Most psychological experiments base their conclusions on samples of what are termed "WEIRD" people – that is, people from Western, educated, industrialized, rich, democratic cultures. However, anthropologist Joseph Henrich and psychologists Steven Heine and Ara Norenzayan of the University of British Columbia in Canada have shown that WEIRD people are not at all representative of humanity at large.
Perhaps the most striking example the researchers give concerns the famous Müller-Lyer illusion in which two line segments of equal length with arrow-like ends pointing towards or away from their centres are shown to subjects who are asked if one line looks longer than the other. While WEIRD people tend to see significant differences in the line lengths, most other cultures are taken in to much lesser degrees, and the San people of the Kalahari in southern Africa seem essentially immune. As the paper says in its closing words: "The sample of contemporary Western undergraduates that so overwhelms our database is not just an extraordinarily restricted sample of humanity; it is frequently a distinct outlier vis à vis other global samples. It may represent the worst population on which to base our understanding of Homo sapiens."