Abstract
Female-biased sex ratios are common among arthropods that mate in small groups with episodic dispersal1–3. When breeding groups are founded by more than one female, individual (within-group) selection favours an unbiased (1:1) sex ratio, regardless of group size and degree of inbreeding (or sibmating), and regardless of the intensity of local competition among siblings for mates. On the other hand, female-biased sex ratios are favoured by group selection in randomly structured populations. Existing treatments of the evolution of female-biased sex ratios1,2,4–6 correctly predict the result of evolution, but do not make clear that it is a balance between opposing forces at two levels of selection3. I now show this to be so.
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