We examine a public goods game in 83 communities in northern Liberia. Women contributed substantially more to a small-scale development project when playing with other women than in mixed-gender groups, where they contributed at about the same levels as men. We try to explain this composition effect using a structural model, survey responses, and a second manipulation. Results suggest women in the all-women condition put more weight on co-operation regardless of value of public good, fear of discovery, or desire to match others' behaviour. Game players may have stronger motivation to signal public-spiritedness when primed to consider themselves representatives of the women of the community.