Sunshine, fertility and racial disparities

Abstract

This research investigates the effect of sun exposure on fertility, with a special focus on how its effects and consequences for birth outcomes may differ by race. Sun exposure is a key mechanism for obtaining Vitamin D, but this process is inhibited by skin pigmentation. Vitamin D has been linked to male and female fertility and risk of miscarriage, and Vitamin D deficiency is more prevalent among blacks than whites. Using 1989-2004 individual live births data from the Natality Detail Files, county-level, monthly conceptions are estimated as a function of monthly solar insolation, temperature and humidity, as well as month, time and location fixed effects and controls. Insolation has positive, statistically significant effects on fertility for both non-Hispanic blacks and whites, but the effects are stronger and the pattern of effect different for black mothers than white mothers. Poisson estimates from the main model suggest that a 1kWh increase in average daily insolation in the conception month - approximately the difference in sunshine experienced in the typical September vs. October - increases non-Hispanic black conceptions by 1% and non-Hispanic white conceptions by 0.6%. Allowing insolation's effect to differ by maternal characteristic suggests that the racial differences are not being driven by differences in socioeconomic status (SES). Models that allow for more complicated timing of insolation's effect further suggest that insolation pushes black (white) conceptions into the unfavorable (favorable) season of birth. These estimated effects and our decomposition analyses suggest that insolation - and the implied Vitamin D deficiency underlying its effect-helps explain why black conceptions are more likely to display a seasonal pattern that is disadvantageous to birth outcomes.

Keywords: Fertility; Racial differences; Season of conception/birth; Solar insolation; Sunshine; Temperature.