The vast majority of psychological studies recruit test subjects who are Western, educated, industrialized, rich and democratic – or "WEIRD." Which is... well, weird, given that these people represent a very small segment of the global human population, from a cultural standpoint.
You might expect culture to have very little impact on the results of a psychological study. If you assume that psychology is informed primarily by biological hardwiring, for instance, then it doesn't matter if your test subject is a frat bro senior at Alabama State or a Hamer tribeswoman from Ethiopia's Omo Valley. But a growing body of evidence suggests that this is far from the case.
Much of this evidence is presented in a 2009 publication titled "The Weirdest People in the World." Authored by Jospeh Heinrich, Steven J. Heine and Ara Norenzayan of the University of British Columbia, the paper coins the "WEIRD" acronym, and reviews the evidence for how WEIRD people differ from other populations. A must-read feature published earlier this year at Pacific Standard gives a tidy summary of their results (emphasis added):
It is not just our Western habits and cultural preferences that are different from the rest of the world, it appears. The very way we think about ourselves and others—and even the way we perceive reality—makes us distinct from other humans on the planet, not to mention from the vast majority of our ancestors. Among Westerners, the data showed that Americans were often the most unusual, leading the researchers to conclude that “American participants are exceptional even within the unusual population of Westerners—outliers among outliers.”
Given the data, [Heinrich and his colleagues] concluded that social scientists could not possibly have picked a worse population from which to draw broad generalizations. Researchers had been doing the equivalent of studying penguins while believing that they were learning insights applicable to all birds.
If you haven't read Pacific Standard's review of the research, do so now. It's long, but well worth the read. If you're pressed for time, or you've already read it and are looking for a quick refresher, here's a great visual overview of Henrich's paper. The infographic (which borrows its title straight from the headline of the PS feature) includes numerous examples of how WEIRD test subjects have been found to differ from the rest of the world:
Infographic Source: Best Psychology Degrees