Although international relations often involves a trade of favors or services, political scientists have not often used exchange theory. Social exchange is motivated by the prospect of mutual gain. The exercise of power entails exchange of needed resources for compliance with the influencer's wishes. The timing of repayment and explicitness of obligation are important dimensions of exchange that vary by issue area and relationship. In sequential exchange, the party that moves first risks being exploited and must therefore trust the other. The parties to an exchange may either leave open or specify what the other should do in return. Reciprocity refers to exchanges which are mutual and perceived by the parties as fair. It is difficult to determine whether exchanges are reciprocal without a common measure of value. Norms and customary expectations determine what is considered fair when there is no standardized measure of value. In negotiations, there are several competing principles of justice. Reciprocity requires that concessions be matched; it does not mean that their magnitude must be equal.