Prejudice is a trait humans have retained from their evolutionary predecessors

A new study led by Yale researchers has found that the tendency to perceive others as 'us versus them' isn't exclusively human but appears to be shared by our primate cousins.

In a series of ingenious experiments, Yale researchers led by psychologist Laurie Santos showed that monkeys treat individuals from outside their groups with the same suspicion and dislike as their human cousins tend to treat outsiders, suggesting that the roots of human intergroup conflict may be evolutionarily quite ancient.

"One of the more troubling aspects of human nature is that we evaluate people differently depending on whether they're a member of our 'ingroup' or 'outgroup,'" Santos said.

"Pretty much every conflict in human history has involved people making distinctions on the basis of who is a member of their own race, religion, social class, and so on.

The question we were interested in is: Where do these types of group distinctions come from?"

The answer, she adds, is that such biases have apparently been shaped by 25 million years of evolution and not just by human culture.

Santos and her lab studied the rhesus macaques living on an island off the coast of Puerto Rico. Like humans, monkeys in this population naturally form different social groups on the basis of family history. In order to assess whether monkeys made the same distinctions between ingroup and outgroup individuals, the researchers used a well-known tendency of animals to stare longer at novel or frightening things than at familiar or friendly things.

They presented subject monkeys with pictures of monkeys who were either in their social group or members of a different group. They found that monkeys stared longer at pictures of other monkeys who were outside their group, suggesting that monkeys spontaneously detect who is a stranger and who is a group member.

The Yale team's results suggest that the distinctions humans make between 'us' and 'them'- and therefore the roots of human prejudice-may date back at least 25 million years, when humans and rhesus macaques shared a common ancestor.

The findings are reported in the March issue of the Journal of Personality and Social Psychology.