A study was conducted to investigate the validity of many of the stereotypes about only children. A sample of 77 undergraduates (30 only, 30 first, and 17 last born) underwent a two-play Prisoner's Dilemma Game, the NASA exercise, and completed a questionnaire. Results indicated that only children were more likely to make cooperative response moves in the Prisoner's Dilemma Game and deviated more from the group decision in the NASA exercise than nononlies. Only children reported having fewer friends and joining fewer clubs than nononlies, but reported comparable numbers of close friends and took a comparable number of leadership positions in the clubs they joined. Results are discussed in terms of effects of siblings on development of interpersonal behavior.