Better stuck together or free to go? Of the stability of cooperation when individuals have outside options

•Should it be easy to leave joint projects? What leads group members to leave?•We study experimentally exit from joint projects when outcome is stochastic.•Subjects leave too often because they overestimate the likelihood their peer will leave.•Low barriers to exit make subjects more sensitive to short-term motivations for exit.•Treatments with low barriers to exit thus generate lower welfare.