
  • Neither changes in parental age, dysgenics, or immigration can explain the findings.
  • Changes in sample composition may conceal a true decline in intelligence test scores.

The present register-based study investigated the secular trend of intelligence test scores during the period from 2006 through 2019 in a Danish population-representative sample, as well as whether the observed trend could be explained by changes in parental age, dysgenics, and immigration or changes in the format of the intelligence test and sample characteristics.

The study population consisted of all Danish men appearing before a draft board during the study period (n = 400,288). Intelligence test scores were obtained by the use of Børge Priens Prøve, typically at age 19. For each of the included draft board cohorts, the intelligence test score mean and standard deviation were estimated.

The results showed that changes in mean intelligence test scores were minimal during the study period. A slight decline was observed from 2006 to 2010. Furthermore, there was a drop of 1.5 IQ points from 2010 to 2011, which coincided with the change in the format of the intelligence test from paper-and-pencil to computer-based, but there was essentially no change after 2011. Neither changes in parental age, dysgenics, or immigration seem to have influenced the observations. However, changes in sample composition may conceal a true decline in intelligence test scores given that a larger proportion of individuals with low intelligence seems to be exempted from testing.

In conclusion, the study findings suggest no systematic change in intelligence test scores during the last decade, but due to changes in sample composition, it cannot be excluded that there has been a negative secular trend.

[Keywords: intelligence, secular trend, Flynn effect⁠, Denmark]

…A slight decline in mean IQ score was observed from 2006 to 2010, which can be seen as a continuation of the decline previously reported between 1998 and 2004 (Teasdale & Owen 2008)…We have had the opportunity to rescale the mean intelligence test scores from the Danish draft board examinations reported by Teasdale & Owen 2008 against our baseline year 1960 to compare them with our observations. The rescaled mean IQ scores are as follows: 1988: 111.0 (SD: 13.0); 1998: 112.4 (SD: 12.7); 2003–4: 111.1 (SD: 12.8). As can be seen, there was an increase from 1988 to 1998 followed by a small decline from 1998 to 2003–4. The mean IQ score in 1998 remains the highest recorded using Danish draft board data, whereas the mean IQ score in 2003–4 is comparable with our mean intelligence test score in 2006. As such, there has been a decline of 1.8 IQ points during the period from 1998 through 2010 followed by a drop of 1.5 IQ points which is probably due to the change in the format of the intelligence test and virtually no change from 2011 through 2019. However, the variance has declined statistically-significantly throughout the study period, corresponding to a decline of 0.15 SD per year (p < 0.001). A previous study has suggested that the negative secular trend observed in developmental test performances may be rooted in declining performances of the top percentiles (Flynn & Shayer 2018), leading to declining variances. If this is also true in our study where the proportion of individuals with low test intelligence scores who were exempted from testing has increased over time, this might explain our observation of no change in mean intelligence test scores, but a declining variance.

“Identification of 370 Genetic Loci for Age at First Sex and Birth Linked to Externalizing Behaviour”, Mills et al 2021

“Identification of 370 genetic loci for age at first sex and birth linked to externalizing behaviour”⁠, Melinda C. Mills, Felix C. Tropf, David M. Brazel, Natalie van Zuydam, Ahmad Vaez, eQTLGen Consortium et al (2021-02-04; ; similar):

Age at first sexual intercourse (AFS) and age at first birth (AFB) have implications for health and evolutionary fitness. In the largest genome-wide association study to date (AFS, n = 387,338; AFB, n = 542,901), we identify 370 independent signals, 11 sex-specific, with a 5–6% polygenic score (PGS) prediction. Heritability of AFB shifted from 9% [CI = 4–14] for women born in 1940 to 22% [CI = 19–25] in 1965. Signals are driven by the genetics of reproductive biology and externalizing behaviour, with key genes related to follicle stimulating hormone (FSHB), implantation (ESR1), infertility, and spermatid differentiation. Polycystic Ovarian Syndrome leads to later AFB, linking with infertility. Late AFB is protective against later-life disease and associated with parental longevity. Higher childhood socioeconomic circumstances and those in the highest PGS decile (90%+) experience markedly later reproductive onset. Results are relevant for improving teenage and late-life health, for understanding longevity, and guiding experimentation into mechanisms of infertility.

“Natural Selection in Contemporary Humans Is Linked to Income and Substitution Effects”, Hugh-Jones & Abdellaoui 2021

“Natural Selection in Contemporary Humans is Linked to Income and Substitution Effects”⁠, David Hugh-Jones, Abdel Abdellaoui (2021; ; similar):

[Censored from bioRxiv⁠; author discussion: 1⁠, 2⁠, 3⁠.] Natural selection has been documented in contemporary humans, but little is known about the mechanisms behind it. We test for natural selection through the association between 33 polygenic scores and fertility, across two generations, using data from UK Biobank (n = 409,629 British subjects with European ancestry).

Consistently over time, polygenic scores associated with lower (higher) earnings, education and health are selected for (against). Selection effects are concentrated among lower SES groups, younger parents, people with more lifetime sexual partners, and people not living with a partner. The direction of natural selection is reversed among older parents (22+), or after controlling for age at first live birth. These patterns are in line with economic theories of fertility, in which higher earnings may either increase or decrease fertility via income and substitution effects in the labour market.

Studying natural selection can help us understand the genetic architecture of health outcomes: we find evidence in modern day Great Britain for multiple natural selection pressures that vary between subgroups in the direction and strength of their effects, that are strongly related to the socio-economic system, and that may contribute to health inequalities across income groups.

Figure 1: Mean polygenic scores (PGS) by birth year in UK Biobank. Points are means for 5-year intervals. Lines are 95% confidence intervals. Green triangles show a statistically-significant linear increase over time (p < (0.05/​33)). Red squares show a statistically-significant decrease.
Figure 7: Mean polygenic score for educational attainment (EA3) of children by household income group. Blue is actual. Grey is hypothetical in the absence of selection effects.

“The Micro-evidence for the Malthusian System: France, 1670–1840”, Cummins 2020

2020-cummins.pdf: “The micro-evidence for the Malthusian system: France, 1670–1840”⁠, Neil Cummins (2020-10-01; ; similar):

I test the assumptions of the Malthusian model at the individual, cross-sectional level for France, 1650–1820. Using husband’s occupation from the parish records of 41 French rural villages, I assign three different measures of status. There is no evidence for the existence of the positive check; infant deaths are unrelated to status. However, the preventive check operates strongly, acting through female age at first marriage. The wives of rich men are younger brides than those of poorer men. This drives a positive net-fertility gradient in living standards. However, the strength of this gradient is substantially weaker than it is in pre-industrial England.

