It takes remarkably little for children to develop in-group preferences. Vittrup's mentor at the University of Texas, Rebecca Bigler, ran an experiment in three preschool classrooms, where 4- and 5-year-olds were lined up and given T shirts. Half the kids were randomly given blue T shirts, half red. The children wore the shirts for three weeks. During that time, the teachers never mentioned their colors and never grouped the kids by shirt color.

The kids didn't segregate in their behavior. They played with each other freely at recess. But when asked which color team was better to belong to, or which team might win a race, they chose their own color. They believed they were smarter than the other color.