Abstract: Three recent scientific reports that purport to show a biological basis for homosexuality have changed the face of pro-gay equal protection litigation by making the argument from immutability more attractive. Professor Janet E. Halley critiques these studies and their reception in legal culture. Because immutability is not a requirement for successful pro-gay litigation, moreover, Professor Halley contends that pro-gay litigators who invoke the argument from immutability do so not only at their option, but at the risk of misrepresenting and dividing the community they hope to represent. She argues that progay legal argument should focus instead on common ground that adequately represents the self-conceptions of both pro-gay essentialists and pro-gay constructivists. And she suggests just such a common ground for more effectively articulating pro-gay equal protection arguments.