Incest

Why is Incest Bad?

There are sound biological reasons that natural selection would have led to the evolution of mechanisms to reduce the probability of mating with a close genetic relative. Throughout the evolutionary history of our species, the selection pressures posed by harmful genetic mutations and disease-causing organisms would have severely negatively affected the health and viability of offspring of close genetic relatives. All else being equal, individuals who avoided mating with a close genetic relative and instead mated with someone who did not share an immediate common ancestor would have left a greater number of healthier offspring. Importantly, the negative consequences of incest are enhanced the more closely related two individuals are, with the most severe consequences occurring between individuals who have a probability of .5 of sharing particular genes (i.e., brother and sister, mother and son, or father and daughter). The deleterious effects drop off as two partners become less closely related. Interestingly, in most (if not all) societies, incest between nuclear family members is forbidden or simply absent. In the United States, incest laws vary by state, but most have sanctions targeting marriage and sexual intercourse with a parent, child, sibling, grandparent, grandchild, niece, nephew, uncle, and aunt. Although some states even sanction first-cousin marriage, none sanctions second-cousin marriage. A variety of studies have documented the negative fitness consequences associated with incest. For instance, in both humans and nonhuman animal species, incest is associated with an increased risk of mortality, mental deficiencies, congenital malformations, and disease. Given these negative consequences, it is likely that evolution engineered mechanisms to prevent individuals from choosing close relatives as sexual partners. But what might such mechanisms look like?

How Do Humans Avoid Incest?

To avoid close genetic relatives as sexual partners, a well-designed mechanism would require (at least) two types of procedures: (1) procedures that categorize individuals according to their probability of relatedness (i.e., procedures for detecting kin), and (2) procedures that use information regarding kinship to regulate sexual attraction. With respect to kin detection, what cues do humans use? Because we cannot see another person's DNA, the best evolution could do is to use cues that were reliably correlated with genetic relatedness in the ancestral past to compute a probability of relatedness. To the extent that different cues identified different categories of kin (e.g., mother, father, sibling, offspring), different detection mechanisms likely exist.

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