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Sex of the fetus is genetically determined such that an equal number of sons and daughters are born in large populations. However, the ratio of female to male births across human populations varies signi fi cantly. Many factors have been impli- cated in this. The theory that natural selection should favour female o ff spring under suboptimal environmental conditions implies that pathogens may a ff ect secondary sex ratio (ratio of male to female births). Using regression models containing 13 potential confounding factors, we have found that variation of the secondary sex ratio can be predicted by seropreva- lence of Toxoplasma across 94 populations distributed across African, American, Asian and European continents. Toxoplasma seroprevalence was the third strongest predictor of secondary sex ratio, β = − 0·097, P < 0·01, after son pref- erence, β = 0·261, P < 0·05, and fertility, β = − 0·145, P < 0·001. Our preliminary results suggest that Toxoplasma gondii in- fection could be one of the most important environmental factors in fl uencing the global variation of o ff spring sex ratio in humans. The e ff ect of latent toxoplasmosis on public health could be much more serious than it is usually supposed to be.