OPINION: Four days before New Zealand's most deadly gun attacks, my opinion column on police carrying guns was published.

In this letter to Canterbury District Commander John Price, I recalled the prevalence of guns in my American childhood and argued that, despite the low-level shootout that had triggered the temporary arming of Christchurch cops, we should be wary of the small steps that normalise the use of firearms.

Then the March 15th massacre happened.

Guns were instantly everywhere. Armed cops from all over the country came to Christchurch in the police's remarkable response. Guns were visible outside of shopping malls, throughout the central city and hospital area, and Al-Noor and Linwood mosques.

We started getting used to the sight of firearms. At public gatherings to commemorate the tragedy, police faced the crowds carrying rifles, pacing back and forth with backs to the stage, eyes scouring the masses.

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I was spending more time at Al-Noor mosque, where several armed officers were posted for months. It was weird having a chat to a friendly cop at the mosque gate as he clutched his semi-automatic rifle.

Many Muslims felt a greater sense of security with this heightened visibility of firearms – there was nothing else to do but respect that.

KAI SCHWOERER/GETTY IMAGES Armed police have become a common sight for many Kiwis.

But for many other New Zealanders, it made us nervous, jittery, as if something really awful was about to happen again. It eroded our inherent trust in other people, it poisoned the peace we hope for in our city streets.

Our nerves in the aftermath of the mosque attacks may've somehow picked up on what social scientists call the "weapons effect". When people see or have access to a gun, their behaviour becomes more aggressive.

A review of over 50 published studies reported that just the sight of weapons increases aggression in both angry and non-angry individuals. In one study, drivers with guns in their cars were significantly more likely to follow another vehicle too closely, make obscene gestures, or both.

In other studies, having a gun was associated with more aggressive thoughts and more hostile views of the world.

JOSEPH JOHNSON/STUFF The presence of armed police in the wake of the March 15 terror attack was reassuring for many members of NZ's muslim community. Pictured is a police officer standing guard at the Al-Noor Mosque during the Duke of Cambridge's visit after the attacks.

Cops, like the rest of us, have brains that link weapons with aggression too, and despite the firearms training that is supposed to mitigate the risk of an armed police officer getting trigger happy, there have been too many deaths* – 18 in the past decade and 14 in the last five years – from police gunfire.

One of these was just two weeks ago, when a Tauranga man was shot dead by police after he riddled a police car with bullets.

Another was David Cerven, the unarmed 21-year-old killed in 2015 after pretending to wield a gun in Myers Park, Auckland.

Barrister Nicholas Taylor, a specialist in firearms law, said the scene immediately prior to the Cerven's shooting was chaotic, and that "there was a cowboyish attitude, the talk of 'tooling up,' and the spraying of rounds".

SIMON O'CONNOR/STUFF The "weapons effect". When people see or have access to a gun, their behaviour becomes more aggressive.

Of course, most trained police officers aren't like this, but if lethal options of policing are instantly at hand, they are more likely to be used instead of the many other policing tactics, such as negotiations, dogs, handcuffing or other restraints.

In their response to the police's new Armed Response Teams, those roaming, gunned-up units that are now being trialled in three districts, researchers from Victoria University's Institute of Criminology laid out the evidence against this approach.

Mutual escalation was one – the idea that when police carry guns, criminals think they need to as well, resulting in more shootouts. The more criminals respond violently, the more police think they need to arm themselves. And on it goes.

There is no better example of this than the US, where 36,000 Americans die each year from gun violence, generating a growing call for removing firearms from the police altogether.

But the researchers argue that "disarming an armed police force is much more difficult than not allowing arms to be routinely used in the first place. Once the genie is out of the bottle, it is hard to put back in."

These experts concluded that based on the evidence, "cops in cars with guns makes communities less safe, not more."

March is nearly here. We'll commemorate the 51 lives lost and hundreds of lives forever damaged due to the accessibility of guns. We'll reflect on the changes we've made since that bloody day, including new restrictions on gun ownership and perhaps a national firearms registry if the Arms Legislation Bill is passed, as planned, by March 15th.

Maybe for a few days police will routinely carry guns to help ensure our Muslim communities feel better protected over the anniversary weekend.

And hopefully, we'll remind ourselves that we cannot, step by step, cop by cop, justify having guns as a part of the experience of living in New Zealand.

* An earlier version of this story incorrectly said about 40 people had been killed by police gunshots in the last decade. This figure was an estimate which also included the number wounded.