beautiful parents

It is commonly believed that whether parents conceive a boy or a girl is up to random chance. Close, but not quite; it is largely up to chance. The normal sex ratio at birth is 105 boys for every 100 girls. But the sex ratio varies slightly in different circumstances and for different families. There are factors that subtly influence the sex of an offspring.

One of the most celebrated principles in evolutionary biology, the Trivers-Willard hypothesis, states that wealthy parents of high status have more sons, while poor parents of low status have more daughters. This is because children generally inherit the wealth and social status of their parents. Throughout history, sons from wealthy families who would themselves become wealthy could expect to have a large number of wives, mistresses and concubines, and produce dozens or hundreds of children, whereas their equally wealthy sisters can have only so many children. So natural selection designs parents to have biased sex ratio at birth depending upon their economic circumstances—more boys if they are wealthy, more girls if they are poor. (The biological mechanism by which this occurs is not yet understood.)

This hypothesis has been documented around the globe. American presidents, vice presidents, and cabinet secretaries have more sons than daughters. Poor Mukogodo herders in East Africa have more daughters than sons. Church parish records from the 17th and 18th centuries show that wealthy landowners in Leezen, Germany, had more sons than daughters, while farm laborers and tradesmen without property had more daughters than sons. In a survey of respondents from 46 nations, wealthy individuals are more likely to indicate a preference for sons if they could only have one child, whereas less wealthy individuals are more likely to indicate a preference for daughters.

The generalized Trivers-Willard hypothesis goes beyond a family's wealth and status: If parents have any traits that they can pass on to their children and that are better for sons than for daughters, then they will have more boys. Conversely, if parents have any traits that they can pass on to their children and that are better for daughters, they will have more girls.

Physical attractiveness, while a universally positive quality, contributes even more to women's reproductive success than to men's. The generalized hypothesis would therefore predict that physically attractive parents should have more daughters than sons. Once again, this is the case. Americans who are rated "very attractive" have a 56 percent chance of having a daughter for their first child, compared with 48 percent for everyone else.