[Keywords: economic history, historical demography, population, Malthus, fertility, mortality, living standards]

“Germline Mutation Rates in Young Adults Predict Longevity and Reproductive Lifespan”, Cawthon et al 2020

“Germline mutation rates in young adults predict longevity and reproductive lifespan”⁠, Richard M. Cawthon, Huong D. Meeks, Thomas A. Sasani, Ken R. Smith, Richard A. Kerber, Elizabeth O’Brien et al (2020-06-19; ⁠, ; similar):

Ageing may be due to mutation accumulation across the lifespan, leading to tissue dysfunction, disease, and death. We tested whether germline autosomal mutation rates in young adults predict their remaining survival, and, for women, their reproductive lifespans. Age-adjusted mutation rates (AAMRs) in 61 women and 61 men from the Utah CEPH (Centre d’Etude du Polymorphisme Humain) families were determined. Age at death, cause of death, all-site cancer incidence, and reproductive histories were provided by the Utah Population Database, Utah Cancer Registry, and Utah Genetic Reference Project. Higher AAMRs were statistically-significantly associated with higher all-cause mortality in both sexes combined. Subjects in the top quartile of AAMRs experienced more than twice the mortality of bottom quartile subjects (hazard ratio [HR], 2.07; 95% confidence interval [CI], 1.21–3.56; p = 0.008; median survival difference = 4.7 years). Fertility analyses were restricted to women whose age at last birth (ALB) was ≥ 30 years, the age when fertility begins to decline. Women with higher AAMRs had statistically-significantly fewer live births and a younger ALB. Adult germline mutation accumulation rates are established in adolescence, and later menarche in women is associated with delayed mutation accumulation. We conclude that germline mutation rates in healthy young adults may provide a measure of both reproductive and systemic ageing. Puberty may induce the establishment of adult mutation accumulation rates, just when DNA repair systems begin their lifelong decline.

“Genome-wide Analysis Identifies Genetic Effects on Reproductive Success and Ongoing Natural Selection at the FADS Locus”, Mathieson et al 2020

“Genome-wide analysis identifies genetic effects on reproductive success and ongoing natural selection at the FADS locus”⁠, Iain Mathieson, Felix R. Day, Nicola Barban, Felix C. Tropf, David M. Brazel, eQTLGen Consortium, BIOS Consortium et al (2020-05-22; ; similar):

Identifying genetic determinants of reproductive success may highlight mechanisms underlying fertility and also identify alleles under present-day selection. Using data in 785,604 individuals of European ancestry, we identify 43 genomic loci associated with either number of children ever born (NEB) or childlessness. These loci span diverse aspects of reproductive biology across the life course, including puberty timing, age at first birth, sex hormone regulation and age at menopause. Missense alleles in ARHGAP27 were associated with increased NEB but reduced reproductive lifespan, suggesting a trade-off between reproductive ageing and intensity. As NEB is one component of evolutionary fitness, our identified associations indicate loci under present-day natural selection. Accordingly, we find that NEB-increasing alleles have increased in frequency over the past two generations. Furthermore, integration with data from ancient selection scans identifies an unique example of an allele—FADS1/​2 gene locus—that has been under selection for thousands of years and remains under selection today. Collectively, our findings demonstrate that diverse biological mechanisms contribute to reproductive success, implicating both neuro-endocrine and behavioural influences.

“Sociology, Genetics, and the Coming of Age of Sociogenomics”, Mills & Tropf 2020

2020-mills.pdf: “Sociology, Genetics, and the Coming of Age of Sociogenomics”⁠, Melinda C. Mills, Felix C. Tropf (2020-05-11; ; similar):

Recent years have seen the birth of sociogenomics via the infusion of molecular genetic data. We chronicle the history of genetics, focusing particularly on post-2005 genome-wide association studies, the post-2015 big data era, and the emergence of polygenic scores. We argue that understanding polygenic scores, including their genetic correlations with each other, causation, and underlying biological architecture, is vital. We show how genetics can be introduced to understand a myriad of topics such as fertility, educational attainment, intergenerational social mobility, well-being, addiction, risky behavior, and longevity. Although models of gene-environment interaction and correlation mirror agency and structure models in sociology, genetics is yet to be fully discovered by this discipline. We conclude with a critical reflection on the lack of diversity, non-representative samples, precision policy applications, ethics, and genetic determinism. We argue that sociogenomics can speak to long-standing sociological questions and that sociologists can offer innovative theoretical, measurement, and methodological innovations to genetic research.

“Identification of 370 Loci for Age at Onset of Sexual and Reproductive Behaviour, Highlighting Common Aetiology With Reproductive Biology, Externalizing Behaviour and Longevity”, Mills et al 2020

“Identification of 370 loci for age at onset of sexual and reproductive behaviour, highlighting common aetiology with reproductive biology, externalizing behaviour and longevity”⁠, Melinda C. Mills, Felix C. Tropf, David M. Brazel, Natalie van Zuydam, Ahmad Vaez, eQTLGen Consortium et al (2020-05-07; ; similar):

The timing of reproductive behaviour—age at first sexual intercourse (AFS) and age at first birth (AFB)—has implications for reproductive health, adolescent development and evolutionary fitness. In the largest genome-wide association study to date (AFS, n = 387,338; AFB, n = 542,901), we identify 370 independent signals, 11 which are sex-specific, with a 5–6% polygenic score prediction. Heritability shifted from 10% for those born in 1940 to 23% for the 1965 birth cohort. Using Genomic SEM⁠, we show that signals are largely driven by the genetics of reproductive biology and externalizing behaviour. This is supported by extensive biological follow-up that isolates key genes related to follicle stimulating hormone (FSHB), implantation (ESR1), infertility (endometriosis, spontaneous abortion) and spermatid differentiation, morphogenesis and binding (KLF17, ZPBP). Later AFB is protective against later-life disease (type 2 diabetes, cardiovascular) and associated with longevity. Those from higher childhood socioeconomic circumstances and polygenic scores in the highest deciles (90%+) experience markedly later reproductive onset. Results are relevant for interventions in teenage sexual, reproductive and mental health, deepen our understanding of the drivers of later-life health and longevity, and fuel infertility and functional follow-up experiments.

“Associations of Autozygosity With a Broad Range of Human Phenotypes”, Clark et al 2019

“Associations of autozygosity with a broad range of human phenotypes”⁠, David W. Clark, Yukinori Okada, Kristjan H. S. Moore, Dan Mason, Nicola Pirastu, Ilaria Gandin, Hannele Mattsson et al (2019-10-31; ; similar):

In many species, the offspring of related parents suffer reduced reproductive success, a phenomenon known as inbreeding depression⁠. In humans, the importance of this effect has remained unclear, partly because reproduction between close relatives is both rare and frequently associated with confounding social factors. Here, using genomic inbreeding coefficients (FROH) for >1.4 million individuals, we show that FROH is statistically-significantly associated (p < 0.0005) with apparently deleterious changes in 32 out of 100 traits analysed. These changes are associated with runs of homozygosity (ROH), but not with common variant homozygosity⁠, suggesting that genetic variants associated with inbreeding depression are predominantly rare. The effect on fertility is striking: FROH equivalent to the offspring of first cousins is associated with a 55% decrease [95% CI 44–66%] in the odds of having children. Finally, the effects of FROH are confirmed within full-sibling pairs, where the variation in FROH is independent of all environmental confounding.

