I mentioned the birthdays example in a talk the other day, and Hal Varian pointed me to some research by David Lam and Jeffrey Miron, papers from the 1990s with titles like Seasonality of Births in Human Populations, The Effect of Temperature on Human Fertility, and Modeling Seasonality in Fecundability, Conceptions, and Births.
Aki and I have treated the birthdays problem as purely a problem in statistical modeling and computation and have not looked at all at work of demographers in this area. So it was good to learn of this work.
Hal also pointed me to a recent paper, Heat Waves at Conception and Later Life Outcomes by Joshua Wilde, Bénédicte Apouey, and Toni Jung, which I looked at and don’t believe at all.
Wilde et al. report that babies born 9 months after hot weather have better educational and health outcomes as adults, and they attribute this to a selection among fetuses, by which the higher temperature conditions make fetal development more difficult so that the weaker fetuses die and it is the stronger, healthier ones that survive. As is typically the case, I’m suspicious of this sort of bank-shot explanation.
Wilde et al. talk about the causal effect of temperature but I’m guessing it can all be explained by selection effects of parents, that different sorts of people get pregnant at different times of the year, with no causal effect of temperature at all. Yes they run some regressions controlling for family characteristics but I get the impression that the purpose of those regressions was just to confirm that their primary findings were OK: As sometimes happens in this sort of robustness analysis, they weren’t looking to find anything there, and then they successfully didn’t find anything. Not what I’d call convincing. The whole thing just seems like massive overreach to me. Also seems odd for them to talk about temperature “shocks”: It’s hardly a shock that it gets warm in the summer and cold in the winter.
I’m not saying that temperature at conception can’t have any effect on fetal health; I just don’t find the particular argument in this paper at all convincing. It’s the learning-through-regression paradigm out of control.
P.S. It’s April, and it just happens that the next available day on the blog is in August. What better time to post something on the effects of heat waves?
P.P.S. See here for further discussion by Joshua Wilde, the first author of the paper I write about above.