Many state decisions, including those to join coalitions, are positively contingent on the actions of other states. Positive feedback leads to difficulties in predicting outcomes, and so calls for analysis of decisionmaking processes in addition to outcomes. The games of coordination, repeated Prisoners' Dilemma, and zero–sum interactions lead to expectation about the intensity of positive feedback in coalition formation; it should increase as the benefits of cooperation do. Statistical measures of contagion can be used to test hypotheses about the incidence of feedback, as illustrated in an examination of international cooperation on economic sanctions. This example supports the hypothesis that international institutions increase positive feedback among their members.