Most of us are familiar with the Muller-Lyer optical illusion above, named after its creator, German psychologist Franz Carl Müller-Lyer.
Like most optical illusions, it is designed to test basic brain and visual functions, helping us learn how and why human senses, cognition, etc. work the way they do. Many folks think the second line is longer than the first, even though both are the same, which purportedly shows that humans are susceptible to certain visual guides like arrows (though explanations for why this happens vary).
But the results do not tell the whole story: while many Westerners fall for this illusion (myself included), a study of 14 indigenous cultures found that none were tricked to the same degree. In fact, some cultures, like the San people of the Kalahari Desert, knew the two lines were equal length.
That’s because most studies claiming to reflect universal traits of human psychology and physiology only do so for a small and specific demographic—people from “WEIRD” societies, or Western, Educated, Industrialized, Rich and Democratic—which represent a tiny minority of all humans (about 12 percent).
The “WEIRD” phenomenon was first described in a 2010 paper from the University of British Columbia in Vancouver, which found that 96 percent of studies in economics, psychology, and cognitive science—such as the ones on optical illusions—were performed on people with European backgrounds. A sample of hundreds of studies in leading psychology journals found close to 70 percent of subjects were from the U.S., and of these, 67 percent were undergraduates studying psychology (which further slants studies to reflect one particular age group).
All this means that a randomly selected American undergraduate is 4,000 times likelier to be a subject in a psych study—and thus reflect all of human nature—than a random non-Westerner.
Yet when scientists perform some of these experiments in other cultures, the results are very different—not just for optical illusions, but for things as diverse as moral reasoning, notions of fairness, and sexual behavior. Even mental disorders seem to manifest differently across cultures and ethnic groups: one small study found that people with schizophrenia in India and Ghana hear friendlier voices than their counterparts in the U.S., suggesting that culture and environment may play a role. (This may account for why Westerners have a harder time with the Muller-Lyer optical illusion than some indigenous people: Most Americans are raised in urban environments where horizontal lines and sharp corners are ubiquitous; this presumably influences us into making optical calibrations that can potentially misfire, which forager societies like the San do not have to worry about.)
In fact, people from WEIRD societies like the U.S. appear to be outliers among humans, with the authors of the UBC concluding that Westerners “are among the least representative populations one could find for generalizing about humans”.
As an writer for NPR blithely noted, “It was not so much that the emperor of psychology had no clothes. It was more that he was dancing around in Western garb pretending to represent all humanity”.
Fortunately, researchers have wizened to these biases over the past decade, carefully adding qualifiers and caveats such as “in college populations” or “in Western society.” But its still easy for journalists, analysts, and casual readers like ourselves to read the findings of these studies and ascribe them to all of humanity. Much of human nature, like humans themselves, is a lot more complicated and multi-variable than WEIRD folks suggest.