Heated debates are taking place over the question: Who is a woman? Many of these are over inclusion criteria for policies that seek to promote equality, safety and/or privacy for girls and women by excluding boys and men. Science cannot resolve these debates, but its concepts and data can offer useful insights and information for policy makers who have to make principled and workable policy decisions about inclusion criteria. To assist policy makers in this difficult task, we begin by reviewing three key concepts that are often misunderstood and conflated: sex, gender, and gender identity. We then review key issues that policy makers should consider: the purpose(s) of the specific policy and whether it relates to sex, gender, and/or gender identity, and the distributions of benefits and costs for all stakeholders. As these considerations sometimes point to a conflict of interests, we end with some suggestions for how such conflicts might be ameliorated. Although we do not offer solutions to these difficult policy decisions, we hope that this article will help reduce misunderstandings, and facilitate open discussion and good decision making in this contentious policy context.