A new study has found that London police officers visibly armed with electroshock 'Taser' weapons were more likely to be assaulted, and used force 48% more often, than those on unarmed shifts.
However, while use of force can include everything from restraint and handcuffing to CS spray, the Tasers themselves were only fired twice during the year-long study period.
Criminologists from the University of Cambridge say the findings suggest that Tasers can trigger the 'weapons effect': a psychological phenomenon in which sight of a weapon increases aggressive behaviour.
While the 'weapons effect' has been repeatedly demonstrated in simulated conditions over the last forty years, this is one of the largest studies to show it "in the field" and the first to reveal the effect in law enforcement.
The Cambridge researchers say their findings, published today in the journal Criminal Justice and Behaviour, may well apply to policing situations in which other forms of weaponry - including the lethal variety - are involved.
"We found that officers are more likely to be assaulted when carrying electroshock weaponry, and more likely to apply force," said lead researcher Dr Barak Ariel from Cambridge's Institute of Criminology.
"It is well established that the visual cue of a weapon can stimulate aggression. 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," he said.
The City of London force is responsible for policing the 'Square Mile' business district in the centre of London. It also holds national responsibility for Economic Crime and prioritises counter-terrorism, violent crime and public order due to its central location.
The force was the first in England and Wales to test "extended deployment" of Tasers - described as "conducted energy devices" in UK policing - to frontline officers. During the rollout, police chiefs allowed Ariel and colleagues to conduct a major experiment.
Between June 2016 and June 2017 the researchers randomly allocated 400 frontline shifts a Taser-carrying officer and compared the results to an equal number of unarmed shifts over the same period. A total of 5,981 incidents occurred during the study.
Use of force by police carrying Tasers was 48% higher than the officers on unarmed shifts. In what researchers call a "contagion effect", even those unarmed officers accompanying Taser carriers on 'treatment' shifts used force 19% more often than those on Taser-free 'control' shifts.
Six physical assaults against police were recorded during shifts with Taser-carrying officers, compared to just three on the unarmed 'control' shifts. While the numbers are small, assaults against officers are rare, and researchers argue that this doubling is significant.
Despite the increased hostility uncovered by the study, actual use of electroshock weapons was minimal over the study period, with just nine "deholsterings" - only two of which resulted in electric shocks applied to a suspect.
"The City of London police rarely discharged Tasers during the study. Yet the very presence of the weapon led to increased hostility between the police and public," said Ariel.
The weapons effect was first shown by psychologist Leonard Berkowitz in 1967, in a laboratory experiment involving the administering of electric shocks in the presence of a rifle - an experiment that Ariel points out has been replicated 78 times.
"For many, a weapon is a deterrence. However, some individuals interpret the sight of a weapon as an aggressive cue - a threat that creates a hostile environment," he said.
"The response is consequently a 'fight or flight' dilemma that can result in a behavioural manifestation of aggression and assault. This is what we think we are seeing in our Taser experiment."
"It would not be surprising to find that serious or violent offenders fit this criteria, especially young males - the very type of suspect that is regularly in direct contact with frontline police."
Half a million police officers in the United States regularly carry Tasers, and electroshock weapons are now becoming part of frontline policing across the UK.
The study author's offer a simple solution to bypass the weapons effect: conceal the Tasers. "The relatively inexpensive policy change of keeping Tasers hidden from sight should not limit efficacy, but could reduce the weapons effect we see in the study," said Ariel.
"This conclusion could be generalised to all types of police armoury, including the lethal firearms carried by police officers. If the presence of weapons can lead to aggression by suspects, so its concealment should be able to reduce aggression and increase officer safety," he said.
Study co-author Chief Superintendent David Lawes, from the City of London Police, said: "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. We have also updated our training package for officers carrying Tasers to make them aware of the findings."
"The use of Tasers have been a proportionate and sensible introduction to policing against a backdrop of unsophisticated terror attacks and an increase in violent crime across London.
"The City of London Police seeks to ensure that any major changes to policy are supported by an evidence base and we wanted to be confident that an extension of Taser deployments to our frontline responders was the right thing to do for both our officers and the public they serve.
"A number of other forces are interested in replicating the study to add to the evidence base and see whether the experiment produces the same results outside of London.
"Across our force, we will continue to use evidence to define how we target problems, which tactics we should use and how we can ensure policing is efficient and safer for both the general public and our officers."