Ingroup bias is the preferential treatment people give to whom they perceive to be members of their own groups.
Experiments in psychology have shown that group members will award one another higher payoffs even when the "group" they share seems random and arbitrary, such as having the same birthday, having the same final digit in their U.S. Social Security Number, or even being assigned to the same flip of a coin.
Ingroup effects appear to be stronger, however, when the group is smaller relative to another high-power group.
This cognitive bias has been studied extensively by Henri Tajfel. It is considered a group-serving bias, as opposed to an outgroup homogeneity bias.