This is a summary of a much longer article linked in this article.
Stanford sociologist Mark Granovetter “thought it was a mistake to focus on the decision-making processes of each rioter in isolation. In his view, a riot was not a collection of individuals, each of whom arrived independently at the decision to break windows. A riot was a social process, in which people did things in reaction to and in combination with those around them. Social processes are driven by our thresholds —which he defined as the number of people who need to be doing some activity before we agree to join them. In the elegant theoretical model Granovetter proposed, riots were started by people with a threshold of zero—instigators willing to throw a rock through a window at the slightest provocation. Then comes the person who will throw a rock if someone else goes first. He has a threshold of one. Next in is the person with the threshold of two. His qualms are overcome when he sees the instigator and the instigator’s accomplice. Next to him is someone with a threshold of three, who would never break windows and loot stores unless there were three people right in front of him who were already doing that—and so on up to the hundredth person, a righteous upstanding citizen who nonetheless could set his beliefs aside and grab a camera from the broken window of the electronics store if everyone around him was grabbing cameras from the electronics store.”
Malcolm Gladwell postulates that the school shootings since Columbine have been a sort of super-slow–motion riot.
“In the day of Eric Harris, we could try to console ourselves with the thought that there was nothing we could do, that no law or intervention or restrictions on guns could make a difference in the face of someone so evil. But the riot has now engulfed the boys who were once content to play with chemistry sets in the basement. The problem is not that there is an endless supply of deeply disturbed young men who are willing to contemplate horrific acts. It’s worse. It’s that young men no longer need to be deeply disturbed to contemplate horrific acts.”
What they suggest is a new type of restraining order that can be used against people when there is evidence (videos, manifestos, etc) that they are considering an act like this.