Indirect Reciprocity and the Evolution of Prejudicial Groups

Prejudice is a human attitude involving generally negative and unsubstantiated prejudgement of others. When acted upon, this results in wide-ranging behaviours such as sexism, ageism and discrimination against sexual preference1,2,3 through to ethnic, racial, nationalistic and religious extremism4,5, with bias and intergroup conflict characterised as a "problem of the century"6. Most recently, prejudice has been highlighted in connection to global political events: for example anti-immigration prejudice was a strong correlate of support for Brexit7. The human disposition to categorize others through their group identity creates an opportunity for discrimination8,9,10. As a consequence of in-group formation11, which occurs through cultural or biological identification with others, or as a consequence of identity-less strangers mutually cooperating12, bias can take hold in two ways.