It is also commonly believed that exactly 50 percent of babies born are boys and 50 percent, girls. Not quite. The normal sex ratio at birth is 105 boys to 100 girls. But the ratio varies slightly in different circumstances and for different couples. What affects the sex of your child?

In 1973, Robert Trivers, one of the greatest evolutionary biologists of his generation, teamed with the mathematician Dan Willard to formulate the Trivers-Willard hypothesis. It predicts that wealthy parents of high status have more sons, while poor parents of low status have more daughters. This is because children generally inherit wealth and status from their parents. Sons from rich families who themselves become wealthy have throughout evolutionary history been able to have a large number of wives, mistresses, and concubines, and produce dozens, even hundreds, of children, whereas their equally wealthy sisters can have only so many children. So wealthy parents should "bet" on sons rather than daughters.

Conversely, poor sons can expect to be completely excluded from the reproductive game, because no women would marry them. But their equally poor sisters can still have some children if they are young and beautiful. So poor parents should "bet" on daughters rather than sons.

Natural selection thus designed parents to have biased sex ratios at birth depending upon their economic circumstances—more boys if they are wealthy, more girls if they are poor.

The precise mechanisms by which parents "select" the sex of the child are unknown. Although the sperm determines the sex of the fertilized egg, the mother may determine the sex of the child through selective : The mother's might impact the intrauterine environment, creating conditions more hospitable to one or the other. My work to date suggests that the mother and the father may both contribute to determining the sex of the child.

Much empirical evidence supports the Trivers-Willard hypothesis. For example, American presidents, vice presidents, and cabinet secretaries have had more sons—but only during the first half of American history.

More recently, I have extended the Trivers-Willard hypothesis to formulate the generalized Trivers-Willard hypothesis, encompassing many factors besides a family's wealth and status. The new hypothesis suggests that if parents have any trait they can pass on to their children that is better for sons than for daughters, then they will have more boys. Conversely, any trait that is better for daughters than for sons will lead to more girls.

These hypotheses do not predict that parents with a given set of traits will produce offspring of only one sex. Imagine that the sex of a child is determined by a toss of a coin, and the "normal" parent has an "unbiased" coin that comes up "boy" 51.22 percent of the time and "girl" 48.78 percent of the time. If a parent possesses some heritable trait that is better for sons than for daughters, then the coin becomes "biased" in favor of sons and now comes up "boy," say, 55 percent of the time.

Brain types are a heritable trait that affects boys and girls differently. Strong "male brains," which are good at systemizing (figuring things out), are more beneficial for sons, while strong "female brains," good at empathizing, are more beneficial for daughters. Since brain types are heritable, the generalized Trivers-Willard hypothesis predicts that parents with strong male brains, such as engineers, mathematicians, and scientists, are more likely to have sons, while those with strong female brains, such as nurses, social workers, and schoolteachers, are more likely to have daughters.

This indeed appears to be the case. Engineers and scientists have 140 boys for every 100 girls; nurses and schoolteachers have 135 girls for every 100 boys.

Similarly, because body size is a distinct advantage in male for mates while conferring no particular advantage for women, tall and big parents have more sons, and short and small parents have more daughters. Because violence was probably a routine means of male competition in the ancestral environment, the tendency toward violence was adaptive for ancestral men but not for ancestral women. Accordingly, violent men have more sons.

Physical is a universally positive quality, but women benefit much more from it than men do, doing well in both long-term and short-term , while attractive men tend to do well only in short-term mating. So the generalized Trivers-Willard hypothesis predicts that beautiful parents have more daughters, and this indeed is the case.

Finally, your level of sexual promiscuity can also influence the sex of your child. Men can potentially and dramatically increase the number of children they have by being sexually promiscuous; if a man has 1,000 sexual partners in a year, he can potentially have 1,000 children (more realistically, about 30) by the end of the year. But a sexually promiscuous woman can still have only one child by year's end no matter how many partners she has. So men benefit enormously from sexual promiscuity, but not women . And since the level of sexual promiscuity is heritable, promiscuous parents beget promiscuous children.

The generalized Trivers-Willard hypothesis would therefore suggest that sexually promiscuous parents are more likely to have sons than those who are more sexually restrained. An analysis of two nationally representative samples of Americans, each with tens of thousands of people, confirms the prediction. Even when one takes into account many social demographic factors, sexually promiscuous parents are significantly more likely to have a boy. If you increase your level of sexual promiscuity from the 50th percentile (the median) to the 84th percentile, you increase the odds of having a son by 12 to 19 percent.

It therefore appears that sexual promiscuity is yet another factor that might subtly influence the sex of your child, along with your brain type, body size, tendency toward violence, and physical attractiveness. There may be other heritable traits that affect the sex of your child. Only more research will tell.

Satoshi Kanazawa is a blogger for PT. Read his blog: The Scientific Fundamentalist

Like Father, Like Son

The generalized Trivers-Willard hypothesis argues that parental characteristics that tend to give males an advantage will result in more male offspring.

Factors favoring sons:

Higher social status

Greater wealth

Strong male brain (being "nerdy")

Larger body size (by height and weight)

Tendency toward violence

Tendency toward sexual promiscuity

Factors favoring daughters: