FTA:

They did, however, show significant differences in group bonding: Students who performed the painful tasks reported a greater degree of bonding than did those who performed the pain-free versions, even after the researchers accounted for participant age, gender, and the size of the group. […] “This finding puts the ‘pain as social glue’ hypothesis to a rigorous test, highlighting that people not only feel closer to others, but are willing to risk their own outcomes to benefit the group,” says Bastian.