Different Kinds of Moms Have Babies at Different Times of Year

(Photo: Dave Herholz)

We’ve written in the past about the relationship between a child’s month of birth and a variety of later outcomes. In SuperFreakonomics, for instance, we wrote about research by Douglas Almond and Bhashkar Mazumder showing that “prenatal exposure to Ramadan results in lower birth weight.” In a Times column called “A Star Is Made,” we examined the link between birth month and accomplishment in sports. We also noted, however, in SuperFreak, that the sports advantage — and probably many other birth-month influences — are relatively small:

But as prevalent as birth effects are, it would be wrong to overemphasize their pull. Birth timing may push a marginal child over the edge, but other forces are far, far more powerful. If you want your child to play Major League Baseball, the most important thing you can do — infinitely more important than timing an August delivery date — is make sure the baby isn’t born with two X chromosomes. Now that you’ve got a son instead of a daughter, you should know about a single factor that makes him eight hundred times more likely to play in the majors than a random boy.

What could possibly have such a mighty influence?

Having a father who also played Major League Baseball. So if your son doesn’t make the majors, you have no one to blame but yourself: you should have practiced harder when you were a kid.

That said, there is a rather large body of literature on the topic of birth month and its relationship to later outcomes. Which is why it’s interesting to see a paper (working version here), just published in The Review of Economics and Statistics, which offers a different angle on all this birth-month conversation. The authors are Kasey S. Buckles and Daniel M. Hungerman:

Season of birth is associated with later outcomes; what drives this association remains unclear. We consider a new explanation: variation in maternal characteristics. We document large changes in maternal characteristics for births throughout the year; winter births are disproportionately realized by teenagers and the unmarried. Family background controls explain nearly half of season-of-birth’s relation to adult outcomes. Seasonality in maternal characteristics is driven by women trying to conceive; we find no seasonality among unwanted births. Prior seasonality-in-fertility research focuses on conditions at conception; here, expected conditions at birth drive variation in maternal characteristics, while conditions at conception are unimportant.