The provocative psychology and the protester's view that violence is justified and appropriate to the circumstances has contributed to pushing stress to the climax.

A curfew was imposed on a series of American cities due to widespread protests and chaos after George Floyd, a black man from Minneapolis, Minnesota, last week was strangled to death by police.

Some protests began in moderation but later turned into violence.

"Things like the death of Floyd can become a trigger because for many people it is a symbolic image of the relationship between the police and the black community," said Professor Clifford Stott, executive

In 2011, riots also broke out in the UK after a man was shot dead by police.

Stott noticed widespread riots in Britain in 2011 because protesters in different cities regrouped because of their ethnicity and shared their hatred with the police.

Violent protests rarely occur if the police have a good relationship with the local community, but the way they react to protesters is also important, the expert said.

"Riot is a product of interaction, mostly derived from the way police treat the crowd," Professor Stott said.

"People 's feelings about violence and confrontation can thus change, for example, they will begin to feel that violence is justified in that situation," Stott added.

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Darnell Hunt, dean of the University of California's School of Social Sciences, believes that US police "intensified their actions" over the weekend, amid a wave of protests.

"Mobilizing the National Guard, using rubber bullets, tear gas, smoke bombs, these police tactics can all exacerbate an already stressful situation," Hunt said.

Ethical psychology can also help explain violent chemical protests, according to Marloon Moojiman, an associate professor of organized behavior at Rice University.

"This feeling can overwhelm the thought of having to keep things within the framework" because "when you think the whole system has been broken, you really want to do something drastic to show that."

Research shows that social media is also more likely to make people more likely to accept violence if they see violent behavior appearing in people who share their moral views.

In the US, hundreds of businesses have been smashed and pillaged by protesters.

"In a way, robbery is a way of showing power, black people can feel they are out of relationship with the police, but in the context of a riot, the rioters

Previous studies have shown that looted places are often associated with large businesses and looting "is often related to the feeling of inequality in capitalist society," he added.

However, both Professor Stott and Professor Hunt noted that the root cause of plundering, foul play in protests and riots is complicated.

According to public order experts, the best way to prevent violence is that the police must show they act lawfully and try to persuade protesters to dialogue.

Professor Lawrence Ho, an expert in public order and policy management at the Hong Kong University of Education, admits that conversation is the key to stopping violence, but the difficulty lies in many protests.

In general, politicians can make the situation better or worse depending on their confrontation or dialogue options.

Professor Hunt said the protests that were taking place in the United States were the most serious since 1968 after Martin Luther King was assassinated.

"George Floyd's incident was not the cause, it was more like a drop of water than a glass of water. We can say that the killing of civilians by the police is a symptom and the underlying cause lies in the thought of white upper skin.