Assaults soared in American cities during Donald Trump's election rallies, reveals new research.

Cities experienced 2.3 more assaults than average on days when hosting presidential campaign rallies for Trump during the lead-up to the 2016 US Presidential Election, according to the first study of its kind.

But rallies involving Trump's Democrat rival Hillary Clinton s were not linked to any increase in assaults, according tothe findings published online by the journal Epidemiology.

Lead author Doctor Christopher Morrison, a fellow at the University of Pennsylvania, said: "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.

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

Focusing on cities of greater than 200,000 people, the team performed a systematic Google search and found publicly available data for 31 rallies in 22 cities for Trump and 38 rallies in 21 cities for Clinton.

Using assault figures from police departments in those cities, including aggravated assaults, simple assaults, and/or battery, the researchers counted assaults on the day of each rally.

For comparison, they also counted assaults on corresponding days of the week for four weeks before and four weeks after each rally.

Rallies in the study were defined as open invitation events that occurred after Trump and Clinton declared their candidacies in spring 2015 and before the US Presidential Election on November 8, 2016, featured a speech by Trump or Clinton, and were not on the same day as a party primary election in the same state as the rally.

The researchers suggest two possible explanations for their findings.

Firstly, all additional assaults could have occurred in and around the venues of candidate Trump's rallies where people invested in the process gathered. This explanation is consistent with news media reports that violence occurred at these specific locations.

Secondly, additional assaults that took place might have occurred elsewhere in the rally cities.

The researchers said that studies informed by theories of "social contagion" find evidence that emotional states can be transmitted through news reports and social media, which could have led to more assaults occurring away from the rally sites.

The researchers noted that Trump rallies were widely broadcast and discussed through news reports and social media, such as a rally in Iowa during which Trump said that he would "knock the crap out of" would-be hecklers at the event.

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

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

The researchers said weather can also influence crime rates, so they consulted National Oceanic and Atmospheric Administration figures for the weather station nearest to each study city to control for temperature and precipitation.

ENDS