You spend your days staring at endless crowds of people, peering into their eyes (figuratively) and tapping into their smartphones (literally), trying to guess which one of them will be the next mass murderer.

You work at a cubicle in a grey government building, employed by one of the intelligence and security agencies charged with spotting the next Parliament Hill shooter, the next Dylann Roof or Anders Breivik, and apprehending them, or changing their course, before they start killing. You have access to extremely confidential information and ever-bigger budgets.

So. How do you spot the terrorist?

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Until recently, there was basically one answer: You focused on those who had the backgrounds and life experiences that you think might to lead to violent terrorism and looked for those who subscribed to the sort of extreme beliefs or radical ideas that other terrorists had expressed.

On the assumption that this was the only viable approach, your agency requested increasingly invasive powers and laws to ban the expressing of extreme ideas – resulting in legislation such as Canada's Bill C-51 and Britain's 2006 Terrorism Act upon which it was modelled. And your government spent money on what's known as "countering violent extremism," making propaganda videos, like Canada's "Extreme Dialogue" series, designed to stop certain ideas and influences.

But even as these controversial laws and programs were taking shape, a quiet revolution was taking place among the analysts who do the hard work of counterterrorism analysis inside the agencies of Canada, the United States and Britain.

A couple of years ago, those analysts began asking the question: What if we have it backward? Could it be that terrorists are not people with extreme ideas trying to build up the courage to turn them into murder, but rather violence-prone people hunting for some excuse to turn their proclivities into deeds?

This was in part because the old "violent extremism" approach was failing to produce results. Studies of thousands of known terrorists and killers have identified little that will predict violent behaviour. Religious upbringing doesn't make people more likely to commit attacks. Nor does poverty. Nor does age, neighbourhood, ethnicity, social class, marital status, education level or immigration status. Even extremism itself: People who hold fundamentalist Islamic beliefs or racist right-wing beliefs are not hugely more likely than anyone else to commit an attack.

But a new type of analysis was producing results – one that started to attract the attention of Canadians in the wake of last fall's Parliament Hill shootings and in other countries at around the same time or earlier.

Analysts began looking at the work of Paul Gill, a criminologist at the University College of London. In a highly influential 2014 paper titled "Bombing Alone: Tracing the Motivations and Antecedent Behaviours of Lone-Actor Terrorists," Dr. Gill and his colleagues analyzed known terrorists not by what they thought or where they came from, but by what they did.

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In the weeks before an attack, terrorists tend to change address (one in five) or adopt a new religion (40 per cent of Islamic terrorists and many right-wing terrorists did so). And they start talking about violence: 82 per cent told others about their grievance; almost seven in 10 told friends or family that they "intended to hurt others."

A huge proportion had recently become unemployed, experienced a heightened level of stress or had family breakdowns. And most had done things that looked like planning – including contacting known violent groups.

In other words: People who commit violent terror attacks, it turns out, are not identifiable by the ideas they hold, but rather by the things that they do. The violence comes first, the thinking second.

Analysts in Canada and elsewhere came to realize this, from their own analyses, before they were aware of Dr. Gill's work – and their findings matched his very precisely.

This new approach, which has come to be widely adopted within counterterrorism circles in Western countries during the past 24 months, has changed the intelligence-gathering needs of agencies: They aren't so interested in trying to monitor and change people's thoughts (which involved infiltrating communities, often with disruptive results). Instead, they want to hear about people who have suddenly changed, started talking of violence or dropped out of their usual social circles. It still isn't precise or easy, but it involves less mass intrusion into the privacy and communications of citizens.

Unfortunately, governments, including Canada's, are behind the curve: Just as their terrorism experts and security employees have abandoned policies which resemble the policing of thoughts, they're passing disturbing laws to make such obsolete practices easier.