The most dramatic effects of social categorization have been demonstrated using the minimal intergroup paradigm pioneered by Tajfel and his co workers. Here participants are simply informed that they have been divided into groups, often on a relatively arbitrary basis. These groups are truly minimal, involving no group activity or contact between members. Yet when these participants are asked to evaluate anonymous ingroup and outgroup members or apportion rewards between them, they do so in a biased and discriminatory fashion, favoring ingroup over outgroup members.

Oxford Handbook of Political Psychology, p560

Nothing most of you didn’t know already, but still. I think it’s good reminding oneself of how instinctively full of prejudices we all are. That awareness might actually make us better people.. And as Voltaire said, Prejudice is the reason of fools.