This book chapter explores evolution and morality by considering the appeal to nature, and in particular to how evolution has shaped female and male brains differently, to explain evident sex differences and the persistence of sex inequality. It uses as illustrative the popularizing accounts of male and female brains found in Louann Brizendine, The Female Brain and The Male Brain, and the portrayal in such accounts of fundamental male and female differences in human mate selection and parenting. Drawing on the work of scientist and philosophers, the chapter critiques these accounts for engaging in an increasingly popular “neurosexism.” Such neurosexism has historical antecedents in law and social thought, e.g., earlier views of how women’s lesser and distinctive brain capacity constrained their ability to participate equally with men in the professions or politics or the like. The chapter then considers the relevance of claims about female and male brains and sex differences rooted in evolution to ongoing issues of law and policy, such as the form that marriage should take (whether it should be open to same-sex couples and whether it is ideally egalitarian) and governmental responsibility concerning work/family conflict. It concludes by noting a striking pendulum swing on the question of gender differences as fixed or malleable: in the wake of the recent economic recession, dubbed the “man-cession,” there is a noteworthy shift away from skepticism about feminist social engineering in the area of gender roles in the home and the workplace and toward calls for men to “adapt” to new economic and social conditions, for society to reimagine masculinity, and for the U.S. to look to how other nations have encouraged this adaptation.