WHILE DONALD INSISTS ON THE OPPOSITE:

Researchers have spent 50 years studying the way crowds of protesters and crowds of police behave -- and what happens when the two interact. One thing they will tell you is that when the police respond by escalating force -- wearing riot gear from the start, or using tear gas on protesters -- it doesn't work. In fact, disproportionate police force is one of the things that can make a peaceful protest not so peaceful. But if we know that (and have known that for decades), why are police still doing it?

"There's this failed mindset of 'if we show force, immediately we will deter criminal activity or unruly activity' and show me where that has worked," said Scott Thomson, the former chief of police in Camden, New Jersey.

"That's the primal response," he said. "The adrenaline starts to pump, the temperature in the room is rising, and you want to go one step higher. But what we need to know as professionals is that there are times, if we go one step higher, we are forcing them to go one step higher."

Interactions between police and protesters are, by their very nature, tough to study. Even when researchers get a good vantage point to observe protests in the real world -- for example, by embedding within a crowd -- the data that comes out is more descriptive and narrative as opposed to quantitative. Some kinds of protests are highly organized with top-down plans that are months in the making. Others, like many of the events across America this past week, are spontaneous outpourings of grief and anger. The social and political context of the time and place also affect what happens. Even a single protest isn't really a single protest. "You have lots of mini protests happening in many places," said Edward Maguire, a professor of criminology and criminal justice at Arizona State University. "There's different dynamics. Some peaceful. Some not. And different police tactics." In Baltimore on Saturday, for example, a police lieutenant mollified a crowd by reading out loud the names of victims of police brutality, while protesters outside City Hall threw bottles at police in riot gear and police used tear gas on the crowd, WBFF-TV reported.

But just because there's no data about protests that can be easily compared in a chart doesn't mean we're bereft of information, said Pat Gillham, a professor of sociology at Western Washington University. There's 50 years of research on violence at protests, dating back to the three federal commissions formed between 1967 and 1970. All three concluded that when police escalate force -- using weapons, tear gas, mass arrests and other tools to make protesters do what the police want -- those efforts can often go wrong, creating the very violence that force was meant to prevent. For example, the Kerner Commission, which was formed in 1967 to specifically investigate urban riots, found that police action was pivotal in starting half of the 24 riots the commission studied in detail. It recommended that police eliminate "abrasive policing tactics" and that cities establish fair ways to address complaints against police.

Experts say the following decades of research have turned up similar findings. Escalating force by police leads to more violence, not less. It tends to create feedback loops, where protesters escalate against police, police escalate even further, and both sides become increasingly angry and afraid.

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