PhysiognomyMacbeth said "there's no art to find the mind's construction in the face," but Shakespeare had almost certainly heard of physiognomy, the medieval "science" of determining someone's character based on their appearance.
The premise is pretty straightforward. Physiognomists believe that personality traits are evidenced in the face — not by a simple "smiling-snarling" analysis, but stuff like the length of your nose and the angle of your cheeks.
If you're thinking "that's pretty stupid," give yourself a gold star. If you're thinking this sounds like a flimsy bit of sophistry intended to justify a shallow societal tendency to ostracize ugly people, give yourself a gold star and a master's degree in the history of philosophy.
Although the roots of the practice go back nearly as far as written history, physiognomy as a formal pretend science dates back as far as the 14th century, when people with names like Giambattista della Porta and Barthélemy Coclès wrote long and lovingly detailed treatises on how to evaluate people by their faces.
Because these were the early days of fake science, Coclès and Porta didn't know to dress up their findings in semantic frippery and bogus data. They just put forth the very basic premise that you could figure out someone's personality by estimating which animal they most looked like. (In fairness, it should be noted that Porta was otherwise a brilliant early scientist whose writings about magic and alchemy actually foreshadowed a lot of modern knowledge. Everyone has a bad day.)
So, if you looked like a pig, you were sloppy and brutish. If you looked like, I don't know, a gazelle, you were swift and graceful. You get the idea anyway.
As scientific concepts go, this was not among the most inspired. "Hey, piggy" works very well in the second-grade, but it's not a philosophy to live by.
As you might expect, physiognomy became very popular, particularly as a tool for racists seeking pretend scientific justification for being such hateful assholes. Physiognomy continues to perform that function to this day.
Around the 18th century, in an environment of mounting hostility between people of African descent and the white folk who were brutally enslaving them, the latter group began trying to concoct rationalizations for why they were such dickwads. Typically, these were along the following lines:
Charming. But blacks were hardly the only targets of the physiognomist racists. Why single out Africans, when there are plenty of Jews and Asians around? Physiognomy is a central belief of the Christian Identity white racist movement in the U.S., and it was a favorite past-time of the Nazis as well.
When the Nazis took Vienna, they rounded up hundreds of Jews and analyzed their faces, looking for the "perfect Jew" to use as a model for identifying and studying the race. They measured their noses, their chins, their lips, and made latex masks from 19 carefully selected faces, which were then used as the guideline for further racial "research" by Dr. Josef Mengele, among others.
Today, physiognomy endures primarily as an anachronism, used by people whose beliefs are so monumentally stupid that the "science" of physiognomy actually makes them look better than they would in a vacuum.
In addition to the Christian Identity types, there are a few remaining New Age types who still think there's something to the idea. Even on the relative scale, this subset people tend toward the inordinately gullible and simple-minded (i.e., people who spend money on certified crystals from Atlantis).
Of course, it's somewhat disingenuous to say "you can't judge a book by its cover," because everyone does it all the time. And not just books.
We make pretty much all our decisions on the basis of how things look. We buy houses, cars, cheeseburgers and mates on the basis of how they look. Which is why we so often get stuck with bad drywall, low gas mileage, high cholesterol and "It's not you, it's me."
But the enlightenment of the modern age at least allows us to boast that very few of us would even argue that it's a good idea, let alone a science. Yay, progress!