“Throwing the Baby Out With the Bath Water: Could Widespread Neutering of Companion Dogs Cause Problems at a Population Level?”, Dawson et al 2019

“Throwing the Baby Out With the Bath Water: Could Widespread Neutering of Companion Dogs Cause Problems at a Population Level?”⁠, Jessica K. Dawson, Tiffani J. Howell, Matthew B. Ruby, Pauleen C. Bennett (2019-07-22; ⁠, ; backlinks; similar):

In many countries where companion dogs are popular, owners are strongly encouraged to neuter their dogs. Consequently, millions of dogs are neutered each year. In recent times considerable attention has been paid to the possible effects of such procedures on canine health and welfare. Less scrutinized are the potential ramifications of widespread neutering on the breeding of dogs and their continued success as human companions. This paper summarizes research investigating factors influencing the breeding and rearing of dogs most suited to companionship roles in contemporary, typically high-density, communities, and briefly reviews current breeder practices. It then argues that a fundamental shift to promote inclusion of “proven” companion dogs in the gene pool, as opposed to dogs meeting conformation or working/​sporting standards, is required to successfully meet the needs of modern urban dog owners. A new model is proposed, whereby responsible owners and breeders work together to produce dogs most suited for life as human companions.

…The demonstrated importance of genetics and early environment in determining behavioral predispositions makes it imperative to consider where companion dogs come from. Prior to the widespread introduction of neutering practices, dogs often bred indiscriminately, and people typically obtained their dogs for free from neighbors whose bitch had produced a litter (47). While this was problematic in terms of creating dog overpopulation, it meant that most of the dogs who produced offspring were well suited to the demands of the lives they were expected to lead. Those who weren’t well-suited were disposed of. Today, strong demand for companion dogs, coupled with rapid urbanization, increased concern regarding the welfare of animals, particularly companion dogs, and high neutering rates, has resulted in a multimillion-dollar industry involving the selective breeding and selling of puppies (48). Widespread neutering means that humans intentionally control nearly all dog breeding in developed countries…As described previously, in many developed countries, neutering companion dogs is considered an important aspect of responsible ownership. Hence, the very best companion dogs in the general community, those owned by responsible citizens who choose their dogs carefully and ensure they are reared correctly, are almost certainly those most likely to be neutered. Conversely, it is those companion dog owners who fail to perform the “responsible” behavior of neutering their dog who are perhaps most likely to breed. These “breeders” may also choose not to perform other “responsible” behaviors, such as selecting their dog carefully, testing it for genetic disorders, or evaluating the dog’s suitability as a companion prior to allowing it to reproduce. In other words, they may not thoroughly consider the genetic and environmental factors known to be critical to optimal puppy development.

Second, we advocate that all dogs should be independently tested for suitability before being bred—much as breeders now advertise that their puppies’ parents are successful show dogs, or that they are free from known genetic disorders, so they should be encouraged to advertise that independent testing has shown their breeding dogs to be well-suited behaviourally to life as human companions. We anticipate that responsible breeders would be willing to pay for this independent certification, much as they presently pay for genetic tests, eye screening and tests for hip dysplasia. Several behavioral tests exist to measure specific traits, such as the Socially Acceptable Behavior test (64), which measures aggression, or the Dog Mentality Assessment test (65), which examines levels of playfulness, curiosity, aggression, sociability, and chase-proneness. In the USA, the Canine Good Citizen program, administered by the American Kennel Club, takes <30 min to administer and is designed to identify dogs that meet ten objectives consistent with being a good companion dog. Any one of these tests could be used as a basis for developing an assessment suited to breeding dogs—dogs that are not themselves good companions are less likely to produce puppies able to excel at this role.

“The Genetics of Human Fertility”, Kim & Lee 2019

2019-kim.pdf: “The genetics of human fertility”⁠, Yuri Kim, James J. Lee (2019-06-01; ):

Heritable variation in fitness—survival and reproduction—is the fuel of evolution by natural selection. Many human societies have dramatically reduced mortality before and during the prime reproductive years, making fertility a reasonably good proxy for the whole of fitness in much of our species. For this reason, empirical knowledge regarding the genetics of fertility must be an essential part of any framework for understanding past and ongoing trends in human adaptive evolution. Here we use R. A. Fisher’s analysis of human fertility as a starting point and find strong support from more recent research for his main contentions: fertility is a moderately heritable trait, where much of the genetic influences are shared with psychological characteristics.

“How Intelligence Affects Fertility 30 Years On: Retherford and Sewell Revisited—With Polygenic Scores and Numbers of Grandchildren”, Menie et al 2019b

2019-woodley-2.pdf: “How Intelligence Affects Fertility 30 Years On: Retherford and Sewell Revisited—With Polygenic Scores and Numbers of Grandchildren”⁠, Michael A. Woodley of Menie, Heiner Rindermann, Jonatan Pallesen, Matthew A. Sarraf (2019; ; similar):

Using newly available polygenic scores for educational attainment and cognitive ability, this paper investigates the possible presence and causes of a negative association between IQ and fertility in the Wisconsin Longitudinal Study sample, an issue that Retherford and Sewell first addressed 30 years ago. The effect of the polygenic score on the sample’s reproductive characteristics was indirect: a latent cognitive ability measure, comprised of both educational attainment and IQ, wholly mediated the relationship. Age at first birth mediated the negative effect of cognitive ability on sample fertility, which had a direct (positive) effect on the number of grandchildren. statistically-significantly greater impacts of cognitive ability on the sample’s fertility characteristics were found among the female subsample. This indicates that, in this sample, having a genetic disposition toward higher cognitive ability does not directly reduce number of offspring; instead, higher cognitive ability is a risk factor for prolonging reproductive debut, which, especially for women, reduces the fertility window and, thus, the number of children and grandchildren that can be produced. By estimating the effect of the sample’s reproductive characteristics on the strength of polygenic selection, it was found that the genetic variance component of IQ should be declining at a rate between −0.208 (95% CI [−0.020, −0.383]) and −0.424 (95% CI [−0.041, −0.766]) points per decade, depending on whether GCTA-GREML or classical behavior genetic estimates of IQ heritability are used to correct for ‘missing’ heritability.

“Cat Psychology & Domestication: Are We Good Owners?”, Branwen 2018

Cat-Sense: “Cat Psychology & Domestication: Are We Good Owners?”⁠, Gwern Branwen (2018-11-03; ⁠, ⁠, ⁠, ⁠, ; backlinks; similar):

Extended book review of Bradshaw 2013 (Cat Sense) on the connections between cat psychology, evolution/​genetics, history of domestication or lack thereof, & possible dysgenics, highlighting modern maladaptivity of cat psychology, with fulltext bibliography of key references.

I review John Bradshaw’s book on domestic cat psychology, Cat Sense, after difficulties dealing with my own cat. Bradshaw reviews the history of domestic cats from their apparent Middle Eastern origins as a small solitary desert predator to their domestication in Ancient Egypt where breeding millions of cats for sacrifice may have played a critical role (as opposed to any unique role as a vermin exterminator) through to the modern day and psychological studies of the learning abilities and personalities of cats, with particular emphasis on cat social skills in “cat colonies” & plasticity in kittenhood. As Bradshaw diagnoses it, these are responsible for what ability they have to modern pet life, even though they are not bred for this like dogs; every tame cat still has the feral cat in them, and are in many ways unsuited for contemporary living, with disturbing hints that human lack of selective breeding plus recent large-scale spay/​neuter population control efforts may be producing a subtle dysgenic effect on domestication, and this double neglect & backfire may be responsible for disturbingly high rates of cat maladaptation & chronic stress diseases.

