We already know that climate change brings extreme weather, but new research reveals it is also rewriting human demography. According to a massive new study published in the Proceedings of the National Academy of Sciences (PNAS), exposure to extreme heat during pregnancy significantly alters the human sex ratio at birth, resulting in fewer baby boys.
By analyzing high-resolution temperature data alongside 5 million live births in 33 sub-Saharan African countries and India, an international team of researchers — including Portland State University demographer Joshua Wilde — discovered that days with a maximum temperature above 20°C (68°F) are negatively associated with male births.
However, the researchers found that the reason for this demographic shift varies drastically depending on the region, highlighting a complex web of biological vulnerability and human behavior.
The Biological Toll: Sub-Saharan Africa
In sub-Saharan Africa, the reduction in male births is driven by biology. The researchers found that exposure to extreme heat during the first trimester of pregnancy leads to maternal heat stress, which increases the risk of miscarriage. According to the "frail male" hypothesis, male fetuses are biologically weaker and require greater maternal investment to survive. Consequently, when a mother's body is strained by extreme heat, male pregnancies are disproportionately lost. This biological filtering is especially prevalent among highly vulnerable populations, specifically mothers residing in rural areas and those with less formal education.
The Behavioral Disruption: India
In India, heat also leads to fewer male births, but the mechanism is behavioral rather than purely biological. In regions of India with a strong cultural preference for sons (particularly in the north), the sex ratio is historically skewed toward males due to the practice of sex-selective abortions targeting female fetuses.
The researchers discovered that when extreme heat hits during the second trimester — the window when fetal sex can be reliably determined by ultrasound — the number of male births drops. The heat disrupts daily life, limiting mobility, reducing income generation and creating barriers to accessing medical clinics. Because families cannot easily access or afford sex-selective abortions during heat waves, more female fetuses survive to term. This inadvertently balances the sex ratio, meaning proportionally fewer males are born.
The Big Picture
Ultimately, this landmark study proves that the impacts of rising global temperatures go far beyond crop failures and property damage. Extreme heat is a profound stressor on maternal and fetal health, and it possesses the power to manipulate deeply rooted social practices. As the climate continues to warm, these hidden biological and behavioral shifts will continue to quietly reshape the structure of the human population.
