Human assortative mating and genetic equilibrium: An evolutionary perspective

Assortative mating in human and other animal species is reviewed, and early imprinting and learning within the family unit act as the proximate mechanisms to establish the criteria for optimal mate selection.

Abstract

Assortative mating in human and other animal species is reviewed. Assortive mating is most often positive (between individuals sharing phenotypic similarity), occurs on a variety of normal and abnormal traits, strengthens the mating bond, and increases fertility. These results can best be understood if it is assumed that natural selection operates to increase genetic homology between mates and between parents and off-spring. The outcome of selection for positive assortment is to increase the genetic relatedness among family members, thus facilitating communication and altruism and increasing inclusive fitness without an additional reproductive effort. The opposing evolutionary vector is to restrict excessive homozygosity and consequent inbreeding deppression by minimizing matings between individuals of greatest similarity (e.g., members of nuclear families). The result of these opposing evolutionary vectors is a negative relationship between the degree of positive assortative mating and heritability (h2): individuals gravitate toward those of similar but not identical phenotypic (and genetic) similarity. Individuals may assess their own and their potential mates' homozygosity and mate so as to maximize genetic homogamy and still avoid excessive inbreeding. Early imprinting and learning within the family unit act as the proximate mechanisms to establish the criteria for optimal mate selection. Predictions that follow from this model are advanced, and several problems are discussed. Laboratory and field work with animals will be especially valuable in advancing our understanding of assortative mating.