“A Systematic Review of the State of Literature Relating Parental General Cognitive Ability and Number of Offspring”, Reeve et al 2018

2018-reeve.pdf: “A systematic review of the state of literature relating parental general cognitive ability and number of offspring”⁠, Charlie L. Reeve, Michael D. Heeney, Michael A. Woodley of Menie (2018-11-01; ; similar):

Highlights:

  • The relationship between general cognitive ability and reproduction is reviewed.
  • There is an inverse relation between cognitive ability and number of children.
  • The effect is stronger among females than males.
  • The effect appears to be increasing in strength over time.
  • Notable limitations of the current literature are reviewed.

The purpose of this study is to conduct a systematic review of the literature on the relationship between general cognitive ability and fertility among modern humans. Our goals were to (a) evaluate the state of the extant literature, and (b) provide a quantitative summary of effect sizes to the extent possible (given the limitations of the literature). A thorough search identified 17 unique datasets that passed the inclusion criteria. Using a Random Effects Model to evaluate the data, the overall weighted effect was r = −0.11, although the data also indicated a sex effect (stronger correlations among females than males), and a race effect (stronger correlations among Black and Hispanic populations compared to Whites). Importantly, the data suggest the correlation has been increasing in strength throughout the 20th century (and early 21st). Finally, we discovered several notable limitations of the extant literature; limitations that currently prohibit a psychometric meta-analysis⁠. We discuss these issues with emphasis on improving future primary studies to allow for more effective meta-analytic investigations.

[Keywords: intelligence, cognitive ability, g, reproductive success, meta-analysis, dysgenic trend]

“Evidence of a Nonadaptive Buildup of Mutational Load in Human Populations over the past 40,000 Years”, Aris-Brosou 2018

“Evidence of a nonadaptive buildup of mutational load in human populations over the past 40,000 years”⁠, Stéphane Aris-Brosou (2018-04-25; similar):

The role played by natural selection in shaping present-day human populations has received extensive scrutiny [1, 2, 3], especially in the context of local adaptations [4]. However, most studies to date assume, either explicitly or not, that populations have been in their current locations long enough to adapt to local conditions [5], and that population sizes were large enough to allow for the action of selection [6]. If these conditions were satisfied, not only should selection be effective at promoting local adaptations, but deleterious alleles should also be eliminated over time. To assess this prediction, the genomes of 2,062 individuals, including 1,179 ancient humans, were reanalyzed to reconstruct how frequencies of risk alleles and their homozygosity changed through space and time in Europe. While the overall deleterious homozygosity consistently decreased through space and time, risk alleles have shown a steady increase in frequency. Even the mutations that are predicted to be most deleterious fail to exhibit any significant decrease in frequency. These conclusions do not deny the existence of local adaptations, but highlight the limitations imposed by drift and range expansions on the strength of selection in purging the mutational load affecting human populations.

“Signatures of Negative Selection in the Genetic Architecture of Human Complex Traits”, Zeng et al 2018

2018-zeng.pdf: “Signatures of negative selection in the genetic architecture of human complex traits”⁠, Jian Zeng, Ronald Vlaming, Yang Wu, Matthew R. Robinson, Luke R. Lloyd-Jones, Loic Yengo, Chloe X. Yap et al (2018-01-01)

“The Contemporary Evolution of Fitness”, Hendry et al 2018

2018-hendry.pdf: “The Contemporary Evolution of Fitness”⁠, Andrew P. Hendry, Daniel J. Schoen, Matthew E. Wolak, Jane M. Reid (2018; backlinks; similar):

The rate of evolution of population mean fitness informs how selection acting in contemporary populations can counteract environmental change and genetic degradation (mutation, gene flow, drift, recombination). This rate influences population increases (eg. range expansion), population stability (eg. cryptic eco-evolutionary dynamics), and population recovery (ie. evolutionary rescue). We review approaches for estimating such rates, especially in wild populations. We then review empirical estimates derived from two approaches: mutation accumulation (MA) and additive genetic variance in fitness (IAw). MA studies inform how selection counters genetic degradation arising from deleterious mutations, typically generating estimates of <1% per generation. IAw studies provide an integrated prediction of proportional change per generation, nearly always generating estimates of <20% and, more typically, <10%. Overall, considerable, but not unlimited, evolutionary potential exists in populations facing detrimental environmental or genetic change. However, further studies with diverse methods and species are required for more robust and general insights.

“The Transition to Modernity and Chronic Disease: Mismatch and Natural Selection”, Corbett et al 2018

2018-corbett.pdf: “The transition to modernity and chronic disease: mismatch and natural selection”⁠, Stephen Corbett, Alexandre Courtiol, Virpi Lummaa, Jacob Moorad, Stephen Stearns (2018-01-01)

“Sexual Dimorphism in the Genetic Influence on Human Childlessness”, Verweij et al 2017

“Sexual dimorphism in the genetic influence on human childlessness”⁠, Renske M. Verweij, Melinda C. Mills, Felix C. Tropf, René Veenstra, Anastasia Nyman, Harold Snieder (2017-07-05; ; similar):

Previous research has found a genetic component of human reproduction and childlessness. Others have argued that the heritability of reproduction is counterintuitive due to a frequent misinterpretation that additive genetic variance in reproductive fitness should be close to zero. Yet it is plausible that different genetic loci operate in male and female fertility in the form of sexual dimorphism and that these genes are passed on to the next generation. This study examines the extent to which genetic factors influence childlessness and provides an empirical test of genetic sexual dimorphism. Data from the Swedish Twin Register (n = 9942) is used to estimate a classical twin model, a genomic-relatedness-matrix restricted maximum likelihood (GREML) model on twins and estimates polygenic scores of age at first birth on childlessness. Results show that the variation in individual differences in childlessness is explained by genetic differences for 47% in the twin model and 59% for women and 56% for men using the GREML model. Using a polygenic score (PGS) of age at first birth (AFB), the odds of remaining childless are around 1.25 higher for individuals with 1 SD higher score on the AFB PGS, but only for women.

We find that different sets of genes influence childlessness in men and in women. These findings provide insight into why people remain childless and give evidence of genetic sexual dimorphism.

“The Genomic Health of Ancient Hominins”, Berens et al 2017

“The genomic health of ancient hominins”⁠, Ali J. Berens, Taylor L. Cooper, Joseph Lachance (2017-06-02; similar):

The genomes of ancient humans, Neandertals, and Denisovans contain many alleles that influence disease risks. Using genotypes at 3180 disease-associated loci, we estimated the disease burden of 147 ancient genomes. After correcting for missing data, genetic risk scores were generated for nine disease categories and the set of all combined diseases. These genetic risk scores were used to examine the effects of different types of subsistence, geography, and sample age on the number of risk alleles in each ancient genome. On a broad scale, hereditary disease risks are similar for ancient hominins and modern-day humans, and the GRS percentiles of ancient individuals span the full range of what is observed in present day individuals. In addition, there is evidence that ancient pastoralists may have had healthier genomes than hunter-gatherers and agriculturalists. We also observed a temporal trend whereby genomes from the recent past are more likely to be healthier than genomes from the deep past. This calls into question the idea that modern lifestyles have caused genetic load to increase over time. Focusing on individual genomes, we find that the overall genomic health of the Altai Neanderthal is worse than 97% of present day humans and that Ötzi the Tyrolean Iceman had a genetic predisposition to gastrointestinal and cardiovascular diseases. As demonstrated by this work, ancient genomes afford us new opportunities to diagnose past human health, which has previously been limited by the quality and completeness of remains.

