Sex difference in the brain is of great interest, because it is believed to reveal the “real” or biologically predetermined basis for differences between men's and women’s behavior. However, current neuroscience research does not support this conception. First, most male/female brain differences are attributable to body size; thus, all brain structures are 10% larger in males, but after accounting for individuals’ total brain volume, sex/gender explains only ~1% of the variance in structural volumes at both the cortical and subcortical level. Other differences, such as higher ratio of gray:white matter and density of interhemispheric connections in women, are also found to be due to brain size rather than sex per se.Second, functional measures of brain activity (fMRI) have not revealed reliable differences in the neural circuits that process verbal, spatial, and emotional information, even though men and women as groups perform differently on such tasks. Finally, it is important to appreciate that brain structure and function are both influenced by experience, or neuroplasticity, so even when small differences are identified, it is not possible to determine whether they were induced by “Nature” (sexually differentiated genes and hormone levels) or “Nurture” (gender enculturation). Overall, measures of brain structure and function exhibit far more overlap than difference between males and females. In spite of much hype, current brain findings do not explain any of the well-described male/female differences in behavior, interests, or mental health.