Nancy Cott’s book Public Vows, which examined the history of marriage as an American public institution supported and regulated by the state, became influential in the legal campaign for same-sex couples’ marriage rights. It led her to become the lead author of amicus curiae briefs in numerous state and federal courts. These briefs explained the numerous deep changes in marriage that state legislatures and courts had already made during two hundred years of American history. She testified as an expert witness in several state and federal cases, including the well-publicized and successful California case Perry v. Schwarzenegger (2010). When the U.S. Supreme Court definitively enabled equal marriage rights (2015) in the case called Obergefell v. Hodges, the decision cited her book Public Vows several times, as well as citing the historians’ amicus brief she had developed.