New research reveals that extreme heat is literally changing the human population's sex ratio — but for two completely different reasons. A massive study of 5 million births in sub-Saharan Africa and India, published recently in PNAS, shows that hot days during pregnancy result in significantly fewer male births.

In sub-Saharan Africa, the cause is biological. Heat stress during the first trimester increases the rate of miscarriage. Because male fetuses are biologically more fragile, they are disproportionately lost to maternal heat stress.

In India, however, the cause is behavioral. Heat waves during the second trimester disrupt access to medical services and financial resources, inadvertently reducing the rate of sex-selective abortions (which typically target girls).

Co-authored by researchers including Portland State University's Joshua Wilde, the study highlights how climate change is quietly acting as both a biological filter and a disruptor of human behavior.