Assortative mating

Assortative mating is a mating pattern, and a form of sexual selection, in which individuals with similar phenotypes or genotypes pair up with one another more often than would happen under random mating. Animals commonly mate assortatively based on traits such as body size, coloration and other visual markings, and secondary sexual characteristics such as crest size. The pattern is called positive assortative mating when similar individuals pair together, and negative, or disassortative, mating when individuals with dissimilar traits pair up instead. Eastern and western bluebirds show color-based assortative mating, with brightly colored males pairing with brightly colored females while duller individuals pair among themselves, and eastern bluebirds also mate assortatively for territorial aggression because of competition for limited nesting sites; in humans, studies dating back to 1903 have documented correlations in height and arm span between spouses, along with a tendency for people to select partners of similar socioeconomic status, education and religion.