Thoughts on Birth-month Research

New research on why people born in winter are less successful suggests that it may have nothing to do with birth timing.  They found that winter babies are more likely to have young, less educated, single mothers–and all of those things are known to be correlated with lower academic and economic success.  This makes me think about several things:

First of all, it’s good to know that my December-born son doesn’t have the deck stacked against him because of his timing.

It’s exciting to see this explanation for a skew I’d noticed in some data at work.  Our research participants, now adults, were selected from the public schools 22 years ago using weighted random sampling: After a screening interview, all of the “high risk” boys and an equal number of the rest of the boys were included in the group that would be interviewed more extensively.  Among the risk factors considered were maternal age, parental education, and number of biological parents in the home.  Well, when I look at our participants’ birth dates, 20.8% were born in December or January.  (If births were distributed equally among the months, 16.7% would be in those two months.)  I remember reading somewhere that fewer Americans are born in April than any other month, and sure enough, April is the least popular month among our participants…but I’ve always wondered about the popularity of December and January in this particular sample.  Now I have a possible explanation!  In a quick look at relationships between birth month and other things, I didn’t see a clear correlation with our “high risk” score or maternal age, but boys born in January are less likely than any other month to have had fathers living in the home at the time they were born and more likely to be living with neither biological parent at the time of their screening interview.

Also interesting to me is that all of the graphs in the article show the opposite extremes for babies born in May.  Not only was I born in May (so I’m not surprised that it’s the “best” month!) but I found when I was attending a highly demanding, expensive university that a statistically surprising number of my friends there had birthdays in May.  Apparently we’re part of a larger trend!

I think it’s kind of odd that the researchers speculate on school-related effects with the assumption that winter-born children are the oldest in their grade.  That used to be more true, but over the last 40 years most American districts have changed to requiring kids to be 5 years old by September 1 to start kindergarten, and those that don’t have a September cutoff usually use October or November, not December or January…so any effect based on school policies would be shifting from winter babies to autumn babies by now.

Prior to this article (which happened to be linked to something else I was reading), I hadn’t known that there was a correlation between birth month and various problems. It’s interesting, but it’s also a very good example of how correlation does not imply causation: Just because one trait is more common among people who share another trait, doesn’t mean that one trait causes the other.  These relationships often are caused by lurking variables, other traits those people share which were not measured by the research.  In this case, maternal age, education, and family configuration appear to be the lurking variables.  What remains mysterious is why those mothers are more likely to have babies born in winter.  Hmmm…

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