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Extending Trivers’s genius
In my previous post, I introduce the Trivers-Willard hypothesis, which predicts that, relative to the baseline offspring sex ratio of 105 boys for 100 girls, wealthy families are more likely to have sons while poor families are more likely to have daughters. More recently, there has been a theoretical extension of the original Trivers-Willard hypothesis, called the generalized Trivers-Willard hypothesis.
The idea behind the new hypothesis is the same as that behind the old one, but it extends the idea to many other factors besides the 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, if parents have any trait they can pass on to their children that is better for daughters than for sons, then they will have more girls. Parental wealth and status are just two of the traits they can pass on to their children that are more beneficial for sons than for daughters, but there are many other factors.
It is important to note that neither the original Trivers-Willard hypothesis nor the generalized Trivers-Willard hypothesis predicts that parents with a given set of heritable (and immutable) traits will produce offspring of only one sex or the other. Assume that the sex of a child is determined by a toss of an imaginary coin, and the “normal” or baseline parent has an “unbiased” coin that comes up “boy” 51.22% of the time and comes up “girl” 48.78% of the time (to reflect the “normal” baseline secondary sex ratio of 105 boys to 100 girls). 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% of the time. Such a parent is therefore significantly more likely to have a boy than the “normal” or baseline parent. However, it does not mean that such a parent will only have boys. It is still possible (albeit less likely than for others) for a parent with a probability of having a boy of .55 to have a girl, or even three girls in a row.
Brain types are a good example of heritable trait which affects boys and girls differently. Strong “male brains,” which are good at systemizing (figuring things out), are more beneficial for sons than for daughters, while strong “female brains,” which are good at empathizing (relating to people), are more beneficial for daughters than for sons. Since brain types are heritable, the generalized Trivers-Willard hypothesis would predict 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. While the sex ratio at birth among the general population is .5122 – 105 boys for every 100 girls – the data show that the sex ratio among engineers and other systemizers is .5833, that is, 140 boys for every 100 girls. The comparable sex ratio among nurses and other empathizers is .4255, that is, 135 girls for every 100 boys.
By the same token, as I mention in a previous post, tall and big parents have more sons and produce more male fetuses (because body size was a distinct advantage in male competition for mates in the ancestral environment, while body size has no particular advantage for women), and short and small parents have more daughters and produce more female fetuses. (And, as I explain in another post, this observation can potentially explain why more boys are born during and after major wars.) Because violence was probably a routine means in the male competition for mates in the ancestral environment (as it is among our primate cousins), tendency toward violence was adaptive for ancestral men but not for ancestral women. Accordingly, violent men have more sons, both in the United Kingdom and the United States, and, as I elaborate in an earlier post, this can potentially explain why many battered women may choose to stay with their abusive mates (so that they can produce violent and hence intrasexually competitive sons).
In my next post, I will discuss yet another factor which subtly influences the sex of the offspring: Parent’s physical attractiveness.
