The protests that took place on the streets of London on the eve of the G20 summit in April lived up to many people's expectations. Around 2,000 protesters turned up, and were heavily marshalled by police. There was a bit of trouble, but the police tactics - specifically, the decision to corral the entire crowd into a small area near the Bank of England, an approach known as "kettling" - kept a lid on the violence.
That, at least, is the official version of events, and it reflects a belief about crowds that is shared by police, governments and to a large degree the general public across the world: that they are hotbeds of trouble and must be contained. The "unruly mob" concept is usually taken as read and used as the basis for crowd control measures and evacuation procedures across the world. Yet research into how people behave at demonstrations, sports events, music festivals and other mass gatherings shows not only that crowds nearly always act in a highly rational way, but also that when facing an emergency, people in a crowd are more likely to co-operate than panic. Paradoxically, it is often actions such as kettling that lead to violence breaking out.
"In many ways, crowds are the solution," says the psychologist Stephen Reicher, who studies group behaviour at the University of St Andrews in Scotland. Rather than being prone to irrational behaviour and violence, members of a crowd undergo a kind of identity shift that drives them to act in the best interests of themselves and everyone around them. This identity shift is often strongest in times of danger. "The 'mad mob' is not an explanation, but a fantasy," Prof Reicher says.
All that said, there is no question that being part of a group can sometimes lead people to do appalling things that they would usually abhor. Examples of crowd-fuelled violence abound, from Hutu death squads in the Rwandan genocide to racist lynch mobs in the southern states of the US. Likewise, the cover that crowds offer can attract individuals who are intent on causing trouble. We can all too easily be led astray by the influence of others.
However, crowd violence is actually extremely rare. "If 100 football matches happen on a Saturday and there is violence at one of them, we know which will appear on the front pages the next day," says Prof Reicher. Widespread panic during crowd emergencies is also uncommon and only occurs in special circumstances, such as when escape routes start to close, says Tricia Wachtendorf of the Disaster Research Center at the University of Delaware.
Evidence against the irrationality of crowds has been building for some time. In a study to be published in the British Journal of Social Psychology, a team led by John Drury at the University of Sussex, talked to survivors of 11 crowd-based disasters or near disasters, including the 1989 Hillsborough stadium crush that killed 96 football fans, and a free concert by Fatboy Slim on Brighton beach in 2002 that was swamped by 250,000 people, four times as many as expected, and led to around 100 injuries. In each case, most interviewees recalled a strong sense of unity with those around them as a result of their shared experience. Rather than being competitive or antagonistic, people did their best to be orderly and courteous - and went out of their way to help strangers. Researchers think that without such co-operation, more people could have been injured and killed.
The team found a similar pattern of solidarity and co-operative behaviour in a study of the suicide attacks in London on July 7 2005, which led to crowds of commuters being trapped underground. Having established that unruly mob behaviour is the exception, researchers are now getting to grips with the psychological processes that can transform hundreds or thousands of individuals into a unit. The key, according to Dr Drury, Prof Reicher and others, is the recognition that you share something important with those around you, which forces you to identify with them in a meaningful way.
"People start agreeing with each other, trusting each other," Prof Reicher says. At the point when members of a crowd start to share a common social identity, the crowd goes from being a mere physical entity to a psychological unit, according to Clifford Stott at the University of Liverpool, who specialises in the behaviour of football crowds. The tendency of people to form strong social bonds while experiencing terror together appears to be a universal human trait. "This is well known in traditional societies where boys going through puberty rituals in the transition to manhood are often put through frightening experiences," says Robin Dunbar, who studies the evolution of sociality at the University of Oxford.
What are the lessons from all this? One of the most important is that the current approach to managing crowds, which is all about control and containment, can be counterproductive. Police tend to assume that people in crowds are prone to random acts of violence and disorder, and treat them accordingly. But aggressive policing is likely to trigger an aggressive response as the crowd reacts collectively against the external threat. This is why many researchers consider kettling to be a bad idea.
Emergency services should also take note: in a situation such as a terrorist attack or fire, a crowd left to its own devices will often find the best solution. Attempts to intervene to prevent people panicking, such as restricting their movement, could make panic more likely. If you find yourself in a crowd emergency, the worst thing you can do is resist the group mentality. One of Dr Drury's conclusions from his research into disasters is that the more people try to act individualistically - which results in competitive and disruptive behaviour - the lower everyone's chances of survival are. This is what some researchers believe happened in August 1985 when a British Airtours plane caught fire on the runway at Manchester Airport, killing 55. Non-cooperative behaviour among passengers may have made it harder for people to reach the exits.
It can be hard to shake off the idea of crowds as inherently violent but it is worth remembering that they have also been responsible for just about every major societal change for the good in recent history, from the success of the US civil rights movement to the overthrowing of communist regimes in eastern Europe. Good leadership and individual heroics are all very well, but if you are looking for a revolution - or even just a good way out of a difficult situation - what you really need, it seems, is a crowd.