Dr. Rufus Johnstone

Department of Zoology, University of Cambridge

Dr. Rufus Johnstone

Department of Zoology, University of Cambridge

The evolution of homosexual mating preferences

Rufus A. Johnstone & Bram Kuijper

Explaining the widespread occurrence of homosexual mating preferences in humans represents a challenge to evolutionary biology, because these preferences are often correlated with reduced reproductive success. Numerous hypotheses have been proposed, suggesting possible adaptive benefits of homosexuality, or indirect selective effects that might explain its prevalence. We observe that, regardless of the validity of any particular hypothesis, local kin competition within social groups is predicted to influence the evolution and prevalence of homosexuality, through its influence on the fitness costs and benefits of reproduction or non-reproduction by males and females. We develop a general model of the evolution of homosexuality in a group-structured population, incorporating demographic factors such as sex-specific dispersal, as well as individual circumstances such as birth-order. We show that this kin-competition model explains several of the patterns observed in the distribution and prevalence of homosexuality, such as the effects of birth order and family size and sex ratio, as well as making testable predictions regarding cross-cultural variation, for which there is some support. Moreover, these predictions are robust to different assumptions about the genetic basis and adaptive benefits of homosexuality. Our results illustrate that in a highly social species such as our own, how selection acts on the reproductive decisions of one individual is closely tied to the decisions of others in the same social group. Consequently, we argue that demographically explicit models that incorporate local kinship dynamics and kin competition are essential to understand the evolution of behaviour and life history traits in humans and social animals more generally