The effects of international institutions on state behavior make up a key research agenda in international relations scholarship. Because states self-select into treaties, we cannot infer that these commitments have causal effects unless we address this selection effect. I explain the significant limitations of the methods used thus far to overcome this problem and argue that a more effective approach must take into account states' treaty preferences. I describe a novel combination of ideal point estimation and propensity score matching that can estimate the probabilities of treaty commitment and use them to test hypotheses. I use this procedure to the test the effects of three key international human rights treaties. My results provide significant new findings regarding the effects of these important agreements.