The word-based task was under way. The inmate was being shown a series of words and phrases, and was supposed to rate each as morally offensive or not. There were three kinds of phrases: some were intended as obvious moral violations, like “having sex with your mother”; some were ambiguous, like “abortion”; and some were morally neutral, like “listening to others.” The computer software captured not only the inmate’s response but also the speed with which he made his judgment. The imaging technology recorded which part of the brain was involved in making the decision and how active the neurons there were.

Neurons in the brain consume oxygen when they are “firing,” and the oxygen is replenished by iron-laden hemoglobin cells in the blood. The scanner’s magnet temporarily aligns these iron molecules in the hemoglobin cells, while the imaging technology captures a rapid series of “slices”—tiny cross-sections of the brain. The magnet is superconductive, which means it operates at very cold temperatures (minus two hundred and sixty-nine degrees Celsius). The machine has a helium cooling system, but if the system fails the magnet will “quench.” Quenches are an MRI technician’s worst fear; a new magnet costs about two million dollars.

The inmate wore a helmet with a head coil for receiving magnetic data and, on the inside, a screen on which words were projected. A sensor measured “skin conductance”—palm sweat. During the functional imaging scans, there was a series of high-pitched beeps, then a loud drilling sound. And during the brain-anatomy scans the machine made a low, rapid thumping, like a metal heartbeat. As the inmate’s brain was scanned, he crossed his feet at the ankles and then uncrossed them. His toes wiggled.

Psychopaths are as old as Cain, and they are believed to exist in all cultures, although they are more prevalent in individualistic societies in the West. The Yupik Eskimos use the term kunlangeta to describe a man who repeatedly lies, cheats, steals, and takes sexual advantage of women, according to a 1976 study by Jane M. Murphy, an anthropologist then at Harvard University. She asked an Eskimo what the group would typically do with a kunlangeta, and he replied, “Somebody would have pushed him off the ice when nobody else was looking.”

The condition was first described clinically in 1801, by the French surgeon Philippe Pinel. He called it “mania without delirium.” In the early nineteenth century, the American surgeon Benjamin Rush wrote about a type of “moral derangement” in which the sufferer was neither delusional nor psychotic but nevertheless engaged in profoundly antisocial behavior, including horrifying acts of violence. Rush noted that the condition appeared early in life. The term “moral insanity” became popular in the mid-nineteenth century, and was widely used in the U.S. and in England to describe incorrigible criminals. The word “psychopath” (literally, “suffering soul”) was coined in Germany in the eighteen-eighties. By the nineteen-twenties, “constitutional psychopathic inferiority” had become the catchall phrase psychiatrists used for a general mixture of violent and antisocial characteristics found in irredeemable criminals, who appeared to lack a conscience.

In the late nineteen-thirties, an American psychiatrist named Hervey Cleckley began collecting data on a certain kind of patient he encountered in the course of his work in a psychiatric hospital in Augusta, Georgia. These people were from varied social and family backgrounds. Some were poor, but others were sons of Augusta’s most prosperous and respected families. Cleckley set about sharpening the vague construct of constitutional psychopathic inferiority, and distinguishing it from other forms of mental illness. He eventually isolated sixteen traits exhibited by patients he called “primary” psychopaths; these included being charming and intelligent, unreliable, dishonest, irresponsible, self-centered, emotionally shallow, and lacking in empathy and insight.

“Beauty and ugliness, except in a very superficial sense, goodness, evil, love, horror, and humor have no actual meaning, no power to move him,” Cleckley wrote of the psychopath in his 1941 book, “The Mask of Sanity,” which became the foundation of the modern science. The psychopath talks “entertainingly,” Cleckley explained, and is “brilliant and charming,” but nonetheless “carries disaster lightly in each hand.” Cleckley emphasized his subjects’ deceptive, predatory nature, writing that the psychopath is capable of “concealing behind a perfect mimicry of normal emotion, fine intelligence, and social responsibility a grossly disabled and irresponsible personality.” This mimicry allows psychopaths to function, and even thrive, in normal society. Indeed, as Cleckley also argued, the individualistic, winner-take-all aspect of American culture nurtures psychopathy.

The psychiatric profession wanted little to do with psychopathy, for several reasons. For one thing, it was thought to be incurable. Not only did the talking cure fail with psychopaths but several studies suggested that talk therapy made the condition worse, by enabling psychopaths to practice the art of manipulation. There were no valid instruments to measure the personality traits that were commonly associated with the condition; researchers could study only the psychopaths’ behavior, in most cases through their criminal records. Finally, the emphasis in the word “psychopath” on an internal sickness was at odds with liberal mid-century social thought, which tended to look for external causes of social deviancy; “sociopath,” coined in 1930 by the psychologist G. E. Partridge, became the preferred term. In 1958, the American Psychiatric Association used the term “sociopathic personality” to describe the disorder in its Diagnostic and Statistical Manual of Mental Disorders. In the 1968 edition, the condition was renamed “general antisocial personality disorder.”

