Many brain and behavioral disorders differentially affect men and women. The new National Institutes of Health requirement to include both male and female animals in preclinical studies aims to address such health disparities, but we argue that the mandate is not the best solution to this problem. Sex differences are highly species-specific, tied to the mating system and social ecology of a given species or even strain of animal. In many cases, animals poorly replicate male-female differences in brain-related human diseases. Sex/gender disparities in human health have a strong sociocultural component that is intimately entangled with biological sex and challenging to model in animals. We support research that investigates sex-related variables in hypothesis-driven studies of animal brains and behavior. However, institutional policies that require sex analysis and give it special salience over other sources of biological variance can distort research. We caution that the costly imposition of sex analysis on nearly all animal research entrenches the presumption that human brain and behavioral differences are largely biological in origin and overlooks the potentially more powerful social, psychological, and cultural contributors to male-female neurobehavioral health gaps.