One of the consistent findings in the field of reproductive medicine is that older parents are more likely to have daughters. Why?
Previous studies have repeatedly found that one of the reliable predictors of the sex of the offspring is the age of the parent. Older parents are significantly more likely to have daughters than younger parents.
The National Child Development Study replicates these findings from earlier studies. As the following graph shows, the association between the age of the parent and the sex of the first child is not monotonic, but there is a general decline in the proportion of sons as the parents get older. Teenage parents are particularly likely to have sons, with the proportion of sons at .5327, and older parents over the age of 40 are significantly less likely to have sons, with the proportion of sons at .3557. Two-thirds of children born to parents over 40 are girls! The bivariate correlation between the probability of having a son and the age of the parent is significantly negative (r = -.030, p < .001, n = 9,301). Each year in the parent’s age decreases the odds of having a son as the first child by 1%.
As you can see in the following two graphs, the association between the age of the parents and the sex of the first child is stronger among women than among men. In fact, the bivariate correlation between the probability of having a son and the age of the parent is only statistically significantly negative among women (r = -.34, p < .05, n = 4,864), not among men (r = -.024, ns, n = 4,437). Among women, each year in age decreases the odds of having a son as the first child by 1.2%. However, the graph below clearly shows that fathers over the age of 40 are significantly less likely to have sons, with the proportion of sons at .3592.
Given the prevalence of age homogamy, where the age of the mother and the age of the father are generally positively correlated such that younger women are typically married to younger men and older women are typically married to older men, the slight sex difference in the pattern is not important. The overall picture is that the older the parents (both the mother and the father), the more likely they are to have a daughter. The question is: why?
Because both the quality of the eggs and the quality of the sperm decrease with age, it is tempting to explain the declining likelihood of having a son among older parents potentially in terms of such quality of gametes (although I’m not aware of any argument that suggests that lower-quality gametes are more likely to produce girls). However, such explanations, even when correct, are proximate, not ultimate. They answer the question of how; they don’t answer the question of why. The lower quality of gametes, if it indeed lowers the probability of producing boys, is the mechanism that evolution employs to make sure that older parents are more likely to have daughters. But such a proximate mechanism does not explain why evolution “wanted” to make sure that older parents are more likely to have daughters, in other words, why it is adaptive for older parents to have daughters, not sons. That requires an ultimate evolutionary explanation.
As I explain in an earlier post, parental investment is much more crucial for the future reproductive success of sons than for that of daughters. Sons’ reproductive success largely hinges on the status and resources that they inherit from their parents, particularly, their fathers. This is why the presence of sons deters divorce and the departure of the father from the family. Sons therefore need parents to invest in them, to make sure that they inherit the status and the resources of the family.
In sharp contrast, daughters’ future reproductive success is largely determined by their youth and physical attractiveness. Once they are conceived with particular genes that influence their physical attractiveness, there is very little that parents can do to increase their daughters’ future reproductive success, beyond keeping them alive and healthy. There is absolutely nothing that parents can do to affect the progression of time that determines the daughters’ age, nor is there anything they can do after the conception to influence the daughters’ physical appearance (once again, beyond keeping them healthy).
The problem with older parents, of course, is that they are more likely to die sooner. If the parents die before the children reach sexual maturity, it will have a greater negative impact on sons’ future reproductive success than on daughters’. This may be one evolutionary, ultimate reason why older parents are more likely to have daughters. Parents may be evolutionarily designed to have more daughters when they are older, so that, when they die, they are less likely to leave sons who have not sexually matured. Being orphaned young is bad both for boys and girls, but it’s much worse for boys than for girls.