The effectiveness of policing

of many species of ant, bee and wasp do not lay eggs, despite having functional ovaries 1 , but the selective causes of this extreme form of altruism are unclear 2–7. Here we show that workers forego reproduction in response to the threat of their eggs being killed, or 'policed' , by nestmates. Our results indicate that social coercion helps to explain worker altruism and cooperation in modern-day insect societies 3–5. Why, in some species, do most workers forego direct reproduction? One possibility is that worker altruism is voluntary: in this scenario, high genetic relatedness should drive the evolution of altruism 2–7 and worker sterility 7 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 sanctions 4–6. In many species, worker-laid eggs are killed by the queen or by other workers 1,8,9 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 6. The role of sanctions in promoting worker sterility has long been suspected 8,9 , but has never been tested in a comparative study. We therefore studied ten single-queen species, nine Vespidae wasps (for example, see Fig.1) and the honey-bee Apis mellifera. These are the only species for which there are data to quantify both the proportion of non-egg-laying workers, a measure of worker altruism, and two key predictor variables: relatedness among workers, which depends on the frequency of queen mating, and the effectiveness with which worker-laid eggs are policed by nestmates 1,8,9. We analysed the data by using individual species as data points and by phylogenetically independent contrasts (PICs) to control for phylogenetic non-independence. (See supplementary information.) Figure 2 shows that, as predicted, fewer workers reproduce when the effectiveness of policing worker-laid eggs is higher (P = 0.00004, Fig. 2a; using PICs: P = 0.000006, see supplementary information). This supports the hypothesis that worker altruism is enforced. Contrary to the voluntary-altruism hypothesis 7 , however, higher relatedness does not lead to increased altruism. In fact, the reverse is true — a larger proportion of the workers reproduce in species where relat-edness is high (P = 0.004, Fig. 2b; using PICs: P = 0.04; see supplementary information). However , this is predicted by policing theory, because low relatedness more strongly selects for …