So What's Wrong with Incest?

"Julie and Mark are brother and sister. They are traveling together in France on summer vacation from college. One night they are staying alone in a cabin near the beach. They decide that it would be interesting and fun if they tried making love. At the very least, it would be a new experience for each of them. Julie was already taking birth control pills, but Mark uses a condom too, just to be safe. They both enjoy making love, but they decide never to do it again. They keep that night as a special secret, which makes them feel even closer to each other. What do you think about that? Was it ok for them to make love?"

That's the way Jonathan Haidt began an article on the psychology of moral judgment. He reported that most people immediately condemn what Julie and Mark did as wrong, but they struggle to give reasons for this judgment. I have had the same reaction from students in my classes when I present this story. They react with disgust. But it's hard for them to give reasons to justify their disgust.

Haidt, a professor of psychology at the University of Virginia, argues that this illustrates the primacy of emotion in moral judgment. Against the tendency to explain morality as caused by moral reasoning, he suggests that moral reasoning usually comes after we have already made a moral judgment from our emotional reaction. He also claims that such a view of moral judgment as initially caused by emotional intuition is confirmed by biological psychology.

I agree. In fact, I think the incest taboo is one of the clearest illustrations of how a moral rule can be explained by a Darwinian view of human biological nature. In The Descent of Man, Darwin acknowledged that "of all the differences between man and the lower animals, the moral sense or conscience is by far the most important," and explaining the uniqueness of human moral experience created the "greatest difficulty" for his theory of evolution.

Proponents of the "theory of special creation" would say that that the human moral sense or conscience shows that human beings have been specially created in God's image, so that He has implanted a conscience in them. God's specific commands against incest are found in the Old Testament, in the 18th chapter of Leviticus, which was long adopted in Western legal codes as a set of rules for forbidden marriages.

But Darwin rejected the belief "that the abhorrence of incest is due to our possessing a special God-implanted conscience." He saw the rules for forbidden incestuous marriages as a subject for scientific investigation. During his lifetime, there was an intense debate in England over whether the marriage of first cousins should be prohibited as incest. Darwin had a personal stake in this, since he had married his first cousin (Emma). He proposed that Parliament should authorize a study of cousin marriages to see if their offspring suffered from inherited defects. Although his proposal was rejected, his son George did a study that concluded that the risk for the offspring of cousin marriages was often low.

In his book The Variation of Animals and Plants Under Domestication, Darwin has a chapter "On the Good Effects of Crossing, and On the Evil Effects of Inbreeding." He surveys the experience of animal breeders in discovering the bad effects of inbreeding. And he wonders whether natural selection could have shaped a natural aversion to incest among human beings. "Although there seems to be no strong inherited feeling in mankind against incest, it seems possible that men during primeval times may have been more excited by strange females than by those with whom they habitually lived. . . . If any such feeling formerly existed in man, this would have led to a preference for marriages beyond the nearest kin, and might have been strengthened by the offspring of such marriages surviving in greater numbers."

In Xenophon's Memorabilia (IV.iv.19-23), Socrates identifies the "unwritten laws" legislated by the gods as laws that could not be disobeyed without natural penalties. He speaks of the incest taboo as one of those "unwritten laws," because those committing incest tend to produce defective offspring. Darwin's evolutionary explanation of this would illustrate, then, how Darwinian science might support the traditional idea of "natural law" or "natural right." (I have argued this in a book chapter: "The Incest Taboo as Darwinian Natural Right," in Arthur Wolf and William Durham, eds., Inbreeding, Incest, and the Incest Taboo: The State of Knowledge at the Turn of the Century [Stanford University Press, 2005].)

Edward Westermarck elaborated Darwin's reasoning for the biological evolution of the incest taboo in his book The History of Human Marriage (first published in 1889). Westermarck's theory can be summarized in three propositions. First, inbreeding tends to produce physical and mental deficiencies that lower Darwinian fitness. Second, as a consequence, natural selection has favored an emotional disposition to feel a sexual aversion to those with whom one has been raised in early childhood. Third, this natural aversion to incest creates moral disapproval that is expressed as an incest taboo.

This put Westermarck in conflict with Sigmund Freud's Oedipal theory of human psychology and culture, because Freud insisted that the inclincation to incest was natural to human beings, and that the taboo against incest arose as a purely cultural construction that human beings created to repress their natural desires for incest, this cultural repression of human nature being necessary for civilization. So morality, as Freud understood it, required a conquest of human nature by human culture. By contrast, Westermarck believed that morality was a cultivation of natural human emotions, so that the incest taboo was a cultural expression of a natural human disposition shaped in human evolutionary history.

