As Temperatures Rise, Fewer Boys Are Born
Climate change is not only melting glaciers and intensifying heat waves. It could also be silently reshaping how populations are composed. For centuries, the ratio of males to females at birth—the so-called sex ratio at birth (SRB)—has fascinated scientists and demographers. For a long time, it was thought to be stable, almost immutable, genetically determined. But in recent decades, it has emerged that social, environmental, and cultural factors can alter it.
A new study published in the Proceedings of the National Academy of Sciences (PNAS) adds an unexpected element: extreme heat during pregnancy is associated with a decrease in male births. It does so through an analysis unprecedented in its scope, linking high-resolution climate data with information on approximately 5 million births in 33 countries in sub-Saharan Africa and India.
Two regions, two models
The research, by Jasmin Abdel Ghany (University of Oxford), Joshua Wilde (University of Oxford), Anna Dimitrova (Barcelona Institute for Global Health), Ridhi Kashyap (University of Oxford), and Raya Muttarak (Department of Social and Political Sciences, Bocconi University), addresses a question that has been little explored until now: how does exposure to heat during pregnancy affect the sex of the child?
The authors explain from the outset that “While some evidence suggests that sex ratios at birth (SRBs) are shaped by environmental and social factors, little is known about the relationship between temperature and sex ratios at birth.” Filling this gap, the study shows that “days with a maximum temperature above 20°C are negatively associated with male births in both regions.” Above 20°C, therefore, the probability of a male birth decreases. But the mechanisms differ significantly between sub-Saharan Africa and India.
Sub-Saharan Africa: the biological weakness of males
In sub-Saharan Africa, the effect is concentrated in the first 3 months of pregnancy. Here, heat seems to increase prenatal mortality, especially among male fetuses. This interpretation fits into the so-called “fragile male” hypothesis: in adverse environmental conditions, male fetuses are biologically more vulnerable.
The study observes fewer male births after high exposure in the first 12 weeks, consistent with an increase in miscarriages linked to maternal heat stress. The effect is stronger among women living in rural areas, with low levels of education, and with higher birth order children (fourth or higher). In these groups, a significant increase in days above 30C in the first 3 months can reduce the male-to-female ratio by up to approximately 2.5 fewer males per 100 females. As the authors summarize, “heat exposure harms maternal health, increases prenatal mortality.” Here, heat acts as a direct biological factor, testing mothers’ ability to adjust temperature and protect pregnancy.
India: climate intersects with a preference for male children
In India, however, the picture is more complex and reveals an intertwining of climate and social norms. Here, the reduction in male births is concentrated in the second trimester, i.e., the time window when it is possible to know the sex of the fetus. The authors show that “in regions with high son preference, elevated temperatures during windows where sex-selective abortions could take place reduce these abortions.” In other words, during periods of intense heat, there is a decrease in selective abortions of female fetuses, a practice documented in some areas of India with a strong preference for male children.
The effect is particularly marked among women with at least four children, over the age of 30, and especially among mothers without sons in the northern states, where the preference for males is historically stronger. In these subgroups, an increase in hot days in the second trimester is associated with a substantial reduction in the male/female ratio. Heat, therefore, acts not only through biology, but also by influencing reproductive behavior and family decisions, perhaps through effects on income, mobility, or access to health services.
A surprising threshold: 20 degrees
An interesting aspect is that there is no linear gradient: it is not 'hotter = stronger effect'. The crucial threshold seems to be 20C. Above this level, the sex ratio at birth becomes less favorable to males, but further increases in temperature do not proportionally amplify the effect.
This has implications for future projections. The authors note that, in the regions studied, global warming will lead mainly to more days above 30°C rather than a shift from below to above 20°C. Consequently, we cannot be certain that future climate change will produce further imbalances in the sex ratio at birth in these areas.
Climate, health, and inequality
Perhaps the most significant finding of the study is that climate can affect the very composition of the population. And it does so in different ways, affecting already vulnerable groups more: women living in rural areas, with low levels of education, large families. In a warming world, heat is not just a matter of comfort or economic productivity. It can affect maternal health, fetal survival, and even gender-based discriminatory practices. The sex ratio at birth, a seemingly technical demographic indicator, thus becomes a lens through which to look at the interaction between climate change, biology, and social equilibria.
