Social psychological research has shown that humans are inclined to separate into groups and form identities based on group membership. Once part of a group, members tend to evaluate in-group members positively and out-group members negatively. When concrete rewards are involved, they reward in-group members more generously than out-group members.
It turns out that people will do this based on almost any characteristic and, in fact, will even do so when they are grouped randomly and informed that the groups are random. This is called the minimal group paradigm.
xkcd reminds us how silly this is:
Via Missives From Marx.Lisa Wade, PhD is a professor at Occidental College. She is the author of American Hookup, a book about college sexual culture, and Gender, a textbook. You can follow her on Twitter, Facebook, and Instagram.