Asymmetric relations: Internal conflicts and the horror of incest

Incest can increase an individual's inclusive and individual fitness if the opportunity cost of incest is sufficiently small. As a corollary, genetic conflicts are possible between the two parties of an incestuous dyad if they have different opportunity costs. The opportunity costs of casual sex usually are greater for females than for males. Therefore, females are predicted to be more averse to incest than are males. Genetic conflicts also are possible between an individual's maternal and paternal genomes because of asymmetries of relatedness. Large asymmetries of maternal and paternal relatedness are present between half-siblings, and in the relationship of offspring to parents. These asymmetries raise the possibility of strong internal conflicts between an individual's maternal and paternal genomes over the costs and benefits of incest, Taboos and other social sanctions may serve to increase male opportunity costs and thus serve as a disincentive to incest. (C) 1999 Elsevier Science Inc.