Psychologists have long known that many of our prejudices operate automatically, without us even being aware of them.  Most people, even those who care deeply about equality, show some level of prejudice towards other groups when subjected to formal psychological testing.   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,” to study if our basic tendency to see the world in terms of “us” and “them” has ancient origins.  Their research suggests that monkeys not only distinguish between insiders and outsiders, but also associate insiders with good things and outsiders with bad things.  Overall, these results support an evolutionary basis for prejudice.

http://www.scientificamerican.com/article.cfm?id=evolution-of-prejudice&WT.mc_id=SA_20110420