Data from self-report and observational studies make it clear that problematic social anxiety confers interpersonal constraint that may contribute to the interpersonal dysfunction reported by individuals with social anxiety disorder. Direct observation of interpersonal behavior in established relationships is rare and difficult to obtain for practical reasons. We tested a flexible iterated prisoner's dilemma computer task to examine whether it might capture interpersonal constraint related to social anxiety. Our first study suggested that, at least for men, the task directly captures social anxiety's constraint on responses to friendly giving. Our second study more strongly supported the assertion that the task directly assesses interpersonal constraint. The data support a model in which social anxiety directly confers trait-like tendencies toward interpersonal constraint and indirectly constrains behavior in specific interactions. These studies provide further evidence that constrained responses to positive social behaviors are characteristic of people with problematic social anxiety and may be particularly important to dysfunctions in the close relationships of these individuals.