Malnutrition, sex ratio, and selection: a study based on the great leap forward famine.

This study tests the evolutionary hypothesis that maternal nutritional condition can influence offspring sex ratio at birth in humans. Using the 1959-1961 Chinese Great Leap Forward famine as a natural experiment, this study combines two large-scale national data sources and difference-in-differences method to identify the effect of famine-induced acute malnutrition on sex ratio at birth. The results show a significant famine-induced decrease in the proportion of male births in the 1958, 1961, and 1964 in the urban population but not in the rural population. Given that both the urban and rural populations suffered from the famine-induced malnutrition, and that the rural population experienced a drastic famine-induced mortality increase and fertility reduction, these results suggest the presence of a short-term famine effect, a long-term famine effect, and a selection effect. The timing of the estimated famine effects suggests that famine influences sex ratio at birth by differential implantation and differential fetal loss by fetal sex.