The emerging phenomenon of genetic paternity testing shows how good science and useful social reform can run off the rails. Genetic paternity testing enables us to sort out, in a transparent and decisive way, the age-old but traditionally never-quite-answerable question of whether a child is genetically related to the husband of the child's mother. Given the impossibility of settling this question for certain, British and American law has long held that a biological relationship must almost always be assumed to exist. According to what is known as the “marital presumption” or “presumption of legitimacy,” a child born to a woman within a marital relationship is assumed to be the biological child of the woman's husband unless he was absent, impotent, or sterile. In other words, if paternity was not a physical impossibility for the husband, there was a nearly irrebuttable presumption that he was the father of the child. The husband was locked into the role of fatherhood.I am grateful to the participants of Genetic Ties and the Future of the Family, a research project run conjointly by The Hastings Center and the Institute for Bioethics, Health Policy, and Law at the University of Louisville School of Medicine. Discussions held in the course of this project have influenced this paper in various ways. I am especially grateful to Mary Anderlik for detailed comments. An earlier version of this paper was presented at the 2002 annual meeting of the American Society of Bioethics and Humanities and at a colloquium at Oxford University on Genetic Technologies and the Family, and the paper has benefited from comments offered on each of those occasions. Funding for Genetic Ties and the Future of the Family is provided by the National Institutes of Health (grant #HG02485).