This book challenges law's reliance on neurology's brain-sex binary. The brain has become the latest candidate in a historical search for a reliable and fixed biological marker of 'true sex' that has permeated every aspect of Western culture, including law. As definitions of the sexed and gendered body have become ever more contentious, the development and dissemination of brain-sex theories have come to dominate popular understanding of LGBTI+ identities. But, this book argues, the brain is no more helpful than earlier biological measures in ensuring just outcomes. Examining how law determines and differentiates 'male' and 'female' in two contested areas of sexed identity -through a discussion of Australian cases authorising medical interventions to alter the embodied sex characteristics of transgender minors and intersex minors -the book demonstrates an incoherence in the legal understanding of gender identity development. As the brain too fails as a convincing biological anchor for the binary sex categories of male and female, law must, it is argued, retreat from its aspiration to create, define, and regulate artificially bounded sex categories of male and female. This book will be of great interest to scholars and students in a range of disciplines who are working at the intersection of law, gender, and sexuality.