Research using the public goods game to examine behaviour in the context of social dilemmas has repeatedly shown substantial individual differences in patterns of contributions to the public good. We present here a new method specifically designed to capture this heterogeneity in play and classify participants into broad categories or types. Players in groups of four made initial, simultaneous contributions to the public good. Subsequently, players were sequentially told the current aggregate contribution to the public good and allowed to change their decision based on this information. The game continued, with players updating their contribution decision until the game ended at an unknown point. By looking at the relationship between players' contributions and the aggregate value they observed, we were able to cleanly classify 82% of our players into three types: strong free riders (28%), conditional cooperators of reciprocators (29%), and strong cooperators (25%). We also found that scores on some of the personality dimensions we investigated (self-monitoring, self-esteem, neuroticism, and conscientiousness) correlated with player type. Finally, males were found to be more likely to be strong cooperators than females. Copyright © 2001 John Wiley & Sons, Ltd.