In a post earlier this month, Eric Johnson put together an interesting argument on the evolution of collective violence (I recommend reading his whole post).
Johnson opens with some of the arguments that group violence is a consequence of our evolutionary history. One of this arguments is the called the Elaborated Social Identity Model of crowd behaviour:
Each individual remains a rational actor, but has been primed by natural selection to identify with the group during a period of crisis. This well developed ingroup/outgroup bias is what has allowed our species to be the most cooperative of the primates, but certain conditions have the potential to turn us against our own community. ...
“Collective violence,” wrote Harvard primatologist Richard Wrangham in the Annals of the New York Academy of Sciences, shows “a common human pattern evident in societies lacking effective central authority, manifested in ethnic riots, blood feuds, lethal raiding, and warfare.” Such aggression, he says, is directly related to that of nonhuman primates and demonstrates a common evolutionary history.
One piece of evidence in support of this theory of inherent group aggression were a series of attacks by baboons in London zoo, with two-thirds of the 140 baboons dying during the violence. However, subsequent observations of baboons in captivity show how strong the environmental influences are on the actions of the groups:
What Kummer found was that captive baboons showed many more aggressive acts than their free-ranging counterparts (nine times more for females and seventeen and a half times more for males). The massacre of Monkey Hill therefore represents a kind of controlled experiment on the potential dangers of social engineering, one that demonstrates the lethal consequences of flawed assumptions. ...
Since the events of Monkey Hill, hundreds of studies with captive primates have shown that impoverished environments result in heightened aggression and antisocial behavior. Such behavior has been shown to significantly increase under conditions of overcrowding, when there’s a lack of novelty in food, entertainment, or social opportunities, when the population increases and the number of strangers in a colony grows, or, most crucially, when food is limited and/or fluctuates dramatically.
Using these observations, Johnson draws some conclusions about the London riots:
[T]he riot outbreaks were clustered in the most economically deprived regions of the city. It was these regions that would have been most aversely affected by the austerity measures and, with a peak in both food and energy prices occurring at the same time, the environmental conditions were ideal for a triggering event that would push an already stressed population over into social discord.....
For London and the cities throughout North Africa and the Middle East, it appears there was a free choice to riot after all. But the choice didn’t come from the rioters alone, it rose from leaders and policymakers and the larger society as a whole. Riots reveal a colony in discord. Many of us have acknowledged the widening inequality and economic decline of our most impoverished citizens–but we chose to ignore it
These conclusions might seem to flow from the observations of stressed primates, but we have an extra piece of data on the London rioters. Over 70 per cent of those convicted of rioting had prior convictions – 15 on average. While society may influence the chance of a riot, it seems that some people are much more susceptible than others.