“Holocene Selection for Variants Associated With Cognitive Ability: Comparing Ancient and Modern Genomes”, Menie et al 2017

“Holocene selection for variants associated with cognitive ability: Comparing ancient and modern genomes”⁠, Michael A. Woodley Menie, Shameem Younuskunja, Balan Bipin, Piffer Davide (2017-02-21; ; similar):

Human populations living in Eurasia during the Holocene experienced significant evolutionary change. It has been predicted that the transition of Holocene populations into agrarianism and urbanization brought about culture-gene co-evolution that favoured via directional selection genetic variants associated with higher general cognitive ability (GCA). Population expansion and replacement has also been proposed as an important source of GCA gene-frequency change during this time period.

To examine whether GCA might have risen during the Holocene, we compare a sample of 99 ancient Eurasian genomes (ranging from 4,557 to 1,208 years of age) with a sample of 503 modern European genomes, using three different cognitive polygenic scores. Significant differences favouring the modern genomes were found for all three polygenic scores (Odds Ratio = 0.92, p = 0.037; 0.81, p = 0.001 and 0.81, p = 0.02). Furthermore, a statistically-significant increase in positive allele count over 3,249 years was found using a sample of 66 ancient genomes (r = 0.217, pone-tailed = 0.04).

These observations are consistent with the expectation that GCA rose during the Holocene.

“Selection against Variants in the Genome Associated With Educational Attainment”, Kong et al 2017

2017-kong.pdf: “Selection against variants in the genome associated with educational attainment”⁠, Augustine Kong, Michael L. Frigge, Gudmar Thorleifsson, Hreinn Stefansson, Alexander I. Young, Florian Zink et al (2017-01-11; backlinks; similar):

Epidemiological studies suggest that educational attainment is affected by genetic variants. Results from recent genetic studies allow us to construct a score from a person’s genotypes that captures a portion of this genetic component. Using data from Iceland that include a substantial fraction of the population we show that individuals with high scores tend to have fewer children, mainly because they have children later in life. Consequently, the average score has been decreasing over time in the population. The rate of decrease is small per generation but marked on an evolutionary timescale. Another important observation is that the association between the score and fertility remains highly statistically-significant after adjusting for the educational attainment of the individuals.

Epidemiological and genetic association studies show that genetics play an important role in the attainment of education. Here, we investigate the effect of this genetic component on the reproductive history of 109,120 Icelanders and the consequent impact on the gene pool over time. We show that an educational attainment polygenic score, POLYEDU, constructed from results of a recent study is associated with delayed reproduction (p < 10−100) and fewer children overall. The effect is stronger for women and remains highly statistically-significant after adjusting for educational attainment. Based on 129,808 Icelanders born between 1910 and 1990, we find that the average POLYEDU has been declining at a rate of ~0.010 standard units per decade, which is substantial on an evolutionary timescale. Most importantly, because POLYEDU only captures a fraction of the overall underlying genetic component the latter could be declining at a rate that is two to three times faster.

“Genome-wide Analysis Identifies 12 Loci Influencing Human Reproductive Behavior”, Barban et al 2016

2016-barban.pdf: “Genome-wide analysis identifies 12 loci influencing human reproductive behavior”⁠, Nicola Barban, Rick Jansen, Ronald de Vlaming, Ahmad Vaez, Jornt J. Mandemakers, Felix C. Tropf, Xia Shen et al (2016-10-31; ):

The genetic architecture of human reproductive behavior—age at first birth (AFB) and number of children ever born (NEB)—has a strong relationship with fitness, human development, infertility and risk of neuropsychiatric disorders. However, very few genetic loci have been identified, and the underlying mechanisms of AFB and NEB are poorly understood.

We report a large genome-wide association study of both sexes including 251,151 individuals for AFB and 343,072 individuals for NEB. We identified 12 independent loci that are statistically-significantly associated with AFB and/​or NEB in a SNP-based genome-wide association study and 4 additional loci associated in a gene-based effort.

These loci harbor genes that are likely to have a role, either directly or by affecting non-local gene expression, in human reproduction and infertility, thereby increasing understanding of these complex traits.

“How Cognitive Genetic Factors Influence Fertility Outcomes: A Mediational SEM Analysis”, Menie et al 2016

“How cognitive genetic factors influence fertility outcomes: A mediational SEM analysis”⁠, Michael A. Woodley Menie, Joseph A. Schwartz, Kevin M. Beaver (2016-08-18; ⁠, ; similar):

Utilizing a newly released cognitive Polygenic Score (PGS) from Wave IV of Add Health (n = 1,886), structural equation models (SEMs) examining the relationship between PGS and fertility (which is ~50% complete in the present sample), utilizing measures of verbal IQ and educational attainment as potential mediators, were estimated. The results of indirect pathway models revealed that verbal IQ mediates the positive relationship between PGS and educational attainment, and educational attainment in turn mediates the negative relationship between IQ and a latent fertility measure. The direct path from PGS to fertility was non-significant. The model was robust to controlling for age, sex and race, furthermore the results of a multi-group SEM revealed no statistically-significant differences in the estimated path coefficients across sex. These results indicate that those predisposed towards higher IQ by virtue of higher PGS values are also predisposed towards trading fertility against time spent in education, which contributes to those with higher PGS values producing fewer offspring.

“Assortative Mating and Differential Fertility by Phenotype and Genotype across the 20th Century”, Conley et al 2016

“Assortative mating and differential fertility by phenotype and genotype across the 20th century”⁠, Dalton Conley, Thomas Laidley, Daniel W. Belsky, Jason M. Fletcher, Jason D. Boardman, Benjamin W. Domingue et al (2016-05-31; similar):

We describe dynamics in assortative mating and fertility patterns by polygenic scores associated with anthropometric traits, depression, and educational attainment across birth cohorts from 1920 to 1955. We find that, for example, increases in assortative mating at the phenotypic level for education are not matched at the genotypic level. We also show that genes related to height are positively associated with fertility and that, despite a widening gap between the more and less educated with respect to fertility, there is no evidence that this trend is associated with genes. These findings are important to our understanding of the roots of shifting distributions of health and behavior across generations in US society.

This study asks two related questions about the shifting landscape of marriage and reproduction in US society over the course of the last century with respect to a range of health and behavioral phenotypes and their associated genetic architecture: (1) Has assortment on measured genetic factors influencing reproductive and social fitness traits changed over the course of the 20th century? (2) Has the genetic covariance between fitness (as measured by total fertility) and other traits changed over time? The answers to these questions inform our understanding of how the genetic landscape of American society has changed over the past century and have implications for population trends. We show that husbands and wives carry similar loadings for genetic factors related to education and height. However, the magnitude of this similarity is modest and has been fairly consistent over the course of the 20th century. This consistency is particularly notable in the case of education, for which phenotypic similarity among spouses has increased in recent years. Likewise, changing patterns of the number of children ever born by phenotype are not matched by shifts in genotype-fertility relationships over time. Taken together, these trends provide no evidence that social sorting is becoming increasingly genetic in nature or that dysgenic dynamics have accelerated.

