The Scientific Fundamentalist

A look at the hard truths about human nature.

Why boys have cooties (but brothers don’t)

What is the evolutionary function of cooties?

When little boys and girls reach a certain age, they start accusing each other of having “cooties.” They regard each other as yucky and dirty because they have cooties, and avoid any contact with them for fear of contracting cooties themselves. That is, until they suddenly “discover” each other when they hit puberty, then they become crazy about each other and no longer fear cooties. Why does this predictable developmental trajectory happen? Why do little boys and girls believe in cooties?

The key to the mystery of cooties is the Finnish anthropologist Edvard Alexander Westermarck (1862-1939). Westermarck taught at LSE, where I now teach, from 1904 until his retirement in 1930. On a very personal note, he is also my intellectual great-great-great-grandfather, by which I mean he was my mentor’s mentor’s mentor’s mentor’s mentor.

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Westermarck is best known for discovering a phenomenon which is named after him: the Westermarck effect. It refers to the fact that when little boys and girls spend a lot of time together as they are growing up, they will later as adolescents find each other sexually repulsive. It is a mechanism designed for incest avoidance. Since individuals with whom small children come in regular and frequent contact as they grow up are almost always their genetic kin (their parents, siblings and other close family members), it will not be in the genetic interest of the children to be sexually attracted to them. So nature has selected individuals to possess a psychological mechanism that aids in avoiding their close genetic relatives as their suitable sexual mates.

As with all other evolved psychological mechanisms, however, the Westermarck effect sometimes backfires, when it finds itself in an evolutionarily novel environment that evolution could not have anticipated. (Evolution is always backward-looking and can only respond to situations that reliably and consistently existed in the past. Evolution can never anticipate the future, especially in a fast-moving environment like ours in the last 10,000 years.) One example where the Westermarck effect backfires is the Israeli kibbutz. In kibbutzim, all small children, from different families, are raised together communally. As a result of the (mis)operation of the Westermarck effect, Israelis who grew up in kibbutzim do not usually find each other sexually attractive as adults, and thus seldom marry each other, even though they are not genetically related.

Another example is a peculiar custom of adoption and child betrothal among traditional Chinese families. Under this practice, a wealthy Chinese family adopts a girl from a poor family which cannot raise her, and raises her with their own son, who is then betrothed to her. Alternatively, an heirless family adopts a boy so that he could later marry their daughter and carry on their family name. When they later marry as planned, however, the couple usually remains childless, because they find each other sexually repulsive as a result of having grown up together. Despite these exceptions, the Westermarck effect works fine as designed most of the time, because most other humans with whom most children regularly associate as they grow up in most societies under most circumstances are indeed genetic kin who should be avoided as mates.

From the perspective of the Westermarck effect, cooties (and their equivalents throughout the world) are a culturally specific device that reflects the operation of an underlying universal evolved psychological mechanism. Boys and girls in every society are evolutionarily designed to employ such a device (unconsciously) to make sure that they will not spend too much time with each other. Children’s play groups in all human societies are sex-segregated; boys play with boys, and girls play with girls. This will guarantee that boys and girls will later find each other sexually attractive when the time is right, which in the context of the ancestral environment was right at puberty.

This line of reasoning leads me to suggest an interesting new prediction, although it would be difficult to test it empirically. I have not seen any systematic scientific data on children’s alleged affliction with cooties and doubt that such data exist. Scientists tend not to take cooties seriously. Nevertheless, if cooties are a device for children to avoid spending too much time with each other so that they could later select them as sexual mates, then they should not employ the device against their own brothers and sisters, whom they will not be selecting as mates anyway and with whom they should be spending a lot of time. In other words, little girls should allege that little boys have cooties, but not their brothers, and little boys should allege that little girls have cooties, but not their sisters. Brothers and sisters may resent, fight, and even hate each other, but they should not allege cootiesitus against each other. A boy might tell his friends that his sister has cooties, in an attempt to keep them away from her, but he should not use the same affliction as an excuse to stay away from her himself. I would love to hear from parents who have both boys and girls for anecdotal evidence for or against my prediction.

Satoshi Kanazawa is an evolutionary psychologist at LSE and the coauthor (with the late Alan S. Miller) of Why Beautiful People Have More Daughters.

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