The deadly mob violence that wracked England this past week has abated, as police came out in force and used surveillance images to track down and arrest some 1,900 alleged rioters. As London and other cities in the nation recover, officials and the public may be left wondering how to prevent such rioting in the first place. A key misunderstanding, however, seems to pervade popular thinking: that mobs are irrational and are driven to violence by a few bad apples. In fact, the scientific evidence shows that individuals in mobs do behave rationally, although not always wisely. The findings suggest that understanding the logic behind mob behavior may offer ways to short-circuit riots before they start.

The recent riots broke out across England after the shooting of north Londoner Mark Duggan by police last weekend. Many official and media accounts place the blame on "a violent few" who swept thousands of others into a destructive frenzy. These analyses echo the 1896 work of Gustave Le Bon, who published The Crowd: A Study of the Popular Mind. "Crowds, after a period of excitement, enter upon a purely automatic and unconscious state, in which they are guided by suggestion," he wrote.

That idea, however, is a myth; social psychologists debunked it in the 1980s. "When people form a psychological group, what happens isn't that they lose a sense of identity but that they think of themselves in terms of group membership," explains social psychologist Stephen Reicher of the University of Saint Andrews in Scotland.

Individuals cannot act with group goals in mind until they see themselves as members of that group. In situations such as the recent London riots, this group identity seems to form spontaneously, but studies of the riots in England 30 years ago suggest a more complex buildup.

"Riots are the endpoint of a very long and entrenched process of social sense-making," Reicher says. "When an event comes along that clicks in perfectly to this broader social understanding, then suddenly it's much more likely to make you see yourself as a group member."

If the residents of Tottenham, where the first riot broke out, had the general feeling that they were being mistreated by the police, then the Mark Duggan incident provided an egregious and highly public example to confirm that. (Other social and economic factors, of course, were also at play as the rioting spread in the days that followed to other neighborhoods and cities.) When people gathered in the streets to protest, the physical group began to take the form of a psychological group, with shared values and a clearly defined out-group—the police.

The Londoners eventually turned to violence, but not necessarily because of mob madness or because the group was made up of individuals with a tendency toward unfettered aggression. ("While the trouble has been largely blamed on feral teenagers," The Daily Mail reported, "many of those paraded before the courts yesterday led apparently respectable lives.")

"One thing science does tell us is that we can't understand it if we treat it as irrational [or] if we think of these people as a gathering of people with individual predispositions to violence," says Clifford Stott, a social psychologist at the University of Liverpool.

Stott's previous research with John Drury of the University of Sussex (pdf) and others (pdf) has demonstrated that a crowd of nonviolent individuals can become a violent, and not—as previously thought—because a few violent protestors take control of an impressionable mob. Instead, the unilateral force that is sometimes used against a mostly nonviolent crowd can backfire, cementing the unity of the group against the now-violent authorities. This newly combative dynamic can change what's considered acceptable group behavior for everyone and leave group members with an intoxicating feeling of empowerment.

Reicher explains: "Crowd events tend to be mixed events with some people who do intend to be violent and some who don't. The response of authorities is to see the group as a whole as dangerous. At that point, precisely those people not originally violent have the experience of being treated with hostility and often physical force. Under those circumstances, they see the police as illegitimate and violence escalates."

Within this context, a violent crowd is not necessarily out of control but may be acting meaningfully and deliberately, with the "us versus them" values of the group in mind. This behavior is not irrational, Stott says, and "is consistent with how the social identity of the group is defined."

Even what appears to be a clear case of crowd violence can be misleading. A recent review of crowd behavior theories (pdf) pointed to the old idea of a mob, where "individuals lose all sense of self-responsibility... and primitive behavior results." But in reality, any riot includes both collective action and individual acts of opportunism, and these are hard to tease apart. Some individuals, Reicher explains, will use a crowd as a "cover," to do things they would not normally do. These single-minded actions do not necessarily represent the behavior of the group as a whole, even though it can appear so.

In the end, group identity is a precondition for a riot: people will only riot when they think their actions are aligned with the worldview of the group as a whole. Reicher suggests then that the key to preventing riots may be for authorities to regularly engage community members who will publicly oppose violence and looting, shifting the perception of group's needs and desires in advance. This early intervention—similar to approaches used to combat gang violence—could intercept the possibility of a violent group identity before it can begin to form.

"You can only take part in these events to the extent that you believe you have collective support from others," Reicher says. "Nobody riots on their own."