Parochial altruism: does it explain modern human group psychology?

• Evidence for the predictions of the evolution of parochial altruism is reviewed.
• Earlier findings of unconditional intra-group cooperation are questioned.
• Most studies do not support non-instrumental, non-retaliatory aggression.
• Mixed evidence for the relationship between group-based cooperation and aggression.

Parochial altruism — the human inclination toward costly intra-group cooperation and inter-group aggression without expectations of future returns — requires group selection logic to explain its evolution. We examined experimental evidence for three implications of the group selection account: the unconditional nature of intra-group cooperation; the non-instrumental, non-retaliatory, and costly nature of inter-group aggression; and the positive relationship between intra-group cooperation and inter-group aggression. Laboratory experiments revealed no support for the unconditional nature of intra-group cooperation, mostly negative evidence for the non-instrumental, non-retaliatory, and costly nature of inter-group aggression, and mixed evidence for the positive relationship between intra-group cooperation and inter-group aggression. Caution against premature conclusions about the role of group selection in the evolution of parochial altruism is advised.