It is the rule that says: “Don’t have sex with first-degree relatives.” First-degree relatives are the individuals you share 50 percent of your genes with—your parents, children, and siblings.
Indeed, non-human animals have evolved a host of strategies to prevent incest. Even plants possess anti-incest mechanisms.
As University of Miami psychologists Debra Lieberman and Adam Smith pointed out in a recent article in the journal Current Directions in Psychological Science, humans have social and psychological mechanisms to deter incest. With very few exceptions, marriages between brothers and sisters and between parents and their children are verboten in every human culture.
The primary psychological anti-incest mechanism is the yuck response. Even the idea of sex with mom or dad or bro or sis is upsetting to most people.
This raises an interesting question: Just what’s so bad about incest?
Sure, having sex with your dad or your sister seems gross. But why? Some anthropologists have argued that incest taboos are learned social conventions.
This explanation, however, doesn’t make sense to me as it does not explain the widespread existence of anti-incest mechanisms in creatures ranging from cockroaches to chimpanzees.
Second, the incest taboo is about as close to a universal law as human moral rules get. Why should mechanisms for avoiding incest be so widespread both in nature and across human societies?
The answer is simple.
The problem with having sex close with relatives is that there is an astonishingly high chance that your offspring will be born with a serious birth defect.
A group of genetic counsellors reviewed the research on the biological consequences of sex between relatives (consanguineous relationships). They found a surprisingly small increase (about 4 percent) in birth defects among the children of married cousins.
Forcing a woman burdened with the psychological scars of incest to bear a child who has a roughly 50:50 chance of having mental disabilities or a severe birth defect is perhaps the ultimate example of a foolish consistency that appeals to little statesmen.