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. Research across the social and natural sciences has consistently found that the month of a child's birth is associated with later outcomes involving health, educational attainment, earnings and mortality. Much of this work shows that on average individuals born in the winter have worse outcomes (less schooling, lower wages) than other individuals. What drives this association remains unclear. Some prior work has speculated that this association may be driven by social and natural factors (such as compulsory schooling laws, changes in temperature, or exposure to illness) that could affect children born in the winter in particular ways, but there is no consensus about the importance of these or other explanations. Kasey Buckles Department of EconomicsMoreover, most work has explicitly dismissed the possibility that seasonality in outcomes might reflect inherent differences in personal attributes or family background. For example, Hoogerheide et al. (2007) write, "one's birthday is unlikely to be correlated with personal attributes other than age at school entry"; Kleibergen (2002) writes, "quarter of birth is randomly distributed over the population"; and in a survey on the returns to schooling literature, Card (1999) concludes that relationships between wages, education, and season of birth "are probably not caused by differences in family background." These claims are often made (or implicitly relied upon) in the large body of work using season of birth as an instrumental-variable. 1Yet despite the widespread use of season of birth as an instrumental variable and the assertion among researchers that family background is unrelated to season of birth, we know of no rigorous investigation of the relation between season of birth and family background. In this paper we undertake such an investigation. Using data from live birth certificates and the census, we first see whether the typical woman giving birth in...

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 disproportionally 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.

There is enormous geographic variation in the use of cesarean delivery: For births over 2,500 grams, adjusted cesarean rates vary fourfold between low-and high-use areas. Even for births under 2,500 grams, high-use counties have rates that are double those of low-use ones. Higher cesarean rates are only partially explained by patient characteristics but are greatly influenced by nonmedical factors such as provider density, the capacity of the local health care system, and malpractice pressure. Areas with higher usage rates perform the intervention in medically less appropriate populations-that is, relatively healthier births-and do not see improvements in maternal or neonatal mortality. [Health Affairs 25 (2006): w355-w367; 10.1377/hlthaff.25.w355] T h e c e s a r e an s e c t i o n r at e i n t h e u n i t e d s tat e s is much higher than that in other countries.

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