Incest/Inbreeding Taboos

Sociobiologists believe that life—and its evolution— results from the competition between individual species members to spread their genes by producing the most progeny. (Progeny, by definition, carry parental genes to future generations.) The central sociobiological thesis concerning incest/inbreeding avoidance simply states that natural selection favors outbreeding behavior because inbreeding more often results in genetically debilitated offspring—in other words, inbreeding is not the best adaptive strategy for producing the most descendants. Although the deleterious thesis is widely accepted, and has taken on a law-like stature, a careful examination of the simple Mendelian mathematics involved quickly refutes this notion (Shields 1982; Livingstone 1969).

Almost all harmful genes are recessive, requiring that both parents carry the gene to produce offspring that manifest the deleterious effect. Since relatives share some common ancestry, they are more likely than nonrelatives to share the same harmful recessive genes. In this respect, the more closely related the mates, the more common their ancestry, and hence the more likely they will share the same deleterious genes. Thus, mating between relatives is thought to more readily produce genetically harmed descendants.

However, if a society customarily practices inbreeding, such as first and second cousin marriage, harmful recessive genes will quickly pair up and wash out of the gene pool. This occurs because deleteriously effected individuals are far less likely to reproduce and pass along the harmful genes to descendants. The result of systematic and recurring inbreeding in a population is to reduce the "genetic load" (the number of harmful recessive in the gene pool). Thus, inbreeding is no more harmful than outbreeding. Indeed, the advantage of an inbreeding system, especially for slow breeding mammals like humans, is that it preserves genotypes that have already proven successful in the environment.

Inbreeding is no more harmful than outbreeding unless inbreeding is practiced erratically in an otherwise outbreeding population. In outbreeding populations, mates are less likely to share close ancestry and thus the same harmful recessives traits. In this kind of reproductive system, recessive genes do not wash out of the population and thus accumulate as a large genetic load. The result can be the manifestation of harmful characteristics in the offspring of mating relatives.

For most of human history, breeding populations were small and isolated, and the community often practiced cousin marriage. The results were a relatively homogenous population of inbred individuals. Such homogenous populations are also common in other species (Shields 1982). It is unlikely, therefore, that a naturally selected mechanism would evolve to prevent incest/inbreeding.