[Keywords: assortative mating, fertility, polygenic scores, cohort trends]

“Mega-analysis of 31,396 Individuals from 6 Countries Uncovers Strong Gene-environment Interaction for Human Fertility”, Tropf et al 2016

“Mega-analysis of 31,396 individuals from 6 countries uncovers strong gene-environment interaction for human fertility”⁠, Felix C. Tropf, Renske M. Verweij, Peter J. van der Most, Gert Stulp, Andrew Bakshi, Daniel A. Briley et al (2016-05-02; ; similar):

Family and twin studies suggest that up to 50% of individual differences in human fertility within a population might be heritable. However, it remains unclear whether the genes associated with fertility outcomes such as number of children ever born (NEB) or age at first birth (AFB) are the same across geographical and historical environments. By not taking this into account, previous genetic studies implicitly assumed that the genetic effects are constant across time and space. We conduct a mega-analysis applying whole genome methods on 31,396 unrelated men and women from six Western countries. Across all individuals and environments, common single-nucleotide polymorphisms (SNPs) explained only ~4% of the variance in NEB and AFB. We then extend these models to test whether genetic effects are shared across different environments or unique to them. For individuals belonging to the same population and demographic cohort (born before or after the 20th century fertility decline), SNP-based heritability was almost five times higher at 22% for NEB and 19% for AFB. We also found no evidence suggesting that genetic effects on fertility are shared across time and space. Our findings imply that the environment strongly modifies genetic effects on the tempo and quantum of fertility, that currently ongoing natural selection is heterogeneous across environments, and that gene-environment interactions may partly account for missing heritability in fertility. Future research needs to combine efforts from genetic research and from the social sciences to better understand human fertility.

Authors Summary

Fertility behavior—such as age at first birth and number of children—varies strongly across historical time and geographical space. Yet, family and twin studies, which suggest that up to 50% of individual differences in fertility are heritable, implicitly assume that the genes important for fertility are the same across both time and space. Using molecular genetic data (SNPs) from over 30,000 unrelated individuals from six different countries, we show that different genes influence fertility in different time periods and different countries, and that the genetic effects consistently related to fertility are presumably small. The fact that genetic effects on fertility appear not to be universal could have tremendous implications for research in the area of reproductive medicine, social science and evolutionary biology alike.

“Mortality Selection in a Genetic Sample and Implications for Association Studies”, Domingue et al 2016

“Mortality Selection in a Genetic Sample and Implications for Association Studies”⁠, Benjamin W. Domingue, Daniel W. Belsky, Amal Harrati, Dalton Conley, David Weir, Jason Boardman (2016-04-21; ; similar):

Mortality selection is a general concern in the social and health sciences. Recently, existing health and social science cohorts have begun to collect genomic data. Causes of selection into a genomic dataset can influence results from genomic analyses. Selective non-participation, which is specific to a particular study and its participants, has received attention in the literature. But mortality selection—the very general phenomenon that genomic data collected at a particular age represents selective participation by only the subset of birth cohort members who have survived to the time of data collection—has been largely ignored.

Here we test the hypothesis that such mortality selection may significantly alter estimates in polygenic association studies of both health and non-health traits. We demonstrate mortality selection into genome-wide SNP data collection at older ages using the U.S.-based Health and Retirement Study (HRS). We then model the selection process. Finally, we test whether mortality selection alters estimates from genetic association studies. We find evidence for mortality selection. Healthier and more socioeconomically advantaged individuals are more likely to survive to be eligible to participate in the genetic sample of the HRS. Mortality selection leads to modest drift in estimating time-varying genetic effects, a drift that is enhanced when estimates are produced from data that has additional mortality selection.

There is no general solution for correcting for mortality selection in a birth cohort prior to entry into a longitudinal study. We illustrate how genetic association studies using HRS data can adjust for mortality selection from study entry to time of genetic data collection by including probability weights that account for mortality selection. Mortality selection should be investigated more broadly in genetically-informed samples from other cohort studies.

“Older Fathers’ Children Have Lower Evolutionary Fitness across Four Centuries and in Four Populations”, Arslan et al 2016

“Older fathers’ children have lower evolutionary fitness across four centuries and in four populations”⁠, Ruben C. Arslan, Kai P. Willführ, Emma Frans, Karin J. H. Verweij, Mikko Myrskylä, Eckart Voland, Catarina Almqvist et al (2016-03-08; ; similar):

Higher paternal age at offspring conception increases de novo genetic mutations (Kong et al 2012). Based on evolutionary genetic theory we predicted that the offspring of older fathers would be less likely to survive and reproduce, i.e. have lower fitness. In a sibling control study, we find clear support for negative paternal age effects on offspring survival, mating and reproductive success across four large populations with an aggregate N > 1.3 million in main analyses. Compared to a sibling born when the father was 10 years younger, individuals had 4–13% fewer surviving children in the four populations. Three populations were pre-industrial (1670–1850) Western populations and showed a pattern of paternal age effects across the offspring’s lifespan. In 20th-century Sweden, we found no negative paternal age effects on child survival or marriage odds. Effects survived tests for competing explanations, including maternal age and parental loss. To the extent that we succeeded in isolating a mutation-driven effect of paternal age, our results can be understood to show that de novo mutations reduce offspring fitness across populations and time. We can use this understanding to predict the effect of increasingly delayed reproduction on offspring genetic load, mortality and fertility.

“Determinants of Online Sperm Donor Success: How Women Choose”, Whyte & Torgler 2015

2015-whyte.pdf: “Determinants of online sperm donor success: how women choose”⁠, Stephen Whyte, Benno Torgler (2015-10-15; ; similar):

Because the worldwide demand for sperm donors is much higher than the actual supply available through fertility clinics, an informal online market has emerged for sperm donation. Very little empirical evidence exists, however, on this newly formed market and even less on the characteristics that lead to donor success. This article therefore explores the determinants of online sperm donors’ selection success, which leads to the production of offspring via informal donation. We find that donor age and income play a statistically-significant role in donor success as measured by the number of times selected, even though there is no requirement for ongoing paternal investment. Donors with less extroverted and lively personality traits who are more intellectual, shy and systematic are more successful in realizing offspring via informal donation. These results contribute to both the economic literature on human behaviour and on large-scale decision-making.

[Keywords: Online sperm donor market, informal market, offspring, donor success, personality traits]

“The Biodemography of Fertility: A Review and Future Research Frontiers”, Mills & Tropf 2015

“The Biodemography of Fertility: A Review and Future Research Frontiers”⁠, Melinda C. Mills, Felix C. Tropf (2015-09-21; ; backlinks; similar):

The social sciences have been reticent to integrate a biodemographic approach to the study of fertility choice and behaviour, resulting in theories and findings that are largely socially-deterministic. The aim of this paper is to first reflect on reasons for this lack of integration, provide a review of previous examinations, take stock of what we have learned until now and propose future research frontiers. We review the early foundations of proximate determinants followed by behavioural genetic (family and twin) studies that isolated the extent of genetic influence on fertility traits. We then discuss research that considers gene and environment interaction and the importance of cohort and country-specific estimates, followed by multivariate models that explore motivational precursors to fertility and education. The next section on molecular genetics reviews fertility-related candidate gene studies and their shortcomings and on-going work on genome wide association studies. Work in evolutionary anthropology and biology is then briefly examined, focusing on evidence for natural selection. Biological and genetic factors are relevant in explaining and predicting fertility traits, with socio-environmental factors and their interaction still key in understanding outcomes. Studying the interplay between genes and the environment, new data sources and integration of new methods will be central to understanding and predicting future fertility trends.

