  Collective violence in direct confrontations between two opposing groups
happens in short bursts wherein small subgroups briefly attack small numbers of
opponents, while the others form a non-fighting audience. The mechanism is
fighters' synchronization of intentionalities during preliminary interactions,
by which they feel one and overcome their fear. To explain these bursts,
subgroups' small sizes and leaders' role, a social influence model and a
synchronization model are compared.
