Abstract
The ‘generalized Trivers-Willard hypothesis’ (gTWH) proposes that heritable traits associated with reproductive success of one sex will be positively associated with a genetic tendency to produce offspring of that sex. However, unlike the original Trivers-Willard hypothesis, the predictions of gTWH are proposed to be borne out regardless of environmental conditions. This is a problem because it ignores the influence of the hypothetical genetic variance in offspring sex-ratio on population operational sex-ratio and thus offspring's likely success in finding a mate. Accordingly, there is a notable lack of evidence to support the existence of such heritable variation in offspring sex-ratio in humans or other mammals. The genetic tendency for all individuals within populations of birds and mammals to produce a male offspring with the same probability as one another is well-established. In fact it is a cornerstone of population sex-ratio theory, upon which is built hypotheses of facultative (environmental) sex-ratio adjustment, including the original Trivers-Willard hypothesis. I therefore suggest that any phenotypic correlations between offspring sex-ratio and traits that may be associated with the reproductive success of offspring of one sex are most likely to be environmental in origin.
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