Teen parents are more likely to produce boys while middle-aged parents are more like to produce girls. Satoshi Kanazawa over at Psychology Today breaks down this pattern by the numbers and comes up with a couple stunning observations:
- Two-thirds of parents over 40 have girls.
- Each year in the parent's age decreases the odds of having a son as the first child by 1 percent.
I'm a case in point since my mother was around 40 when she became pregnant with me. Of course, this isn't an absolute rule, but a statistical pattern of likelihood. For instance, a small, unscientific poll over at Babycenter found that a few more women over 30 had boys, not girls, and the proportion of girls vs. boys among women who gave birth in their teens and 20s was equal. Nevertheless, that pattern among large data sets (ie the National Child Development Study) is strong enough to indicate that there might be some sort of evolutionary adaptation at work here -- at least that's what Kanazawa thinks.
He posits that explanation for the Younger Parents = Boys and Older Parents = Girls puzzle boils down to reproductive success. From this vantage point, it makes sense that boys are born to younger folks who will probably live longer and offer greater parental resources. Kanazawa explains, "Sons' reproductive success largely hinges on the status and resources that they inherit from their parents, particularly, their fathers." Girls, however, are a lot more low-maintenance it seems. As long as our parents bestow good lookin' genes, someone will want to make babies with us. Piece of (cringe-worthy) cake! Granted, this evolutionary psychologist also penned "Why Beautiful People Have More Daughters."
The correlating factors among parents and offspring gender also aren't isolated to parental age. For instance, a 2008 study found a link between maternal diet and gender, possibly because certain foods might make a woman's body more receptive to an X or Y chromosome provided by sperm. I'm sure I could find an encyclopedia's-worth of similar research, too, if I kept hunting.
While it's interesting to note a link between parental age and baby gender, the "why?" behind is probably far more complex than the evolutionary idea that as long as a girl is pretty, she'll be fine.