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In 1941, American psychiatrist Hervey Cleckley published a seminal book about psychopaths called The Mask of Sanity, in which he described an intelligent and cunning person skilled at manipulating others and indifferent to their pain. A man like this, Dr. Cleckley explained, finds no real meaning in love or horror or humour, as if "colour blind" to human feeling.

Succeeding on his superficial charm and purity of focus, he walks the paces of a normal person, yet carries "disaster lightly in each hand." A man like this can wear the uniform of social responsibility, even pilot planes for the Queen of England, and be quite a different beast inside.

These traits come remarkably close to describing the horror of Russell Williams.

"I don't know the answers," the former Air Force commander said in his confession, when asked if he'd reflected on his crimes. "And I am pretty sure the answers don't matter."

Society would beg to differ. Since his arrest, we've been wondering: How did he get away with it? Most of those grim details we now know. But science is closing in on the answer to a more compelling question: What made him become the kind of man who would want to?

Dr. Cleckley used interviews, observation and medical records to learn about his patients, but today, brain imaging offers scientists a new way to peer behind the mask. A growing number of them now see psychopathy as a neurodevelopmental disorder, one in which a combination of genetic and environmental factors, such as neglect or poor bonding with parents, lead to deficits in the brain. And if biology is to blame, can society hold the psychopath responsible?

The brain deficits that neuroscientists have documented affect the ability of psychopaths to feel emotions and learn from their mistakes - as if they have a learning disability that impairs their emotional development, says Kent Kiehl, a neuroscientist at the University of New Mexico. The differences have been seen in the brain images of children as young as 5.

Dr. Kiehl has been amassing the brain scans of convicted criminals, at least 15 per cent of whom are estimated to be psychopaths. He and other researchers are now turning their attention to the study of children, searching for the triggers and types of experience that shape the brain of a child or adolescent at risk of becoming a psychopath.

So far he has imaged the brains of 200 young offenders in the American criminal justice system, including many in maximum security, he says. They are assessed for psychopathy using the standard checklist for juveniles, developed by Canadian psychologist Robert Hare and his colleagues.

He is not ready to report his results, but Dr. Kiehl says his hypothesis is that their brains will share many of the same features he has documented in adult psychopaths in U.S. prisons, although the impairment will not be as severe, and thus perhaps more easily corrected.

The brains of psychopaths seem to be stunted in the machinery involved in humanity's ability to feel empathy and kindness, even love. In adult psychopaths, the almond-shaped structure called the amygdala that generates emotions like fear and is also involved in learning, is significantly smaller. They appear to have weaker connections in the inner recesses of the brain that make up the paralimbic system, which involves emotions and self-control. (Psychopathic traits have also been seen in patients with damage to this area.)

There also appear to be differences in the corpus callosum, which joins the right and left hemispheres of the brain - which has been linked to their impressive ability to lie and cheat and manipulate people, an evolutionary advantage in a world that rewards those who get the upper hand, says Yu Gao, a researcher in New York who studies the neurobiology of psychopathy. Think of serial killer Ted Bundy, who sexually assaulted and murdered at least 30 women in the 1970s, and yet earned glowing letters of reference from his boss, the Governor of Washington.

There is also strong evidence that psychopaths tend to stay focused on a task - they can't or don't stop despite signals and cues from the environment that other people would find hard to ignore. Those skills may make them capable, if callous, CEOs. In the extreme cases, it may make them heartless to the pleading cries of their murder victims.

While Dr. Kiehl does his work in prisons, Dr. Gao and her colleagues recruited volunteers for some of their brain imaging studies from temporary employment agencies in Los Angeles. They found more of them had been in jail compared to the general population and some scored high on measures of psychopathic personality traits, reporting that they had done things such as running over someone in their car because they were mad at them.

"There are plenty of perfectly normal people in temp agencies - our goal was to get a good range of people then to correlate brain imaging results with measures of psychopathic personality," says Dr. Gao's colleague Andrea Glenn, a doctoral student at the University of Pennsylvania.

Ms. Glenn is interested in the genetic factors that appear to play an important role in psychopathy. "I think the main thing that we know is that psychopathic traits are moderately to highly heritable," she says. A number of candidate genes have been identified, but a large number of genes likely influence the condition.

"By identifying some of them we may be able to understand which individuals have which factors, and may be able to tailor treatments to a given individual's biology."

But first, researchers also need to understand how those genes are linked to brain deficits that have been documented in psychopathy and the role environmental factors play.

