It is of particular importance to gain a more detailed picture of how the brains of males and females differ, because several psychiatric disorders and conditions differ in their prevalence between the sexes. For instance, rates of Alzheimer’s disease are higher in females than males, prompting a recent call for the prioritization of biomedical research into sex differences in measures relevant to this disorder (Mazure and Swendsen 2016). Females also show a higher prevalence of major depressive disorder (Rutter et al. 2003; Gobinath et al. 2017), whereas males display higher rates of disorders such as autism spectrum disorder (Baron-Cohen et al. 2011), schizophrenia (Aleman et al. 2003) and dyslexia (Arnett et al. 2017). Improving therapeutic strategies for these conditions will almost certainly require accurate quantitative estimates of where and how the sexes differ normatively. Moreover, although many psychological sex differences are small (consistent with the “gender similarities hypothesis”; Hyde 2014), some behaviors and traits do show reliable and substantial differences. For instance, performance on mental rotation tasks (Maeda and Yoon 2013) and physical aggression (Archer 2004) are on average higher in males, whereas self-reported interest in people versus things (Su et al. 2009) and the personality traits of neuroticism and agreeableness are on average higher in females. A full explanation of these cognitive and behavioral phenomena might benefit from a better understanding of brain sex differences. UK Biobank received ethical approval from the Research Ethics Committee. The present analyses were conducted as part of UK Biobank application 10 279. All participants provided informed consent to participate.