Ratifying core conventions adopted by the International Labor Organization (ILO) creates legal obligations to improve labor standards in the domestic economy, notably with regard to union rights, minimum age and discrimination in employment, and forced labor. Why and when do states choose to ratify them? Two influential theoretical approaches lead to the expectation that states are influenced by the ratification behavior of other states. Drawing on rationalist institutionalism, the authors expect states to use institutions such as the ILO to improve or consolidate their preferred standards domestically while reducing the risk of suffering competitive disadvantages in world markets. In this view, ILO conventions are devices for the prevention and mitigation of regulatory races to the bottom among trade rivals. Drawing on sociological institutionalism, they expect states to ratify ILO conventions if doing so conforms to a norm of appropriate behavior that is prevalent in a state's peer groups. This article develops observable implications of these hypotheses and tests them by applying spatial regression models to seven core ILO conventions and 187 countries between 1948 and 2009. The analysis yields strong evidence in support of both hypotheses.