Extreme rituals entail excessive costs without apparent benefits, which raises an evolutionary cost problem (Irons, 2001). It is argued that such intense rituals enhance social cohesion and promote cooperative behaviors (Atran & Henrich, 2010; Durkheim, 1912). However, direct evidence for the relation between ritual intensity and prosociality is lacking. Using economic measures of generosity and contextually relevant indicators of group identity in a real-world setting, we evaluated pro- social effects from naturally occurring rituals that varied in severity.