Cleckley’s book fell out of favor, and Cleckley described himself late in life as “a voice crying in the wilderness.” When he died, in 1984, he was remembered mostly for his popular study of multiple-personality disorder, written with Corbett Thigpen, “The Three Faces of Eve.”

In 1960, Robert Hare took a job as the resident psychologist in a maximum-security prison about twenty miles outside Vancouver. On his first day, a tall, slim, dark-haired inmate came into his office and said, “Hey, Doc, how’s it going? Look, I’ve got a problem. I need your help.” Hare later wrote of this encounter, “The air around him seemed to buzz, and the eye contact he made with me was so direct and intense that I wondered if I had ever really looked anybody in the eye before.” Hare asked the inmate, whom he called Ray in his account, to tell him about his problem. “In response, he pulled out a knife and waved it in front of my nose, all the while smiling and maintaining that intense eye contact,” Hare wrote in his 1993 book, “Without Conscience: The Disturbing World of the Psychopaths Among Us.” Ray said he was planning to use the knife on another inmate, who was making overtures to his “ ‘protégé,’ a prison term for the more passive member of a homosexual pairing.” Ray never harmed Hare, but he successfully manipulated him throughout Hare’s eight months at the prison, and two and a half years later, after Hare had joined the faculty at the University of British Columbia, Ray, now paroled, tried to register there with a forged transcript.

Hare wasn’t familiar with the psychopathy literature when he was working at the prison. Later that year, he moved with his family to London, Ontario, where he pursued a Ph.D. at the University of Western Ontario. (When his brakes failed at the first steep hill on the trip east, he recalled that Ray had worked on his car in the prison garage.) His dissertation was on the effects of punishment on human learning and performance. One day in the library, he came across “The Mask of Sanity.” Reading Cleckley’s case histories put Hare in mind of Ray, and of other types he had encountered in the maximum-security prison. Were these men psychopaths? Over the next year, Hare read not only Cleckley but also the early literature Cleckley had synthesized. After receiving his doctorate, in 1963, and returning to Vancouver, he set about what would be his life’s work: the study of psychopathy, and the creation of the Psychopathy Checklist, the twenty-item diagnostic instrument that Kiehl is using at Western.

Thanks to the checklist, scientists working in different places can be confident that the subjects they are studying are taxonomically similar. The PCL also has a wide variety of forensic applications. It is employed throughout Canada in parole-board hearings and is gaining popularity in the U.S. In the thirty-seven states that allow the death penalty, a high psychopathy score is often used by prosecutors as an “aggravating factor” in the penalty phase of capital cases. Psychopathy scores have also been used in child-custody cases; a high score may result in one parent’s loss of custody. Hare’s influence on the field of psychopathy is profound. Today, Hare’s former students hold important administrative positions throughout the Canadian prison system, and are prominently represented in the next two generations of psychopathy researchers around the world.

One day when Kent Kiehl was eight years old, his father, Jeff, a copy editor at the Tacoma News Tribune, came home talking about a local man named Ted Bundy. “This was a guy who had grown up just down the street,” Kiehl told me, “and he had supposedly killed all these women.” Bundy, whose family moved to Tacoma when he was a child, is known to have sexually assaulted and murdered at least thirty women in the nineteen-seventies. But to outward appearances he was an exceptionally promising young man. He had received glowing letters of recommendation both from a psychology professor at the University of Washington, where he was an undergraduate (“he is exceedingly bright, personable, highly motivated, and conscientious”), and from the Republican governor of Washington, Dan Evans, for whom he worked. His good looks, charm, and verbal skills—qualities that made him such an effective predator—convinced many in the Tacoma community that he was innocent, up until the time he was convicted of murder and sentenced to death, in 1979. Bundy was executed in Florida in 1989.

Kiehl’s father was a sports fanatic, but he suffered from a form of muscular dystrophy that made it difficult for him to walk. (He died when Kiehl was twenty-two.) “My dad’s greatest wish for me was that I play college football,” Kiehl told me. Kent fantasized about playing for the University of Washington Huskies in front of fifty thousand fans. He ended up at the University of California at Davis, where he played in front of a couple of thousand. He enrolled as a pre-med, planning to be a doctor, but was having trouble balancing his academic and athletic commitments. When he blew out his knee at the start of his sophomore year, his choice became clear.

