The Evolution of Prejudice

by Daisy Grewal

Scientific American
April 5, 2011

Psychologists have long known that many people are prejudiced towards others based on group affiliations, be they racial, ethnic, religious, or even political. However, we know far less about why people are prone to prejudice in the first place. New research, using monkeys, suggests that the roots lie deep in our evolutionary past.

Yale graduate student Neha Mahajan, along with a team of psychologists, traveled to Cayo Santiago, an uninhabited island southeast of Puerto Rico also known as “Monkey Island,” in order to study the behavior of rhesus monkeys. Like humans, rhesus monkeys live in groups and form strong social bonds. The monkeys also tend to be wary of those they perceive as potentially threatening.

To figure out whether monkeys distinguish between insiders (i.e. those who belong to their group) and outsiders (i.e. those who don’t belong), the researchers measured the amount of time the monkeys stared at the photographed face of an insider versus outsider monkey. Across several experiments, they found that the monkeys stared longer at the faces of outsiders. This would suggest that monkeys were more wary of outsider faces.

However, it is also possible that outsiders simply evoke more curiosity. To rule this out, the researchers took advantage of the fact that male rhesus monkeys leave their childhood groups once they reach reproductive age. This allowed the researchers to pair familiar outsider faces (monkeys that had recently left the group) with less familiar insider faces (monkeys that had recently joined the group). When presented with these pairs, the monkeys continued to stare longer at outsider faces, even though they were more familiar with them. The monkeys were clearly making distinctions based on group membership.

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