The theory is that neglect, abuse and early trauma somehow desensitize children to the feelings of others, says Dr. Kiehl, but it still has not been proven. Not all psychopaths had horrible childhoods. Some come from stable families. Millions of children are abused he says, but don't become psychopaths.

In one of her studies, Dr. Gao found that children who lived apart from their parents in the first three years of life were more likely to have psychopathic personalities. This suggests that failure to bond may play a role, she says. She also found that adults who reported they were neglected by their mothers when they were children were also more likely to have difficulty with empathy, and other psychopathic traits.

But every child showing signs of callousness and fearlessness isn't a psychopath in the making - although it certainly increases the odds. It is rare for people to become callous and unfeeling as adults if they began life as caring, empathetic children, says Paul Frick, a psychologist at the University of New Orleans, who studies anti-social behaviour and develops therapies for anti-social children. These troubled kids learn to conform quickly, often even fooling researchers by posing as model citizens until the end of the day, when, denied a reward, they become nasty intimidators even with adults.

But one study that followed 12-year-olds with these traits into adulthood found that only about 20 per cent met the measurement for psychopathy. Genes may lay the foundation, but environment builds upon it. A fearless child with callous traits who lives in a stable, supportive home with a parents that can afford to send him skiing as an outlet for his risk-taking has better outcomes than one raised in a poor family where the parents have few resources.

In the past, it was argued that psychopaths could not be treated - therapy sessions appeared to have no impact on their recidivism rates, and they often emerged having learned new skills about human nature that made them better manipulators. Some new research, however, has shown progress in teaching empathy to young children, as well as the benefits of very intense therapy for adult criminals.

Now brain research suggests there may some day be a way to prevent or ameliorate the symptoms with therapies designed to improve their capacity for empathy. But does that mean, as some defense lawyers have argued in the United States, that they suffer from a mental illness that should absolve them of criminal culpability?

Even as neuroscientists aim for new treatments, many psychiatrists remain resistant to the idea that psychopathy should be identified as a disorder. It is not included in the Diagnostic and Statistical Manual of Mental Disorders, which instead used a more general broad, term "anti-social personality disorder." This frustrates researchers like Dr. Kiehl, who says it is time for the medical community and the criminal justice system to see it as a brain disorder like other mental illnesses. "Everyone understands if you have a child with low IQ they aren't as responsible and don't make the same choices. What about a child that has an emotional IQ in the same low range?"

But Stephen Porter, a psychologist at the Centre for the Advancement of Psychological Science and Law at the University of British Columbia, argues the opposite position - that, in fact, the ability to feel no emotion, makes them more able to react rationally, and has no impact on their ability to understand right from wrong. "They are every bit as rational as any human being, if not more so, because they don't have the noise of human emotion," says Dr. Porter. In his research, he has studied psychopaths in Canadian prisons, analyzing their crimes. He found that while psychopaths committed thefts and assaults without much premeditation, they were far less likely to kill in passion. Their murders were almost always carefully planned and executed. He describes them as "selective impulsive," that is, they carefully weigh the costs and benefits of their deeds - the likelihood of being caught, the steeper punishment of life in prison for murder if they are careless. He points as well to research that has shown they are more than twice as likely to be granted parole. "They can put on Academy Award-winning performances for the parole board," he says. And for juries.

Though never officially diagnosed as a psychopath, Mr. Williams appears to fit the profile. The way he toyed with his victims like playthings, unable to offer mercy: "Have a heart," one pleaded, as he unflinchingly duct-taped her nose to suffocate her. The singular attention with which he cataloged photos and video of his crimes. The fleeting shows of remorse. (He feels "disappointed," he tells police in his confession, about what he's done.)

All the while, he wore the mask of sanity, in place for so long and so well, that he tortured and killed, and then went to work to discuss whether the military should buy a new aircraft transport. That's also the measure of a cunning psychopath: He's often the last guy you'd ever suspect.

However, Mr. Williams is a curiosity in some respects - for his apparent late start into heinous crime, the quiet worry he expressed, upon realizing he was caught, for his wife (and her shiny new floor that might be scratched in a police search of their Ottawa home), even his ability to hide himself in an environment that required taking orders without protest. Read what you will into his tearful statement of regret at his court sentencing on Thursday.

By some estimates, as many as one per cent of men are psychopaths working and living among us. "Many of us will come across a psychopath at some time in our lives, and you really have to know who to defend yourself against these perfect actors," says Dr. Porter. "We have to have some kind of ammunition."

When asked about his past early in his confession, Mr. Williams told police, "it will be very boring." That's one statement science has clearly revealed to be a lie.