One punch or one stone thrown is all it can take to spark a riot and the dark side of human nature to spill over, according to New Zealand psychologists.

Thousands of people have taken to the streets in a number of cities in Britain, setting cars and buildings on fire, looting stores and causing chaos.

And it only takes one to get everybody started, psychologists here say.

Psychologist Russell Read, based in Invercargill, said those involved in the riots would know that their actions were wrong. However, many of them would not have acted if they had not seen others move first.

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"It's a copycat thing. You know what it's like at school. You get a kid who gets teased and then a whole group gets involved," Mr Read said.

"You know it's wrong but it's okay because other people are doing it ... it only takes one person to throw the first stone."

University of Auckland associate professor in clinical psychology Ian Lambie acknowledged that the police shooting of Mark Duggan would be fuelling some people in the mob.

However, it was likely that most people were already prone to anti-social and aggressive behaviour and the riots had given them an opportunity to vent their rage.

"They're a disaffected group in society and this is the opportunity for them to voice their aggressions," Dr Lambie said.

"There's these sorts of people and then there's basically hooligans who will jump on the bandwagon just for the hell of it."

South Auckland clinical psychologist Barry Kirker said it was normal for people to think about stealing, burning or throwing something.

"But they just don't have the context to do so - it's not allowed. The riots have provided a context for them to do so.

"And actually, it takes an unusual person to just stand there while everybody else is helping themselves."

Dr Lambie said the use of cellphones, social networking sites and YouTube videos played a huge role in the spread of the riots from city to city.

Coverage of the riots in the media were also other reasons why the riots were continuing.

Dr Clifford Stott, a senior lecturer in social psychology at the University of Liverpool and an expert in the psychology of riots, said that while the behaviour of people smashing up their "own communities" may seem irrational, they were meaningful targets for the rioters.

He said the shooting dead of a man by police in Tottenham "represented for many within his community the antagonistic relationship" they had with police.

He said it was also highly relevant that people had taken the opportunity to target shops selling high-end electrical goods, clothes and jewellery.

"In this age of austerity, such items are becoming increasingly unobtainable."