• Choose a category
• All
• Classic-Fiction

By Root 570 0

simple predictions: stepsiblings should generally not be found to marry unless they were brought up apart. Very close childhood friends should also generally not be found to marry. Here the best evidence comes from two sources: Israeli kibbutzim and an old Chinese marriage custom. In kibbutzim, children are reared in crèches with unrelated companions. Lifelong friendships are formed, but marriages between fellow kibbutz children are very rare. In Taiwan, some families practise shim-pua marriage in which an infant daughter is brought up by the family of the man she will marry. She is therefore effectively married to her stepbrother. Such marriages are often infertile, largely because the two partners find each other sexually unattractive.4 Conversely, two siblings reared apart are surprisingly likely to fall in love with each other if they meet at the right age.

All of this adds up to a picture of sexual inhibition between people who saw a great deal of each other during childhood; fraternal incest, as Westermarck suggested, is therefore prevented by this instinctive aversion that siblings show to each other. But Westermarck’s theory would also predict that if incest does occur, it will prove to be between parent and child, and specifically between father and daughter, because a father is past the age at which familiarity breeds aversion and because men usually initiate sex. That, of course, is the commonest form of incest.5

This contradicts Freud’s idea that incest taboos are there because people need to be told not to commit incest. Indeed, Freud’s theory requires that evolutionary pressures have not just failed to generate some mechanism to avoid incest, but have actually encouraged maladaptive incestuous instincts, which the taboos repress. Freudians have often criticized the Westermarck theory on the grounds that it would obviate the need for incest taboos at all. But in fact incest taboos that outlaw marriage within the nuclear family are rare. The taboos that Freud observed are nearly always concerned with outlawing marriage between cousins. In most societies there is no need to outlaw incest within the nuclear family because there is little risk of it happening.6

So why are the taboos there? Claude Lévi-Strauss invented a different theory called the ‘alliance theory’, which stressed the importance of using women as bargaining chips between tribes and therefore of not letting them marry within the tribe, but since no two anthropologists can agree on exactly what Lévi-Strauss meant, it is hard to test his idea. Nancy Thornhill has argued that the so-called incest taboos are actually rules about marriage customs invented by powerful men to prevent rivals from accumulating wealth by marrying their own cousins. They are not about incest at all, but power.7

Teaching Old Chaffinches New Tricks

The incest story neatly demonstrates the interdependence of nature and nurture. The incest avoidance mechanism is socially induced: you become sexually averse to your siblings during your childhood. In that sense there is nothing genetic about it. And yet it is genetic, for it is not taught: it just develops within the brain. The instinct not to mate with childhood companions is nature, but the features by which you recognize them are nurture.

It is critical to Westermarck’s argument that this aversion to mating with familiar people wears off for new acquaintances in later life. Otherwise, people would become averse to mating with their spouses within weeks of marrying them, which they plainly do not. Biologically, this is not hard to arrange. One of the most striking features of animal brains is the ‘critical period’ of youth during which something can be learnt and after which the learning is not erased or superseded. Konrad Lorenz discovered that chicks and goslings ‘imprint’ on the first moving thing they meet, which is usually their mother and rarely an Austrian zoologist, and thereafter prefer to follow that object. But chicks of a few hours old will not imprint, nor those of two days old. They are at their most sensitive