Since the women's health movement blossomed in the 1970s, there has been an ever-increasing trend toward examining all aspects of human health for evidence of sex differences. But some of the movement's major achievements - such as a federal mandate to collect and analyze data by sex in all health research - may paradoxically turn out to be obstacles for understanding health differences between and within sex/gender groups. Building on her earlier work in Brain Storm: The Flaws in the Science of Sex Differences and using examples from both physical and mental health research, this 2011 Silver Science lecture by Rebecca Jordan-Young reviews some basic questions about measurement in "sex-specific" medicine that could revolutionize the field and yield research and clinical practice that is actually far more specific and scientific than the current approach. What kind of variable is "sex," and can it be measured separately from "gender"? When we have information on specific biological mechanisms underlying health differences, what does the variable "sex" add to our analyses? Introduced by BCRW Acting Director Elizabeth Castelli, Rebecca Jordan-Young delivered this lecture on October 11, 2011.