allowed to compete in women’s sports?6 How should we regard the marriage of a woman to a transman or vice versa? Understandably, differing answers to these questions tend to polarize people. But it’s important to realize that behind the surface polarization lie two very different understandings of what gender is and how it is determined. The older understanding (sometimes called biological essentialism) claims that a person’s gender is determined by the objective fact of their biological sex. Where