A study published Friday found correlation between presidential campaign rallies for Republican Donald Trump and increases in violence in host cities. Those holding a Trump rally saw an average of 2.3 more violent assaults reported on the day of the event than on an average day.

The researchers at Perelman School of Medicine at the University of Pennsylvania found Hillary Clinton rallies showed no increase in assaults according to the results published in Epidemiology.

"News media sources reported there were violent incidents at some campaign rallies, but it was difficult to gauge whether there really was a systematic problem, and if so, how many additional assaults were associated with each rally," said lead author for the study, Christopher Morrison, PhD and Masters of Public Health.

To prevent similar violence in the future, it is important to understand the underlying causes of this behavior, perhaps including the role that political rhetoric might play in normalizing or promoting violence."

Rallies in the study were confined to open invitation events that occurred after Donald Trump and Hillary Clinton declared their candidacies in spring 2015, but before the U.S. Presidential Election on November 8, 2016. They each featured a speech by Trump or Clinton and did not coincide with any party primary election in the same state as the rally.

"This research provides evidence that this increase in assaults is associated with candidate Trump's rallies leading up to the election," said senior author Douglas Wiebe, PhD, an associate professor in Epidemiology.

Violent language may have affected the mood and behavior of rally attendees, as well as those exposed to the rally through news reports and social media."

“It appeared to be a phenomenon that’s unique to Donald Trump’s rally,” added Morrison.

During a rally in Cedar Rapids, Iowa, then candidate Trump remarked he would "knock the crap out of" would-be hecklers at the event. In the month of March 2016 alone, a planned Chicago Trump rally was canceled due to violence, at a rally in North Carolina an anti-Trump protester was punched and another was assaulted at a rally in Arizona.

Stories of violence at Trump rallies plagued the president's campaign. Many pointed to statements made by the candidate himself.

“Maybe he should have been roughed up,” Trump said of a protester who was reportedly punched and kicked in November 2015. A few months later, Trump remarked,

I’d like to punch him in the face, I’ll tell ya.”

At one rally Trump even offered to pay legal fees for any of his supporters arrested for assault during the rally.

To determine whether an actual shift in violence occurred, the researchers compiled a list of 31 Trump rallies and 38 Clinton rallies held in cities with online assault data. They compared total reported assaults on the day of a rally to those reported on the corresponding day of the week for four weeks before and after the event.

Cities in the study reported an average of 19.4 assaults. On the day of a Trump rally, that number rose to 21.7 violent assaults. The pattern held consistently even when researchers controlled for the influence of factors like population, data source and the day of the week.

Two potential causes for the increase in violence were offered by the researchers. One offers a direct correlation between the rallies and assaults, resulting in clashes at or near the rally venue. The second points to a phenomenon known as "social contagion". Social contagion refers to attitudes and actions influenced by reports in media, including social media. Aggressive language by Mr. Trump and violence from his supporters or his opponents might have influenced people elsewhere in the city.