A century and a half ago, Mormons made national headlines by claiming a First Amendment right to practise polygamy, despite criminal laws against it. In four cases, from 1879 to 1890, the United States Supreme Court firmly rejected their claim, and threatened to dissolve the Mormon church if they persisted. Part of the Court's argument was historical: the common law has always defined marriage as monogamous, and to change those rules ‘would be a return to barbarism’. Part of the argument was prudential: religious liberty can never become a licence to violate general criminal laws ‘lest chaos ensue’. And part of the argument was sociological: monogamous marriage ‘is the cornerstone of civilization’, and it cannot be moved without upending our whole culture. These old cases are still the law of the land and most Mormons renounced polygamy after 1890.