Small‐scale cooperation can be sustained by the same mechanisms that give rise to cooperation among nonrelatives in other animals—reciprocity and self‐interested sanctions. Large‐scale cooperation requires something more. If there were no large‐scale cooperation in the Pleistocene, it would be possible to believe that human psychology was shaped in a world of small‐scale cooperation, and this psychology misfires to give rise to modern large‐scale, inclusive fitness reducing cooperation. If foragers did cooperate on large‐scales during the Pleistocene, additional, human‐specific, mechanisms were necessary, and this leads to quite different proposals about the psychology of contemporary human cooperation.
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