[Keywords: fertility, age at first birth, number of children ever born, genetics, behavioural genetics, molecular genetics, natural selection]

“Infertility Etiologies Are Genetically and Clinically Linked With Other Diseases in Single Meta-diseases”, Tarín et al 2015

“Infertility etiologies are genetically and clinically linked with other diseases in single meta-diseases”⁠, Juan J. Tarín, Miguel A. García-Pérez, Toshio Hamatani, Antonio Cano (2015-05-15; ; backlinks; similar):

The present review aims to ascertain whether different infertility etiologies share particular genes and/​or molecular pathways with other pathologies and are associated with distinct and particular risks of later-life morbidity and mortality. In order to reach this aim, we use two different sources of information: (1) a public web server named DiseaseConnect focused on the analysis of common genes and molecular mechanisms shared by diseases by integrating comprehensive omics and literature data; and (2) a literature search directed to find clinical comorbid relationships of infertility etiologies with only those diseases appearing after infertility is manifested. This literature search is performed because DiseaseConnect web server does not discriminate between pathologies emerging before, concomitantly or after infertility is manifested. Data show that different infertility etiologies not only share particular genes and/​or molecular pathways with other pathologies but they have distinct clinical relationships with other diseases appearing after infertility is manifested. In particular, (1) testicular and high-grade prostate cancer in male infertility; (2) non-fatal stroke and endometrial cancer, and likely non-fatal coronary heart disease and ovarian cancer in polycystic ovary syndrome; (3) osteoporosis, psychosexual dysfunction, mood disorders and dementia in premature ovarian failure; (4) breast and ovarian cancer in carriers of BRCA1/​2 mutations in diminished ovarian reserve; (5) clear cell and endometrioid histologic subtypes of invasive ovarian cancer, and likely low-grade serous invasive ovarian cancer, melanoma and non-Hodgkin lymphoma in endometriosis; and (6) endometrial and ovarian cancer in idiopathic infertility. The present data endorse the principle that the occurrence of a disease (in our case infertility) is non-random in the population and suggest that different infertility etiologies are genetically and clinically linked with other diseases in single meta-diseases. This finding opens new insights for clinicians and reproductive biologists to treat infertility problems using a phenomic approach instead of considering infertility as an isolated and exclusive disease of the reproductive system/​hypothalamic-pituitary-gonadal axis. In agreement with a previous validation analysis of the utility of DiseaseConnect web server, the present study does not show an univocal correspondence between common gene expression and clinical comorbid relationship. Further work is needed to untangle the potential genetic, epigenetic and phenotypic relationships that may be present among different infertility etiologies, morbid conditions and physical/​cognitive traits.

“Physical Attractiveness and Reproductive Success in Humans: Evidence from the Late 20th Century United States”, Jokela 2009

“Physical attractiveness and reproductive success in humans: Evidence from the late 20th century United States”⁠, Markus Jokela (2009-09-01; ; backlinks; similar):

Physical attractiveness has been associated with mating behavior, but its role in reproductive success of contemporary humans has received surprisingly little attention.

In the Wisconsin Longitudinal Study (WLS; 1244 women, 997 men born between 1937 and 1940) we examined whether attractiveness assessed from photographs taken at age ~18 predicted the number of biological children at age 53–56.

In women, attractiveness predicted higher reproductive success in a nonlinear fashion, so that attractive (second highest quartile) women had 16% and very attractive (highest quartile) women 6% more children than their less attractive counterparts. In men, there was a threshold effect so that men in the lowest attractiveness quartile had 13% fewer children than others who did not differ from each other in the average number of children. These associations were partly but not completely accounted for by attractive participants’ increased marriage probability. A linear regression analysis indicated relatively weak directional selection gradient for attractiveness (β = 0.06 in women, β = 0.07 in men).

These findings indicate that physical attractiveness may be associated with reproductive success in humans living in industrialized settings.

[Keywords: fertility, interbirth interval, physical attractiveness, reproductive success, offspring sex ratio, sociobiology]

“Secular Declines in Cognitive Test Scores: A Reversal of the Flynn Effect”, Teasdale & Owen 2008

2008-teasdale.pdf: “Secular declines in cognitive test scores: A reversal of the Flynn Effect”⁠, Thomas W. Teasdale, David R. Owen (2008-03-01; ; backlinks; similar):

Scores on cognitive tests have been very widely reported to have increased through the decades of the last century, a generational phenomenon termed the ‘Flynn Effect’ since it was most comprehensively documented by James Flynn in the 1980s. There has, however, been very little evidence concerning any continuity of the effect specifically into the present century.

We here report data from a population, namely young adult males in Denmark, showing that whereas there were modest increases between 1988 and 1998 in scores on a battery of 4 cognitive tests—these constituting a diminishing continuation of a trend documented back to the late 1950s—scores on all 4 tests declined between 1998 and 2003–2004. For 2 of the tests, levels fell to below those of 1988. Across all tests, the decrease in the 5–6 year period corresponds to ~1.5 IQ points, very close to the net gain between 1988 and 1998. The declines between 1998 and 2003–4 appeared amongst both men pursuing higher academic education and those not doing so.

[Keywords: cognitive tests, secular trend, Flynn Effect]

“Feral Cats: Their Role in the Population Dynamics of Felis Catus”, Bradshaw et al 1999

1999-bradshaw.pdf: “Feral cats: their role in the population dynamics of Felis catus⁠, J. W. S. Bradshaw, G. F. Horsfield, J. A. Allen, I. H. Robinson (1999-12; ; backlinks; similar):

The so-called domestic cat occupies an unique position within the truly domestic animals since it freely interbreeds with feral populations, and there is considerable gene flow in both directions. This is possible because the likelihood of an individual cat forming a relationship with people is strongly affected by its experiences during the socialisation period (3–8 weeks of age), although this does not preclude differences between owned and feral populations in the relative frequencies of alleles which affect social behaviour towards humans.

We suggest a hitherto unconsidered reason why a separate domesticated population of cats (apart from pedigree breeds) has not yet emerged: the unusual and stringent nutrient requirements of the cat may historically have militated against successful breeding on a completely human-provided diet, and led to the retention of the ability to achieve a nutritionally complete diet by scavenging and/​or hunting. More recently, the widespread availability of nutritionally complete manufactured foods and veterinary care in western countries appears to be leading towards a rapid change in the population dynamics and population genetics of both owned and feral cats.

