Here’s How The Season Of Your Birth Affects Your Future Health And Success

No, I’m not talking about astrology. (If you’re a regular reader of this blog you can guess what my feelings on astrology might be.) This connection is indirect and has to do with who has babies when:

Research has found that season of birth is associated with later health and professional outcomes; what drives this association remains unclear. In this paper we consider a new explanation: that children born at different times in the year are conceived by women with different socioeconomic characteristics. We document large seasonal changes in the characteristics of women giving birth throughout the year in the United States. Children born in the winter are disproportionally born to women who are more likely to be teenagers and less likely to be married or have a high school degree. We show that controls for family background characteristics can explain up to half of the relationship between season of birth and adult outcomes. We then discuss the implications of this result for using season of birth as an instrumental variable; our findings suggest that, though popular, season-of-birth instruments may produce inconsistent estimates. Finally, we find that some of the seasonality in maternal characteristics is due to summer weather differentially affecting fertility patterns across socioeconomic groups.

Source: “Season of Birth and Later Outcomes: Old Questions, New Answers” from National Bureau of Economic Research Working Paper No. 14573, December 2008

From the paper:

…we find that the fraction of children born to women without a high school degree is about 10 percent higher (2 percentage points) in January than in May. By way of comparison, this 2-percentage-point-effect on the fraction of mothers without a high school degree is about ten times larger than the effect from a one-percentage-point increase in unemployment estimated by Dehejia and Lleras-Muney (2004). We also document a 10 percent decline in the fraction of children born to teenagers from January to May. This effect, which is observed every spring, is about as large as the decline in the annual fraction of children born to teenagers observed over the entire 1990s. We show similar seasonality in maternal characteristics using the 1960, 1970, and 1980 censuses.

A 10% swing? Holy crap. If you have an “at-risk” teenage girl, lock her in the closet during the summer.

There are times I just want to read the abstract and move on, but sentences like “Finally, we find that some of the seasonality in maternal characteristics is due to summer weather differentially affecting fertility patterns across socioeconomic groups” make me very curious…

Given that the relationship between season of birth and later outcomes seems in part driven by fertility patterns among different groups of women, it is natural to ask what causes these different fertility patterns. We explore one possibility, which is that the fertility-decreasing effects of hot summer temperatures disproportionately affect low socio-economic populations. To test this, we document fertility patterns for married and unmarried women, and add controls for weather at conception. We find that including weather controls attenuates the dip in births to unmarried women in the spring (nine months after the peak of summer heat), but does not affect fertility patterns for married women. This suggests that differences in exposure to extreme temperatures can account for some of the relationship between season of birth and family background.

And the possible reasons for this:

Lam and Miron (1996) show that extreme heat may reduce conceptions, in part because heat reduces sperm count and sperm motility. Low SES individuals may be more exposed to temperature extremes, and work has also shown that temperature may have larger effects on the health outcomes of low SES populations than others. If low SES women or their partners are more responsive to summer heat than other women, this may explain the dip in spring births in Figure 3 (nine months after the hottest months of summer)… There is some evidence that women would like to avoid giving birth in the winter—for example, Rodgers and Udry (1988) survey undergraduate students, and find that almost half of the respondents name either December or January as the worst month for birth. The winter dip to married women in Figure 3 could be generated if high-SES women are either more likely to have these preferences, or are better at executing them (perhaps because their births are more likely to be planned).

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