We analyse two team settings in which one member in a team has stronger incentives to contribute than the others. If contributions constitute a sacrifice for the strong player, the other team members are more inclined to cooperate than if contributions are strictly dominant for the strong player.

Laboratory experiments by Fudenberg and Pathak (2010), and Vyrastekova, Funaki and Takeuch (2008) show that punishment is able to sustain cooperation in groups even when it is observed only in the end of the interaction sequence. Our results demonstrate that the real power of unobserved...