There were five hypotheses: shared experience promotes willingness to perform extreme pro-group action; shared negative experiences make individuals contribute more than euphoric experiences; the more intense the experience the stronger the pro-social effects; the effect of shared negative experiences on pro-social behavior is much stronger where groups compete directly against other groups rather than if they cooperate against nature; and the effects of shared negative experience can be stronger than those of kinship. The hypotheses were then tested empirically in a variety of different study populations, including U.S. military veterans of the Vietnam war, college fraternity and sorority members who had undergone hazing, English Premier League football fans, martial arts practitioners of Brazilian Jiu Jitsu who sometimes use painful belt-whipping, and twins to examine the level of fusion. From both the theoretical and empirical research, the study concluded that overall shared negative experiences are a powerful mechanism for promoting pro-social behaviors, which under certain conditions can be extremely costly to the individuals concerned. - www.sciencedaily.com