  During intergroup confrontations, agitating stimuli such as opponents'
threats and provocations can trigger collective violence, even when the usual
mechanisms of cooperation, such as norms with sanctions, are absent. We examine
video recordings of street fights between groups of young men. Collective
violence in their attacks sometimes breaks out in a burst, while at other times
it reaches only a fizzle in which only a few group members participate. An
adapted Ising spin-glass model demonstrates that these two temporal unfoldings
can be predicted by the proportion of unconditional defectors in a focal group.
