RESUMEN
Human brain sex differences have fascinated scholars for centuries and become a key focus of neuroscientists since the dawn of MRI. We recently published a major review in Neuroscience and Biobehavioral Reviews showing that most male-female brain differences in humans are small and few have been reliably replicated. Although widely cited, this work was the target of a critical Commentary by DeCasien et al. (Biol Sex Differ 13:43, 2022). In this response, I update our findings and confirm the small effect sizes and pronounced scatter across recent large neuroimaging studies of human sex/gender difference. Based on the sum of data, neuroscientists would be well-advised to take the null hypothesis seriously: that men and women's brains are fundamentally similar, or "monomorphic". This perspective has important implications for how we study the genesis of behavioral and neuropsychiatric gender disparities.
The idea that men and women have fundamentally different brains has prevailed among scientists and non-scientists alike. MRI studies have been grappling with this for more than three decades but yet to find distinct features that define the "male brain" versus "female brain" across the human species. In seeking the causes of gender disparities in behavior and mental health, scientists need to look well beyond brain structure.