Sex differences in a disease or trait can provide insight into its causes, risk factors, and consequences. The aims of this paper are to: (a) summarize the sex‐specific lifetime prevalence of the most common psychiatric disorders among adults and youth; (b) enumerate hypotheses for sex differences in mental disorders; (c) describe the use of the concepts and tools of genetic epidemiology to evaluate sex differences in psychiatric disorders and (d) examine how traditional family and twin studies, and case‐control genome‐wide association studies (GWAS) can help to elucidate the etiology of psychiatric disorders in the molecular era. In examining the sex‐specific presentation of psychiatric disorders, we will consider sex differences in lifetime prevalence, onset, severity and/or clinical manifestations of these conditions. The hypotheses put forth to explain sex differences in mental disorders include artifactual or methodological differences in the studies or samples, differential expression or severity of disorders in males and females, sex differences in developmental trajectories, environmental factors, and different genetic architecture of the condition in males and females. As described below, the tools of genetic epidemiology, including family and
