The joke goes like this; "So I was fucking my sister the other night and she said, 'Gee you cock is bigger than Dad's,' so I said 'Yeah I know, Mom told me'."
Slip that into one of those moments of silence you get at a dinner party and I can pretty much guarantee you'll get a reaction. In most cases the response will be uproarious laughter, although depending on the audience you might be greeted with frozen silence (it's certainly not a joke to tell at a family gathering). If you do tell it, keep in mind that whatever you do with the rest of your life, whether you're President of the United States or public enemy number one, you will forever be remembered to those gathered at the table as the guy who told "that" joke.
Why is the joke so funny? Well in part it's the shock value. Why is it so shocking? Because incest, both fraternal and parental, is one of the most deeply ingrained sexual taboos. But the joke is also funny because it's absurd. Siblings in general don't have sex with each other or their parents. The very idea is appalling to most people. In the words of a character from Somerset Maugham's The Book Bag, upon discovering an incestuous affair between her husband and his sister; "It's monstrous!"
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Monstrous? Rape is monstrous. Child abuse is monstrous. But sex between consenting adults? The strength of our reaction to incest seems natural because we all feel it. The incest taboo is a universal human phenomenon. Every culture has some proscription against incest. The definition of incest varies from culture. Some cultures prohibit marriage or sex between first-cousins, others, parallel-cousins. But practically every culture has a very strong prohibition against sex between parents and children and between blood siblings.
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But just because everyone agrees that incest is icky, it doesn't explain why it's icky. Many great minds from Sigmund Freud to Margaret Mead have sought a convincing explanation for the incest taboo. Freud believed siblings naturally lusted for each other. He also believed children lusted for their opposite sex parents. What stopped family life from turning into a rutting frenzy was the existence of a very strong prohibitive taboo. As a children grew they internalized this taboo and the superego learned to override the primitive urgings of the id.
The biological benefit of an incest taboo is inarguable. Inbreeding restricts the gene pool, reducing the genetic variety within a population. It also leads to an increase in the frequency of homozygotes (two identical forms of a particular gene). As such, it can lead to the expression of harmful recessive mutations which might otherwise not have been expressed in a heterozygotic pairing. The assumption among Freudians was that tribes and cultures, over time, associated incestuous pairing with mutations and developed prohibitions against it.
The problem with this thinking is that the chance of a deleterious mutation occurring in the product of an incestuous mating is still relatively small. Given enough rolls of the dice, the negative effect of inbreeding becomes apparent, but to the casual observer, the effect is anything but obvious. When one considers that mutations sometimes occurred in non-incestuous progeny and incestuous pairings normally produced healthy offspring, it seems unlikely that every tribe and culture would have noticed the marginal increase in mutations amongst the progeny of incestuous pairings, especially give that few primitive cultures noticed that the net effect of their rain dance was pretty much zip.
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In the late nineteenth-century, a Finnish anthropolgist by the name of Edward Westermarck came up with a different idea. In his The History of Human Marriage (1891), Westermarck hypothesized that humans avoid mating with individuals with whom they have been closely associated with in childhood. In essence, the incest taboo was a biological instinct honed by evolution.
The logic of this is fairly unassailable. While tribes and cultures may not notice the deleterious effects of inbreeding, nature, over the long haul, would. A gene that conferred a disinclination to have sex with our children, parents or siblings would result in greater reproductive survival rates and would accordingly spread through the population.
Freud ridiculed Westermarck's theory on the grounds that if there were no desire to perform the incestuous acts, then there would be no need for an incest taboo. However, this argument assumes that cultural taboos emerge to turn people against their natural tendencies. It is perhaps more likely that cultural taboos and beliefs exist to support our evolved behavior. For instance, human males have a natural tendency to wage war with males outside their own group (or tribe, or country). This is an extremely destructive tendency yet many of our cultural virtues such as patriotism, loyalty and bravery, exist in support of this behavior.
Westermarck's theory fell out of favor for the first half of the 20th century. Then in the 1950s, the anthropologist Melford E. Spiro, in his studies of an Israeli Kibbutz (where children were raised communally in close peer groups), noted that there were practically no instances of intermarriage between his subjects as adults, despite positive pressure from parents and community. Subsequent studies have shown out of the almost 3,000 marriages that occurred across the kibbutz system, only fourteen were between children from the same peer group. Tellingly, of those fourteen, none had been reared together during the first six years of life.
Then in a study of Taiwanese "child marriages," (a cultural practice in which the future wife was brought into the family and raised with her husband to be), Arthur Wolf and Chieh-shan Huang, noted that while marriages took place, they were notoriously difficult to consummate. Further, they found that the earlier in age the children had been brought together, the more likely the marriage would fail.
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Subsequent studies have confirmed and refined the Westermarck effect to the point where it can almost be called a natural law. Generally speaking, a kind of reverse sexual imprinting takes place whereby the earlier and closer the relationship between children of the opposite sex, the greater the repulsion between them as potential sexual partners. Mystery solved.
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