Altruism among non-relatives: alternatives to the ‘Prisonerʹs Dilemma’

Triverʹs model of reciprocal altruism, and its descendants based on the Prisonerʹs Dilemma model, have dominated thinking about cooperation and altruism between non-relatives. However, there are three alternative models of altruism directed to non-relatives. These models, which are not based on the Prisonerʹs Dilemma, may explain a variety of phenomena, from allogrooming among impala to helping by non-relatives in cooperatively breeding birds and mammals