One of the oldest explanations for the incest taboo is inbreeding theory. It focuses on the potentially damaging consequences of inbreeding or marrying within the family. People within the same family are likely to carry the same harmful recessive genes. Inbreeding, then, will tend to produce offspring who are more likely to die early of genetic disorders than are the offspring of unrelated spouses. Recent evidence suggests that inbreeding also tends to increase the likelihood of diseases that affect people later in life, such as heart disease and diabetes.61 For many years, inbreeding theory was rejected because of what is known from dog-breeding, in which, for example, a half-brother and half-sister might be bred (line breeding) or brothers and sisters might be bred (inbreeding). The inbreeding practiced to produce prize-winning dogs, however, is not a good guide to whether inbreeding is harmful; dog breeders have not counted the runts they cull at the expense of producing one superficially show-worthy dog. Nor have they needed to worry about the genetic ailments a dog may eventually display. Indeed, dog-breeders as well as mating humans now have a good deal of evidence that the closer the degree of inbreeding, the more harmful the genetic effects.62
Genetic mutations occur frequently. Although many pose no harm to the individuals who carry a single recessive gene, matings between two people who carry the same gene often produce offspring with a harmful or lethal condition. Close blood relatives are much more likely than unrelated individuals to carry the same harmful recessive gene. If close relatives mate, their offspring have a higher probability than the offspring of nonrelatives of inheriting the harmful trait.
One study compared children produced by familial incest with children of the same mothers produced by nonincestuous unions. About 40 percent of the incestuously produced children had serious abnormalities, compared with about 5 percent of the other children.63 Matings between less closely related kin also show harmful, though not as harmful, effects of inbreeding. The likelihood that a child will inherit a double dose of a harmful recessive gene is lower the more distantly the child’s parents are related. Also consistent with inbreeding theory is the fact that rates of abnormality are consistently higher in the offspring of uncle-niece marriages (which are allowed in some societies) than in the offspring of cousin marriages; for the offspring of uncle-niece marriages, the likelihood of inheriting a double dose of a harmful recessive is twice that for the offspring of first cousins.64
Although most scholars acknowledge the harmful effects of inbreeding, some question whether people in former days would deliberately have invented or borrowed the incest taboo because they knew that inbreeding was biologically harmful. William Durham’s cross-cultural survey suggests that they did. Ethnographers do not always report the perceived consequences of incest, but Durham found that biological harm to offspring was mentioned in 50 percent of ethnographic reports.65 For example, Raymond Firth reporting on the Tikopia, who live on an island in the South Pacific, wrote:
The idea is firmly held that unions of close kin bear with them their own doom, their mara. . . . The idea [mara] essentially concerns barrenness. . . . The peculiar barrenness of an incestuous union consists not in the absence of children, but in their illness or death, or some other mishap. . . . The idea that the offspring of a marriage between near kin are weakly and likely to die young is stoutly held by these natives and examples are adduced to prove it.66
Durham concluded that if the harm of inbreeding was widely recognized, people may have deliberately invented or borrowed the incest taboo.67 But whether or not people actually recognized the harmfulness of inbreeding, the demographic consequences of the incest taboo would account for its universality: Reproductive and hence competitive advantages probably accrued to groups practicing the taboo. Thus, even though cultural solutions other than the incest taboo might provide the desired effects assumed by the family-disruption theory and the cooperation theory, the incest taboo is the only possible solution to the problem of inbreeding.
As is discussed toward the end of the next section, a society may or may not extend the incest taboo to first cousins. That variation is also predictable from inbreeding theory, a fact that lends additional support to the idea that the incest taboo was invented or borrowed to avoid the harmful consequences of inbreeding.