Kiehl had taken a course in psychology under Debra Long, a professor specializing in psycholinguistics, who was also his academic adviser, and he had enjoyed the work they did on emotional processing and the brain. Long asked what inspired him, Kiehl recalled. “I said I want to understand why people do bad things—how someone could get to be like Ted Bundy—and I want to study the brain. So she said, ‘You should combine those two things and study psychopaths.’ ” Kiehl’s first published paper was based on data collected from four hundred and eighty-five undergraduates at U.C. Davis, to whom he distributed a psychopathy checklist adapted from Hare, which they filled out themselves. He found that a high proportion of psychopathic traits were remarkably consistent with a pattern of disruptive behavior.

Kiehl’s early training in psychopathology coincided with the emergence of functional neuroimaging; Kiehl, a techie, quickly became adept at the computer skills necessary to run experiments. The earliest technique, measuring what’s known as an event-related potential, or E.R.P., charts the brain’s electrical activity, using an electrode-studded skullcap. In a landmark 1991 E.R.P. study conducted at a prison in Vancouver, Robert Hare and two graduate students showed that psychopaths process words like “hate” and “love” differently from the way normal people do. In another study, at the Bronx V.A. Medical Center, Hare, Joanne Intrator, and others found that psychopaths processed emotional words in a different part of the brain. Instead of showing activity in the limbic region, in the midbrain, which is the emotional-processing center, psychopaths showed activity only in the front of the brain, in the language center. Hare explained to me, “It was as if they could only understand emotions linguistically. They knew the words but not the music, as it were.”

Since then, cognitive neuroscience has come to be dominated by brain scans, although they are not as widely used in psychopathy research. So far, fMRI studies of psychopaths have only reinforced different models of psychopathy that were in place before fMRI became popular, theories that Kiehl studied while pursuing a doctorate in Hare’s lab at the University of British Columbia. Some scientists think that psychopaths suffer from an extreme and far-reaching attention deficit, which causes them temporarily to forget the moral and social consequences of certain antisocial actions. Joseph Newman, who chairs the psychology department at the University of Wisconsin at Madison, is the leading advocate of this theory. His model is based on traditional research methods, such as lab work using rats with brain lesions, and studies of humans using a well-known card-playing task, in which players gradually start to lose money; the players in the control group stopped as their earnings diminished, but the psychopaths could focus only on the outcome of the next card choice. Another hypothesis is that psychopaths lack fear of personal injury and, more important, moral fear—fear of punishment. David Lykken pioneered this theory in the nineteen-fifties, and it has been taken up by James Blair, Christopher Patrick, and others. The updated version of this model posits that psychopathy is a result of a dysfunction of the amygdala, the almond-shaped bundle of gray matter situated in the midbrain, which is another area instrumental in emotional processing.

When Kiehl arrived at the University of British Columbia, Hare sent him to a new maximum-security prison nearby. Kiehl recalled, “Bob said, ‘There is this new prison that just opened up, with the worst of the worst offenders—you can work there.’ So I submitted my proposal to do E.R.P. studies, and met with the prison psychiatrist, whose name was Johann Brink. He said, ‘I got you this office over here, here are your keys, and there are the inmates’—that was my training. They open the door, and there are fifty guys with tattoos looking at me. My first week, I interviewed a serial killer. He told me he had killed sixteen people and described how he had chopped them up—and I am sitting right across the table from the guy.” [#unhandled_cartoon]

Kiehl’s most memorable “perfect forty” on a PCL was an inmate I’ll call George. Kiehl was at the prison on the morning that George arrived. After being processed, George stripped naked and walked around the track outside the cellblock in the pouring rain. “I was new here,” he later explained to Kiehl, “and I wanted to establish right away that I am a crazy motherfucker so leave me alone.” George described his criminal past in full detail. He started out committing petty crimes as a child and by seventeen had been convicted of arson. In the early nineties, after serving eighteen months in prison for breaking and entering, he moved back in with his mother. One day, the two had a fight, and his mother picked up the phone to call the cops. “Man, can you believe the balls on that chick?” George asked Kiehl. He wrapped the phone cord around his mother’s neck and strangled her. “Then I threw her down the basement stairs, but I wasn’t sure she was dead, so I got a kitchen knife and stabbed her, and her body made these weird noises, I guess gas escaping, but I wasn’t sure, so I grabbed a big propane cannister and bashed her brains in.” Then he went out and partied for three days. “When I came back, that house stank—what a stench.” He cleaned the whole basement with bleach, wrapped up his mother’s body, put it in the trunk of her car, and drove to the mountains.

On the way, a policeman stopped him and asked if he was drunk. George said that he was just looking for a place to pee. So the cop pointed him toward a dirt road, and George went up there and left the body in the woods. It turned out he had dumped his mother in a mountain-pine-beetle-infested area, and a forestry crew came across the body a few days later. “Fucking pine beetles,” George remarked. At first, the police couldn’t find any evidence in the house, but George had neglected to clean the propane tank, which was splattered with his mother’s blood. When he was convicted of manslaughter and sentenced to life imprisonment, George just smiled.