Temperature and Sex Ratios at Birth

Human sex ratios at birth (SRBs) shape population composition and are closely linked to maternal health and gender discrimination. In the context of climate change, SRBs may theoretically be skewed by physiological or behavioral responses to exposure to global warming. However, evidence for this is limited. In this study, we estimate the effect of prenatal exposure to temperature on birth sex by linking survey data on 5 million live births in 33 sub-Saharan African countries and India with high-resolution temperature data. To distinguish between spontaneous and induced abortions, we exploit sociodemographic differentials, exposure timing, and regional differences in son preference. We find that days with a maximum temperature above 20°C are negatively associated with male births in both regions. In sub-Saharan Africa, we observe fewer male births after high first trimester temperature exposure, consistent with increased spontaneous abortions from maternal heat stress. This is particularly true for births by mothers in rural areas, with little formal education, and for higher birth orders. By contrast, in India we find second trimester temperature exposure is associated with fewer male births, consistent with reductions in induced sex-selective abortions against girls. As expected, these reductions are concentrated in higher birth orders and older mothers. We also find large reductions in male births by sonless mothers in northern Indian states, where son preference is greater. These findings demonstrate that climate change harms maternal health, increases prenatal mortality, and reduces engagement with the health system, leading to a complex effect on sex ratios at birth.