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Volume 11, No. 5July/August 2001
PRESIDENT GEORGE W. BUSH has fathered two children, both of them daughters. Former president Bill Clinton has fathered a single child, also a daughter. That makes the forty-second and forty-third commanders in chief somewhat anomalous by historical standards. Of the 150 children sired by previous U.S. presidents, 90 were male and only 60 female. That's three boys for every two girls. Now, this could be a statistical fluke, like flipping a coin 150 times and getting 90 or more heads. But such an outcome is observed very rarely, less than 1 percent of the timeunless, of course, the coin is biased.
American presidents are not the only elite group to produce markedly more sons than daughters; the same goes for European aristocracies and royal families. (Ditto, in the animal kingdom, for socially dominant Peruvian spider monkeys and well-fed opossums.) For oppressed groups, the situation is just the opposite: In racist societies, the subject races tend to have slightly more daughters than sons.
Biologists of a neo-Darwinian bent have a theory to explain all this. Homo sapiens, for most of its evolutionary history, has been a polygamous species. If you are a high-ranking member of such a species, your son is likely to attract many female mates and thus furnish you with far more grandchildren than a daughter. If you are lowly, though, then your son may fail to win any mates at all, whereas your daughter can at least get your genes into the next generation by joining a harem. (Even in monogamous societies, a poor son is often forced to remain single, but a poor daughter might marry a rich man.)
Let's suppose for a moment that this neo-Darwinian story is true. What sort of biological mechanism could be behind the sex-ratio bias? Each potential father, after all, produces as many X sperm as Y sperm; if the former unites with the mother's egg, a girl is conceived; if the latter, a boy. Given these basics of human reproduction, it would seem that any significant deviation from a 1:1 sex ratio could be ascribed only to pure chance. How, then, could your reproductive apparatus ever respond to an environmental variable, such as where you stand in the social hierarchy?
Consider a curious phenomenon known as the "returning soldier effect." During and just after major wars, mothers in the warring nations give birth to substantially more sons than daughters. One is tempted to see this in teleological terms, as though nature were trying to make up for the men lost on the battlefield. That, of course, is nonsense: The extra baby boys can't very well mate with war widows. So what is happening here? One hypothesis, developed by the English biologist William H. James, attributes the skewed sex ratio to hormones. During wartime, women adopt more dominant, "masculine" rolesRosie the Riveter, that sort of thingand this may well tip their hormonal balance. These hormones, it is further conjectured, affect the relative success of the X and Y sperm in getting to the egg.
The key here could be the fact that the male-engendering Y sperm carry a little less DNAand are thus slightly more agilethan their female-engendering X counterparts. The hormones might work by thickening the mucus in the cervix, making it easier for a Y sperm to swim toward its goal. Male hormones may also play a role in some as-yet-ill-understood way. Circumstantial evidence suggests that men with high testosterone levels are more likely to sire sons than daughters, which might explain the presidential record if you subscribe to the alpha-male model of leadership.
Theories of sex determination have been around for ages, often yielding quack advice for parents who yearn for offspring of one sex over the other. Both Aristotle and the Talmud recommended placing the conjugal bed on a north-south axis to conceive a boy. Another ancient notion was that male seed issued from the right testicle, female seed from the left; so, if you wanted to father a boy, you might tie a string tightly around your left testicle prior to sex, or even have it amputated altogether, as some French aristocrats did.
Today there are more reliable ways of exercising control over the sex of one's children. In the United States, a number of sex-selection clinics have opened over the past few years. By using a laser machine that can separate Y sperm from X sperm, these clinics can guarantee with fairly high probability that a couple will conceive a child of the desired sex. And in the great majority of casesmore than two in threethe desired sex is female. "Women are the driving force, and women want daughters," the inventor of one sex-selection method explained to The New York Times Magazine. Since these clients tend to be affluent and presumably of high status, their preference for girls does not match the predictions of evolutionary psychology.
Let me close with a little puzzle having to do with sex selection. Suppose that the Joneses are planning to have two children and that the first child turns out to be a boy. What is the chance that the Joneses will end up with two boys? It is one-half, because that is the chance the second born will also be a boy. Now look at the set of all families with two children. Choose at random a family where at least one of the children is a boy. What is the chance that the other child will also be a boy? Now the answer is only one-third. Why the bias for girls?
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