Cooperation among workers and their seeming altruism result from strict policing by nestmates.Workers of many species of ant, bee and wasp do not lay eggs, despite having functional ovaries, but the selective causes of this extreme form of altruism are unclear. Here we show that workers forego reproduction in response to the threat of their eggsbeing killed, or 'policed', by nestmates. Our results indicate that social coercion helps to explain worker altruism and cooperation in modern-day insect societies. Why, in some species, do most workers forego direct reproduction? One possibility is thatworker altruism is voluntary: in this scenario, high genetic relatedness should drive the evolution of altruism and worker sterility7 because higher relatedness increases the indirect benefit of working. Theoretically, however, worker altruism could also be enforced' and may have evolved in response to social sanction. In many species, worker-laid eggs are killed by the queen or by other workers and, if these sanctions are effective, the advantage to workers of laying eggs is reduced. As a result, more would be selected to work altruistically, rather than to lay eggs.
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