SHARE One of the consistent findings in the field of reproductive medicine is that older parents are more likely to have daughters. 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. Two-thirds of children born to parents over 40 are girls!
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. Among women, each year in age decreases the odds of having a son as the first child by 1. 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.
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.
However, such explanations, even when correct, are proximate, not ultimate. 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. 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. 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. The problem with older parents, of course, is that they are more likely to die sooner.
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.