This article contributes to the burgeoning literature on why states ratify human rights treaties. It first analyses why Ireland, the United Kingdom and the United States did not initially ratify or accede to the 1948 Genocide Convention, and then explores why the three countries eventually did accept it, 20–40 years after it was approved by the United Nations General Assembly. The extent to which material costs and benefits, the logic of appropriateness, and acculturation played a role in each of the three cases is assessed. Acculturation is particularly evident in the Irish case, but it also helps to explain the UK and US acceptance of the Convention.