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 is a professor of sociology at Occidental College and the co-author of Gender: Ideas, Interactions, Institutions. You can follow her on Twitter and Facebook.