Eric Michael Johnson in Scientific American:
Collective violence, extending from riots to warfare, presents a challenge to our ordinary understanding of free will. Actions that would rarely be taken by an individual on their own seem to be embraced when supported by a larger group. This can occur in societies ranging from the communist regime of Soviet Russia to the capitalist free market of modern day England. Given this commonality, perhaps the collective violence of a riot can be best understood as a biological event in which evolved cognitive responses encounter a unique environmental threat. And if that is the case, do individuals caught up in such incidents have any choice in the matter?