Why do individuals of some animal species live in cooperative social groups? An analysis across bird species indicates that whether or not a species is cooperative depends on the sex lives of females. Cooperation is more likely when promiscuity is low, when females mate with only a single male for instance, which means helpers can be more sure that they are helping relatives. Intermediate levels of promiscuity favour the ability to distinguish relatives from non-relatives, but at high levels of promiscuity no form of cooperation is favoured. [Letter p. 969; News & Views p. 930]
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