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The Problem With Incest

Evolution, morality, and the politics of abortion.

Key points

  • Humans have social and psychological mechanisms to deter incest.
  • Anti-incest mechanisms guard against the high chance that one's offspring from such an encounter will be born with a serious birth defect.
  • The argument to deny abortion even in cases of first-degree relative incest to protect the sanctity of life is an example of foolish consistency.
Source: CasimiroPT/Shutterstock

Mr. James Russell of Cashiers, North Carolina, recently justified meat-eating in the pages of Asheville Citizen-Times by arguing that humans are biologically classified as carnivores. His reasoning was simple. The consumption of animal flesh is morally right because it is natural.

Unfortunately, Mr. Russell got his facts wrong. Zoologists place humans in the order Primate (family Hominidea), not in the order Carnivora. Furthermore, like rats, humans are omnivores, not carnivores. But more troubling is Mr. Russell’s belief that humans should look to nature for moral guidance. He justifies meat-eating in humans on the grounds that other animals eat one another. I suspect, however, that he does not approve of gang rape, adultery, cannibalism, and the consumption of feces, all of which are practiced in nature by our four-legged brethren. While moral codes exist in other species, humans have the capacity—and, indeed, the responsibility—to operate on a higher ethical plane.

The (Nearly) Universal Taboo

On matters of morality, I generally agree with Katherine Hepburn who quipped to Humphrey Bogart in The African Queen, "Nature is what we are put in this world to rise above." There is, however, an exception to my contention that humans should not turn to nature for moral guidance. 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 (here). Even plants possess anti-incest mechanisms (here).

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. Psychologist Jonathan Haidt has found that nearly everyone is repelled by the prospect of brother-sister sex, even in hypothetical situations in which there is no chance of pregnancy (here).

The Biological Cost of Incest

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 (here). 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 with close relatives is that there is an astonishingly high chance that your offspring will be born with a serious birth defect. Take the results:

Percent of children with severe birth defects.

Source: A study of Czechoslovakian children whose fathers were first-degree relatives. Fewer than half of the children who were the product of incestuous unions were completely healthy. Forty-two percent of them were born with severe birth defects or suffered early death and another 11 percent mildly impaired mentally. This study is particularly instructive as it included a unique control group—the offspring of the same mothers but whose fathers were not the mothers’ relatives. When the same women were impregnated by a non-relative, only 7 percent of their children were born with a birth defect (Figure 1).

A group of genetic counselors reviewed the research on the biological consequences of sex between relatives (consanguineous relationships) (here). They found a surprisingly small increase (about 4 percent) in birth defects among the children of married cousins. Incest between first-degree relatives, however, was a different story. The researchers examined four studies (including the Czech research) on the effects of first-degree incest on the health of the offspring. Forty percent of the children were born with either autosomal recessive disorders, congenital physical malformations, or severe intellectual deficits. And another 14 percent of them had mild mental disabilities. In short, the odds that a newborn child who is the product of brother-sister or father-daughter incest will suffer an early death, a severe birth defect, or some mental deficiently approaches 50 percent.

Foolish Consistencies and Little Minds

The profound negative effects of incest on unborn children raise the issues of moral consistency and of abortion politics. I understand the pro-life argument. If you believe that human life begins at the moment sperm meets egg, it is perfectly logical to oppose abortion. But at what point do reasonable people temper logical consistency with compassion and common sense?

During the 2012 Republican Party convention in Tampa, the Platform Committee struggled with an aspect of the argument against legal abortion. Just about everyone on the committee agreed that abortion should be banned. But committee members were split over whether official party doctrine should include exceptions to the abortion ban if a fetus was the result of rape or incest. In the end, ideological purity prevailed. The official Republican platform states, “We assert the inherent dignity and sanctity of all human life and affirm that the unborn child has a fundamental individual right to life which cannot be infringed.” No exceptions, period. Even in cases of first-degree relative incest.

I grudgingly admit that the lack of any exception in the official Republican position on abortion is logically consistent with the party's statement on the “sanctity of all human life.” But shouldn't logic sometimes be tempered with compassion? Emerson famously wrote, “A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.”

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.


Human Heredity 21(2):108-28

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