The brain is the most enigmatic of organs – it is really a collection of organs that undergoes a remarkable coordinated development that is driven in part by sex steroids. Today my guest is Margaret McCarthy, one of the leading researchers on sex differences in the brain. Here, we cover the history of research on sex and the brain, including the utility of animal models, the roles of hormones and sensitive periods in brain development, masculinization vs. feminization of the brain, epigenetic regulation of sex differences in the brain, differences between the sexes in the prevalence and age of onset of mental illnesses, and even the role of politics in the field of neurobiology. Margaret received her BS and MA degrees in biology at the University of Missouri in 1981 and 1984, respectively, and her PhD in behavioral neuroscience at Rutgers University in 1989. She then worked at Rockefeller University and the National Institutes of Health before joining the faculty at the University of Maryland, where she holds an endowed professorship and is the Chair of the Department of Pharmacology.