In June 2016, the City of London police force began to equip some of its frontline officers with Tasers, or "conducted energy devices," as they are known in such circles. The move was the first of its kind in England and Wales, and criminologists from the University of Cambridge used the shift to carry out a major experiment on how the public respond to visibly armed officers. Their findings suggest that while they were barely used, the sight of Tasers seems to lead to more aggressive behavior from both sides, with the researchers calling for a rethink on how such items should be carried.

Led by Dr Barak Ariel from the University of Cambridge's Institute of Criminology, the study took place between June 2016 and June 2017, in which time a total of 5,981 incidents with police occurred. During the study, 400 frontline police shifts were allocated a Taser-carrying officer, which were then compared to an equal number of shifts without a visibly armed officer.

This enabled the researchers to tease out some interesting insights into public-police interactions in light of the new weaponry. The researchers found that police armed with Tasers used force 48 percent more often than those who were not, with the use of force defined to include the use of not just Tasers but also physical restraint, handcuffing and CS spray. They say that even unarmed officers accompanying those on shift with Tasers used force 19 percent more often, something they call a "contagion effect."

While the Tasers were deholstered nine times during the study, electric shocks were only applied to suspects twice. Meanwhile, three police offers from the unarmed control group were assaulted throughout the study, compared to six from the taser-carrying group. Though these figures are small, the authors contend that assaults against police officers are very rare and this doubling of the statistic is therefore significant.

"It is well established that the visual cue of a weapon can stimulate aggression," says Ariel. "While our research does not pierce the 'black box' of decision-making, the only difference between our two study conditions was the presence of a Taser device. There was no increase in injury of suspects or complaints, suggesting it was not the police instigating hostilities. The presence of Tasers appears to provoke a pattern where suspects become more aggressive toward officers, who in turn respond more forcefully."

The results of the study tie in with a decades-old concept known as the weapons effect, first demonstrated in laboratory experiments in 1967 using electric shocks in the presence of a rifle. The researchers believe that a similar phenomenon might be at play here, provoking more aggressive responses from suspects who catch a glimpse of the Tasers.

"For many, a weapon is a deterrence," says Ariel. "However, some individuals interpret the sight of a weapon as an aggressive cue – a threat that creates a hostile environment. The response is consequently a 'fight or flight' dilemma that can result in a behavioral manifestation of aggression and assault. This is what we think we are seeing in our Taser experiment."

The researchers point out that around half a million police officers in the US are armed with Tasers, and that as they start to enter use in the UK it might be worth thinking about how they are carried. They put forward what is a pretty simple solution to limiting this weapons effect: conceal them from view.

"Following the findings of the study, we are exploring whether a simple holster change or weapon position move will nullify the weapons effect issue shown in the experiment," says City of London Police Chief Superintendent David Lawes, who is also a study co-author. "We have also updated our training package for officers carrying Tasers to make them aware of the findings."

The team's research has been published in the journal Criminal Justice and Behavior.