these desires with the help of social taboos and rules; second, that people do not find their very close relatives sexually arousing, that the taboo is in the mind. The first explanation is Sigmund Freud ' s. He argued that our first and most intense sexual attraction is toward our oppositesex parent. That is why, he went on to say, all human societies impose on their subjects strict and specific taboos against incest: Since the taboo 'is not to be found in the psychology of the individual, ' there is a ' necessity for stern prohibitions. ' Without those taboos, he implied, we would all be dreadfully inbred and suffer from genetic abnormalities:'
Freud made three unjustified assumptions. First, he equat-ed attraction with sexual attraction. A two-year-old girl may love her father, but that does not mean she lusts after him. Second, he assumed without proof that people have incestuous desires. Freudians say the reason very few people express these desires is that they have 'repressed' them—which makes Freud 's argument irrefutable.
Third, he assumed that social rules about cousins marrying were
' incest taboos. ' Until very recently scientists and laymen alike followed Freud in believing that laws forbidding marriages between cousins were enacted to prevent incest and inbreeding. They may not have been.
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Freud 's rival in this field was Edward Westermarck; in 1891 he suggested that men do not mate with their mothers and sisters not because of social rules but because they are simply not turned on by those they were reared with: Westermarck 's idea was simple: Men and women cannot recognize their relatives as relatives, so they have no way of preventing inbreeding as such. (Curiously, quail are different; they can recognize their brothers and sisters even when reared apart.) But they can use a simple psychological rule that works ninety-nine times out of a hundred to avert an incestuous match. They can avoid mating with those whom they knew very well during childhood. Sexual aversion to one 's closest relatives is thus achieved. True, this will not avert marriage between cousins, but then there is nothing much wrong with marriage between cousins: The chance of a recessive deleterious gene emerging from such a match is small, and the advantages of genetic alliance to preserve complexes of genes that are adapted to work with one another probably outweigh it: (Quail prefer to mate with first cousins rather than with strangers.) Westermarck did not know that, of course, but it strengthens his argument, for it suggests that the only incestuous relations a human being should avoid are the ones between brother and sister, and parent and child.'
Westermarck 's theory leads to several simple predictions: Stepsiblings would generally not be found to marry unless they were brought up apart.
Lifelong friendships are formed, but marriages between fellow kibbutz children are very rare. In Taiwan some families practice '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: Conversely, two siblings reared apart are surprisingly likely to fall in love with each other if they meet at the right age.4
All of this adds up to a picture of sexual inhibition between people who saw a great deal of each other during childhood; frater-
nal incest, as Westermarck suggested, is therefore prevented by this instinctive aversion that siblings have for 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 most common form of incest.'
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 its happening:'
So why are the taboos there? Claude Levi-Strauss invented a different theory called the 'alliance theory, ' which stressed the importance of using women as bargaining chips between tribes and therefore not letting them marry within the tribe, but since no two anthropologists can agree on exactly what Levi-Strauss meant, it is hard to test his idea. Nancy Thornhill of the University of New Mexico has argued that the so-called incest taboos are actually rules about marriage customs invented by powerful men CO prevent rivals from accumulating wealth by marrying their own cousins.
They are not about incest at all but about power:'
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: THE USES OF BEAUTY
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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 wear 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 learned 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 they prefer to follow that object: But chicks a few hours old will not imprint, nor will those two days old: They are at their most sensitive to imprinting at thirteen to sixteen hours old. During that sensitive period they will fix their preferred image of a parent in their heads:
The same is true of a chaffinch learning to sing: Unless it hears another chaffinch, it never learns the species 's typical song. If it hears no chaffinch until it is fully grown, it never learns the right song but produces a feeble half- song. Nor will it learn the song if it hears another chaffinch only when it is a few days old. It must hear a chaffinch