[Keywords: domestication, feral populations, population dynamics, cat]

“DYSGENICS: Genetic Deterioration in Modern Populations”, Writer 1996

1996-lynn-dysgenics.pdf: “DYSGENICS: Genetic Deterioration in Modern Populations”⁠, Writer (1996-01-01; backlinks)

“New Methodology to Reduce the Environment-Heredity Uncertainty About Dysgenics”, Shockley 1970

1970-shockley.pdf: “New Methodology to Reduce the Environment-Heredity Uncertainty About Dysgenics”⁠, William Shockley (1970-01-01)

“Interplay Between Social Ecology and Physiology, Genetics and Population Dynamics of Mice”, Kessler 1966

1966-kessler.pdf: “Interplay Between Social Ecology and Physiology, Genetics and Population Dynamics of Mice”⁠, Alexander Kessler (1966-03-31; ; backlinks; similar):

The interplay between socioecologlcal and biological processes manifests Itself at the level of individuals, populations, and species. The biology of Individuals is deeply modified when they are groups; many of the attributes of populations such as size, distribution, composition, etc. are related to social interactions, and at the level of species, patterns of social relations within groups tend to be structured in ways that influence survival, reproduction, and exchange among populations.

In one experimental approach to these problems, the social ecology of freely growing populations of mice In large enclosures was related to behavioral, physiological, and health changes of individuals, to demographic changes and to changes of gene frequencies. Another experiment examined the process and effects of artificial selection for the same trait in different social environments.

Population Experiment

The population enclosures were octagonal structures subdivided Into central and peripheral sections with a total surface area of 13.3 square feet. From a founder group of mice of known genetic (progeny of a four-way cross among inbred mouse strains C57L/​J, SWR/​J, C3HeB/​FeJ, 129/​J) and environmental background, three equivalent samples of mice were distributed into replicate population enclosures (Pop A and B) and into standard laboratory cages as randomly mated male-female pairs—the control group (Pop C).

During the first year of study, daily observations of the enclosures were made, and several censuses were performed. Identifiable cohorts, animals born during each census interval, were established to provide an additional way of analyzing changes in the populations.

In Pop C, reproduction remained constant and mortality was negligible. Marked changes occurred in Pop A and B. The sizes (1000-A and 800-B mice) and densities (85-A and 60-B mice per square foot) are several times greater than those of any previously reported population of small mammals. However, there would have been 100,000 mice in each enclosure at the end of a year had the populations continued to grow as they did at first. Changes of reproductive physiology constituted prominent aspects of self-regulation in the enclosures. Peak demographic input rates occurred during the third month, but were already associated with decreased productivity per adult female. Analysis of maturation and reproduction pointed to inhibition of reproduction in sexually mature females as the most important factor in the decline of productivity. Pregnancy rates fell steadily and inhibition of full-term gestation occurred. Gonads and reproductive cells of males were adult, but a large proportion of males showed little sexual activity.

Neonatal mortality was particularly striking in Pop B, where 30% of females showed advanced pregnancy during the last 5.5 months with no newborns surviving. About 25% of the mice in the enclosures died during the year. Highest weekly death rates occurred during the first half of the year before peak numbers were present. Autopsies of mice of Pop A revealed little in the way of abnormal findings.

Biomass either paralleled or increased more rapidly than numbers in both enclosures, contrasting with some other population: studies in which growth was impaired with crowding.

Changes of behavior included: 1. disappearance of circadian activity peaks, 2. decline in frequency of fighting per male but an increase in unusual aggressiveness, 3. aberrations of sexual behavior, 4. deterioration of maternal care, 5. cannibalism, 6. striking decrease in social responsiveness.

Cohorts in the populations were biologically distinguishable sub-units in contrast to control cohorts, which showed no such differentiation. Cohorts in Pop A and B differed with respect to reproduction physiology, mortality, and behavior, and intercohort differences persisted at all levels of population density.

Many of the properties of Pop A and B mice changes when the mice were placed in different social environments, attesting to the specificity of the influence of social factors. For example, mice of Pop A, randomly paired in control cages, showed a marked rise in reproduction, and cohorts reproductively inhibited before were most productive in the new social environment. Behavioral tests performed outside the enclosure environment revealed: 1. intercohort differences among Pop A mice contrasted with stereotyped behavior of Pop C mice, and 2. changes in behavior of Pop A mice both immediately after removal from the population and after six weeks in new social conditions. Pop B mice changed their social environment by emigrating into the empty interconnected enclosure of Pop A. Two distinctive sub-populations formed. Greater changes in reproduction, mortality, and behavior occurred in the emigrant subpopulation, which underwent more extensive social reorganization. Immediately following reunion of the two subpopulations, a population crash occurred, possibly related to the sudden changes of social conditions.

Use of genetically defined animals made feasible the study of gene frequency changes. Polymorphism of alleles at the C locus affecting coat color differed between Pop A and B on the one hand and Pop C on the other. Although the magnitude of the upward change of recessive c in Pop A and B was not large, the consistency and similarity of the change in Pop A and B and lack of change in Pop C suggested the action of systematic processes and the probable adaptiveness of the changes. There was little evidence of differential adult reproduction or mortality among the phenotypes but there were suggestions of differential neonatal survival. The relatively slow rate of change of the alleles after the first generation suggested the establishment of a state of balanced polymorphism at the C locus. Hemoglobin allele and genotype frequencies of mice of Pop A alive at the end of the year did not deviate from what might have been predicted on the basis of panmixia.

Selection Experiment

Selection for the same trait in varied environments tends to involve genetic and physiological differences. The question of adaptability to different social environments was studied; heavy body weight at sexual maturity was chosen as the trait for selection; groups of different sizes—pairs or groups of 20–30 mice—were the environmental variables. Sexes were kept separate between weaning and sexual maturity. A within-litter selection method was used.

Crowding depressed weight at sexual maturity but equal improvement with selection occurred in both social environments. Heritability was also equal in crowded and uncrowded groups. Environmental exchange carried out in the sixth and seventh generation suggested that mice selected in crowded environments performed slightly better in both crowded and uncrowded environments.

The large sizes and unusual degree of crowding attained by the freely growing populations in this study compared with previous studies may be related to the types of animals used, to the number of individuals in the founder nuclei, and to the physical structure of the enclosures. Extreme crowding was compatible with general physical health. The decline of fertility and fecundity, the decreased survival of newborns, and the appearance of behavioral aberrations—rather than disease or an increase in adult mortality—represented the major self-regulatory mechanisms that eventually limited population growth. The growth of individuals was not inhibited. Social withdrawal and the decline of social interaction rather than a rise of interaction characterized the populations. Such findings cast doubt about the generality of the so-called “Stress” theory of social ecology that emphasizes increased interaction and pituitary-adrenal hyperactivity as the principal mechanisms involved in self-regulation of vertebrate populations.

Other formulations of mammalian social ecology, such as those that focus on the importance of early development, of spatial requirements, of neurophysiological reactivity, and of communications, constitute additional explanations of the interplay of social and biological processes in crowded populations.

Although man’s potential reactions are more complex and variable than those of lower vertebrates and give prominence to the role of symbols and culture, his social environment is even more fundamental to his entire existence. This, if anything, increases the importance of the interplay of socioecological and biological processes for man.

“Why the Red Delicious No Longer Is”

https://www.washingtonpost.com/wp-dyn/content/article/2005/08/04/AR2005080402194_pf.html: “Why the Red Delicious No Longer Is” (; backlinks)

“You and Some 'cavemen' Get a Genetic Checkup”

https://phys.org/news/2017-08-cavemen-genetic-checkup.html: “You and some 'cavemen' get a genetic checkup”

Miscellaneous