We are concerned that Lise Eliot’s review of Gina Rippon’s book The Gendered Brain (Nature 566, 453–454; 2019) undermines the premise that sex is a biological variable with respect to many medical conditions and drug responses (see J. A. Clayton and F. S. Collins Nature 509, 282–283; 2014).

As president-elect and president, respectively, of the Organization for the Study of Sex Differences, we disagree with Eliot’s claim that the brain is “no more gendered than the liver or kidneys or heart”. We also disagree that sex differences in behaviour are due to cultural effects on newborns, not to biological effects. In our view, these are not mutually exclusive. Sex disparities occur in animal models that are not subject to cultural bias.

The brain, like many organs, shows differences attributable to sex, both during health (see, for example, E. Luders et al. J. Neurosci. 29, 14265–14270; 2009) and during disease. Two-thirds of people with Alzheimer’s disease are women; twice as many men as women have Parkinson’s disease (see, for example, L. J. Young and D. W. Pfaff Front. Neuroendocrinol. 35, 253–254; 2014). And multiple sclerosis affects three times more women than men, although men with the condition develop neurological disability more quickly (see, for example, R. R. Voskuhl and S. M. Gold Nature Rev. Neurol. 8, 255–263; 2012). Sex is a modifier of disease risk and progression.

Studying the effects of sex differences in health and disease will lead to new treatments that target sex hormone and sex-chromosome effects. These will ultimately help people irrespective of their sex.