Over the past century, the evidence for Westermarck against Freud has grown. Arthur Wolf's study of "minor marriages" in China is one line of evidence. A traditional form of Chinese marriage was for parents to give their infant daughter to another family to be raised with the family's infant son, so that when the boy and girl reached maturity, they would be married. Wolf showed that these marriages were generally unsuccessful, because children raised together as siblings developed a sexual aversion to one another.

Another line of evidence came from the experience of the Israeli kibbutzim. In the attempt to create a fully socialist community, the kibbutz would have infant children taken from their families and put in the "children's house," where they would be raised together communally. When they reached sexual maturity, the children were encouraged to find marriage partners among those with whom they had been raised. But the children resisted this, because even though they were not biological relatives, they felt as if they were siblings and thus felt revulsion at the thought of sexual mating with one another.

So it seems that these children in China and in the kibbutzim were manifesting the "Westermarck effect": as a result of an innate disposition shaped by evolutionary history, they developed a sexual aversion to the children with whom they had been raised, even though they were not actually biological siblings.

Another kind of evidence for Westermarck's theory is that it now seems that many primates show incest avoidance. While Freud thought that incest was common among nonhuman animals, we now know that this is not true. For most primate species, males leave their native troop when they reach sexual maturity, which seems to be a mechanism for avoiding excessive inbreeding. For chimpanzees, the females leave at maturity to join another troop. This means that chimp mothers will be in the same troop with their sons. And their sons often do attempt to mount their mothers and sisters, but when the males reach sexual maturity, their mothers and sisters generally push them away. This is what Westermarck's theory would predict. The human incest taboo is humanly unique as a legal and moral norm, but it expresses a natural emotional disposition that can be found in primate evolutionary history.

Freud may have been fooled by his psychoanalytic experience, because in his Vienna, many of his patients had been raised by nurses rather than their parents, and in these conditions, the "Westermarck effect" would not kick in. After all, Oedipus himself was separated from his mother shortly after birth, and so again the sexual aversion to mating with immediate family members would not have been acquired by the Westermarck mechanism.

This Darwinian theory of the incest taboo goes a long way to explaining marriage law. In the United States today, all states prohibit people from marrying their parents, their children, or their siblings. But there is disagreement over cousin marriages. Early in the nineteenth century, all states permitted cousin marriages (as is the case today in Europe). But now cousin marriages are prohibited in 31 states, while being permitted in 19 states. (The debate over cousin marriage is surveyed in Martin Ottenheimer's book Forbidden Relatives [University of Illinois Press, 1996].) There is a good argument for permitting cousin marriage, because we now know that the genetic risk from deleterious recessive genes is often low for cousins, but high for those more closely related. Further, Westermarck's theory would predict that except in cases where cousins are reared together, there will not be a strong moral emotion against cousins mating.

There is great variation in the incest taboos across cultures depending upon the cultural definition of kinship categories.  But the variable boundaries of the incest taboos are extensions of an invariant core centered on the nuclear family.  The emotional reaction is strongest against incest with one's parents, siblings, and children.  The intensity of this emotional reaction fades as one moves away from the nuclear family.

Of course, incest does occur. But the pattern of incestuous behavior follows the predictions of Darwinian theory. Men are more likely to be the perpetrators of incest than are women, because men, unfortunately, are more indiscriminate in their sexual promiscuity.  The most common form of incest is actually child abuse--fathers abusing their daughters.  And this is more likely to occur if the fathers did not participate in the early rearing of the daughters.

Some people might not feel any moral inhibition against incest because they suffer from a psychopathic poverty of moral emotions. But still, the natural aversion to incest among most human beings is so strong that we formulate moral and legal norms against incest and enforce them as expressions of our natural emotional dispositions.

We can see how reason comes into play here. We might, for example, call on our knowledge of the genetics of inbreeding to rationalize our abhorrence of incest. But this is only a rationalization of a moral abhorrence that causes our moral condemnation of incest as an expression of moral emotion.

For more posts on the laws and moral psychology of incest, go here, here, here, here. here, here, here,  here., and here.  I have written about the flaws in Haidt's use of the "Julie and Mark incest story."