Therapeutic community graduates cluster together in social networks: Evidence for spatial selection as a cooperative mechanism in therapeutic communities

•Therapeutic community residents who graduate receive affirmations from more peers who graduate.•This effect extends to a network path length of 2.•Residents who receive more affirmations overall are somewhat less likely to graduate.•We find no evidence that reciprocity in affirmations predicts graduation.•These results confirm both theory and laboratory findings on spatial selection in cooperation.