The reign of King Louis Philippe, the last king of France, came to an abrupt end on Feb. 24, 1848, after days of increasingly violent demonstrations in Paris and months of mounting agitation.

The protesters were fairly orderly at first. But on Feb. 23, the tide turned dark. Soldiers had fired on a crowd, gravely wounding scores of men and women. Blocks away, a journalist was "startled by the aspect of a gentleman who, without his hat, ran madly into the middle of the street, and began to harangue the passersby. 'To arms!' he cried. 'We are betrayed.' " "The effect was electric," the journalist wrote. "Far and wide the word was given that the whole system must fall."

Several decades later, in 1895, those events became grist for Gustave Le Bon's scholarly efforts to understand the mob mentality. Ever since, social scientists have sought to describe the dynamics of humans en masse. Why and how does a crowd become a mob?

The deadly assault on the U.S. Capitol on Jan. 6 has raised these questions again. If the scenes reveal one thing, it is variety. There were people in military gear, carrying guns, zip-ties and maps of the corridors; individuals in Uncle Sam hats and animal-skin costumes; others carrying nooses, planting explosive devices, breaking windows, attacking journalists; and hundreds just milling around. Many have gravitated toward using the term "mob," but the word hardly captures the totality of the events.

"Crowds do not act with one irrational mind," said James Jasper, a sociologist at the City University of New York. "There are many groups, doing different things, for different reasons. That is crucial to understanding how they ultimately behave."

Le Bon, a French intellectual, was not yet 7 during the 1848 rebellion in Paris. But he was repulsed by the entity at its center — the "howling, swarming, ragged crowd," he wrote in 1895. From there he built a theory of crowd behavior that has never quite gone away.

John Minchillo - Associated Press

"The sentiments and ideas of all the persons in the gathering take one and the same direction, and their conscious personality vanishes," he said. "A collective mind is formed."

But in the middle of last century, a major shift in thinking about crowd behavior occurred, integrating two competing principles. One is that, under specific conditions, peacefully minded protesters may act out — for instance, when a barricade is broken by others, when the police strike down someone nearby.

At the same time, as a rule, impulsive violence is less likely to occur in crowds that have some social structure and internal organization. The protests of the civil rights movement were tactical and organized. So were many sit-ins in the 1960s and '70s, against nuclear power and the Vietnam War. Windows were broken, there were clashes with police, but spontaneous mayhem was not the rule.

Calvin Morrill, a professor of law and sociology at the University of California, Berkeley, said, "The idea of the group mind does not give social scientists any room to explain the different levels of organization behind all those protests and what they meant.

"Ever since then, protests, whether nonviolent or not, have included tactics, strategy — and training — precisely to make sure the crowd does not lose its focus."

Experts said the footage from the Capitol reveals little about the strategies of either the crowd or the police, if any were at work. "It just felt like a mishmash of tactics and confusion, as one journalist put it after the Ferguson demonstration," said Ed Maguire, a criminologist at Arizona State University. "No clear structure in the crowd and absolute chaos on the police side: no clear sense of credible incident command, of wearing the right gear, carrying the right weapons. All of that seemed to be missing."

The video footage shows that, when the crowd actually breached the Capitol, many of the invaders weren't sure exactly what to do next. "People seemed surprised they had gotten in," Jasper said, and some "protesters stayed inside the velvet ropes, like tourists, looking around sort of in awe."

With no apparent structure or strategy, the crowd had no shared goal or common plan. The same haphazard quality that had allowed pockets of violence to open was probably part of what ultimately defused it.

"It looked like, in the end, it was just a matter of attrition," Jasper said. "People wanted to go find a bathroom, or a pub, or somewhere to sleep."