Construct · 3 fields · 3 citations
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In evolutionary biology the sex ratio is the proportion of males to females among offspring, and Fisher's principle explains why it so often sits near parity: whenever one sex becomes rarer, parents who produce that sex gain a fitness advantage, which pushes the population back toward equal parental investment in the two sexes. Fisher's The Genetical Theory of Natural Selection gave this frequency-dependent argument its canonical form. The referent is the balance of the sexes as a product of selection, the same quantity demographers track at birth and behavioural ecologists track among breeders.[1]
In demography the sex ratio is the number of males per 100 females in a population or among births; the human sex ratio at birth is characteristically a little above parity (around 105 boys per 100 girls) and shifts with parental and environmental factors. McKeown's 1956 Sources of variation in the human sex ratio at birth examined what moves it. The referent is the measured sex composition of a human population, the same construct the evolutionary account seeks to explain and that behavioural ecology studies in other species.[2]
Behavioural ecologists refined the construct into the operational sex ratio: the ratio of sexually active, ready-to-mate males to receptive females at a given time, which predicts the intensity of competition for mates and the direction of sexual selection. Emlen and Oring's 1977 Ecology, sexual selection, and the evolution of mating systems introduced it as a driver of mating-system diversity. It is the same males-to-females construct, narrowed to the pool actually available to mate, linking the demographic count to the evolutionary logic.[3]
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