President Donald Trump has ordered Marines into Los Angeles in response to immigration protests. The deployment follows days of reported violence, pillaging, and torching across the city, despite National Guard troops already having been sent in. California Governor Gavin Newsom responded by suing the Trump administration, calling the deployment “unlawful.”

Earlier, President Trump defended his combative approach, claiming: “We made a great decision in sending the National Guard to deal with the violent, instigated riots in California: If we had not done so, Los Angeles would have been completely obliterated.”

Meanwhile, there have been several days of riots in Northern Ireland.

The psychological understanding of exactly why crowds turn violent might be vital if the authorities are to get the response right, and there is already a debate as to what the correct answer is, and this arises directly out of competing understandings of why citizens riot.

In the court cases that follow after riots, a large group of people are prosecuted who, apparently, had never been in trouble with the law before. It would appear that some relatively law-abiding citizens can become transformed into even violent looters and arsonists, by some powerful mobilising effect of "crowd psychology."

The anonymity afforded by throngs, plus the hampering of the police getting access to individuals, means some will now "have a go" when they wouldn’t have otherwise. Heaving crowds offer various protections, and it is this, one theory contends, which produces aggression in otherwise relatively passive people.

This is not a dissimilar model to why the internet and social media appear to provoke more aggressive trolling than is apparent in direct human interaction.

But does crowd psychology mean that something primitive is released within us that is lying normally dormant, and that it could happen to anyone given the........

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