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. 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?

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