Composite photo (L to R): Michelle Erstikaitis. (QMI Agency/Handout); Paul Bernardo. (QMI Agency/Handout); Col. Russ Williams. (QMI Agency/Handout)

A new Canadian study suggests that psychopaths are not mentally ill and should be held entirely responsible for their violent and manipulative actions.

Researchers from universities in Ontario, Alberta and Saskatchewan studied 289 murderers, rapists and other violent offenders, and concluded "psychopaths are executing a well-functioning, if unscrupulous strategy."

Psychopaths, with their trademark ruthless, risk-taking and often violent behaviour, "may have evolved to exploit others."

The theory rests in part on the victims of psychopaths.

Mental disorders "disrupt" the mechanism that stops people from hurting their families. But among the violent offenders researchers spoke to, those with a "greater degree" of psychopathy tended not to hurt family members.

"On average, psychopathy is associated with less harm to genetic relatives - that's exactly what you'd expect of healthy people," lead author Daniel Krupp, of Queen's University, told QMI Agency.

They are preserving their genetic material, he said.

But if being a psychopath is beneficial, why wouldn't everyone evolve that way?

Researchers call it "frequency dependent selection."

If most of society is one way - generally co-operative and trusting - there is a niche for a few people to take advantage, Krupp explained.

"It's a sort of balance between the population being exploitable enough for a handful of people to invade ... but if there's too many of them, then selection favours being skeptical of those people," Krupp said.

Meanwhile, the psychopaths among us are difficult to detect because they don't show any of the signs doctors expect to see in mentally disordered people, such as neurological or developmental problems, lower intelligence and difficulty interacting with others.

They may be selfish, antisocial and violent, but they are not dysfunctional, according to Krupp.

"Psychopaths know full well the consequence of their behaviour," he said.

The study appears in the latest issue of the journal Frontiers in Psychology.