Although social anxiety disorder appears to confer impairment in friendships, evidence beyond self-report is minimal. We used the flexible iterated prisoner's dilemma as a simulated interaction with a friend with 27 individuals with the generalized type of social anxiety disorder and 23 demographically equivalent individuals without the disorder. Participants with generalized social anxiety disorder were less giving on the task. Lower giving was also moderately associated with interpersonal variables (e.g., coldness). A trend was also found for participants with generalized social anxiety disorder to show lower assertiveness on the task. The connection between generalized social anxiety disorder and friendship impairment appears likely to be partially explained by interpersonal constraint that is perceived by others as coldness and manifests in a behavioral economics task.