It is argued that an appropriate model for the evolution of sexual investment, at least in higher vertebrates, may be as follows. The primary sex ratio is fixed at unity, parents can recognise the sex of individual offspring, and the returns (in offspring fitness) are different for the two sexes. For this model, it is shown that it is evolutionarily stable to invest differently in sons and daughters. In particular: If for a given investment the probability of survival is lower for one sex, selection favours greater investment in that sex. If one sex has a frequency-dependent component of fitness, such that individuals receiving a greater-than-average investment are fitter, selection favours greater investment in that sex. If the sex of an offspring can be recognised after an investment d, it may be evolutionarily stable to invest only in some fraction r of the more expensive sex, and to abandon a fraction (1-r). However, such behaviour can evolve only if d is a small fraction of the total investment required per offspring.
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