Derry Mainwaring-Knight holds a special place in the annals of con artistry. Fresh out of an English prison in 1984 after serving time for a rape conviction, Mainwaring-Knight convinced a church rector to enlist in his battle against the spread of devil worshippers. The articulate, ingratiating ex-convict offered to start an organization that would purchase and destroy artifacts linked to satanism and black magic.

Within a few months, the dazzled rector had emptied his own pockets and obtained money for Mainwaring-Knight's campaign from many devout church members, including prominent politicians and businesspeople. Mainwaring-Knight collected nearly $400,000 as well as a Rolls-Royce automobile. He spent the money on himself and his girlfriends.

In 1986, the satanic bubble burst. Mainwaring-Knight was hauled into court on 19 counts of fraud. Denying any wrongdoing, he argued that he had no need to trick people out of their money since he made a great living running a prostitution ring. After his conviction, his mother revealed that he had also duped her out of a large sum of cash.

Mainwaring-Knight wasn't just a con man. By all accounts, he had a psychopathic personality. Psychopaths lack a conscience and are incapable of experiencing empathy, guilt, or loyalty. Descriptions of psychopaths callously manipulating, intimidating, or harming others go back hundreds of years.

Psychiatrist Hervey Cleckley wrote The Mask of Sanity (1941, Mosby), a classic textbook on psychopathy. Cleckley portrayed psychopaths as superficially charming, intelligent people who don't feel deep emotions and lie about almost everything because they neither understand nor care about others.

Two conditions-sociopathy and antisocial personality disorder-often get confused with psychopathy. Sociopathy refers to criminal attitudes and behaviors viewed as normal in certain groups, such as street gangs. Sociopaths have a sense of right and wrong that is based on the values of their criminal group.

Antisocial personality disorder, an official psychiatric ailment, is a diagnosis applied to people who commit a broad range of aggressive and criminal acts. Some qualify as psychopaths, but many don't.

Although psychiatrists don't currently label psychopathy as a formal personality disorder, a wave of new research has yielded insights into how psychopaths think and suggested biological and temperamental roots of this condition.

These findings have not only sparked debate among researchers but also attracted widespread interest among lawyers and judges. Courts in the United States and other countries increasingly rely on psychopathy measures to make sentencing judgments. New studies suggest that being labeled a psychopath increases the likelihood that an offender will be locked up indefinitely or even executed.

The jury is still out on the psychopathy's usefulness in law. But the condition deserves intense scientific scrutiny, says psychologist Joseph P. Newman of the University of Wisconsin - Madison. Much new research appears in The Psychopath: Theory, Research, and Practice (2007, H. Hervé and J. Yuille, eds., Lawrence Erlbaum).

"Psychopathy rivals any mental disorder in its negative consequences for the person who has it," Newman contends.

Stone-cold killers

In 2002, psychologist Stephen Porter of Dalhousie University in Halifax, Nova Scotia, interviewed 125 men who were serving time in two Canadian prisons for murder. The 34 men with high scores on a psychopathy test gave him a surprise. Despite many investigators' assumption that psychopathic criminals lack self-control and often act impulsively, most of the psychopathic Canadian killers had planned the ruthless, cold-blooded murders that they had committed.

One psychopathic offender murdered his ex-girlfriend to stop her from interfering with his new relationship. Another psychopathic inmate arranged and committed the murder of his wife to cash in her life insurance policy.

In contrast, a large majority of the nonpsychopathic prisoners had killed someone in the heat of the moment or upon reaching an emotional breaking point.

Porter measured psychopathy using a tool called the Psychopathy Checklist-Revised (PCL-R). This clinical-rating scale, devised by psychologist Robert D. Hare of the University of British Columbia in Vancouver, has served as the gold standard of psychopathy tests for about 20 years.

In this approach, a psychologist or psychiatrist interviews a person and reviews his or her criminal record. The rater then judges whether any of 20 psychopathy-related traits applies to that person. These traits include being superficial, acting grandiosely, lying frequently, showing no remorse, lacking empathy, refusing to accept responsibility for misdeeds, behaving impulsively, and having committed many crimes.

PCL-R scores range from 0 to 40. Most people in the general population score no more than 5 on this test. Hare estimates that 1 percent scores at least 30. Researchers typically use scores of 30 and above to indicate psychopathy, as Porter did.

The average PCL-R scores for men and women in prisons are 22 and 19, respectively. About 15 percent of male offenders and 10 percent of female offenders score 30 or more.

Among psychopaths who kill, a thrill-seeking temperament and sadistic interests form a toxic brew, Porter says. Famous sexual murderers such as Ted Bundy and Albert DeSalvo, who was known as the Boston Strangler, targeted a wide array of victims to fend off boredom, he says.

Psychopaths plan murders with special care because the stakes are so high, Porter argues. Even their impulsive, nonhomicidal offenses, such as robberies and assaults, reflect not an inability to control behavior so much as a lack of interest in controlling it, he suggests.

In interviews with 50 additional men imprisoned for murder, Porter found that psychopaths not only committed the bulk of premeditated homicides but also tried to explain them away as being provoked by others. The men often failed to mention incriminating details contained in police records.

Psychopathic murderers contacted by Porter preferred "hands-on" weapons, such as knives, rather than guns, and they often applied torture, mutilation, or other forms of extreme violence to their victims.

Not all psychopaths resort to violence, however. Highly intelligent people with psychopathic personalities find fertile, nonviolent opportunities in conning and manipulating others, in Porter's view.

There's currently a bull market in corporate psychopaths, according to psychologist Paul Babiak of HRBackOffice, an industrial-consulting firm in Hopewell Junction, N.Y. Organizations undergoing major changes, such as downsizing or mergers, provide a chaotic atmosphere that savvy psychopaths exploit, Babiak holds. They cozy up to a firm's power brokers, manipulate coworkers, and intimidate underlings on their way up the corporate ladder, stealing everything possible along the way.

In today's rapidly changing business world, "increased corporate rewards for risk taking and nonconformity can offer the psychopath faster career movement than before," Babiak says.

No fear

While looming as public threats, psychopaths also stand as scientific mysteries.

Evolutionary psychologists regard psychopathy as an inherited personality style that has evolved because glib, deceitful individuals-as a minority within a larger population of trusting folk-often reproduce with much success.

Other investigators, such as neuroscientist R.J.R. Blair of the National Institute of Mental Health (NIMH) in Bethesda, Md., regard psychopathy as the result of a still-unspecified genetic disorder. The inherited defect interferes with the workings of the brain's emotion system, which is centered in the amygdala, a structure especially concerned with perceiving dangerous situations.

People with psychopathy don't modify behaviors for which they're punished and don't learn to avoid actions that harm others, Blair proposes in the September Cognition. As a result, they fail to develop a moral sense, in his view.

Blair's theory fits with previous observations that psychopaths have difficulty learning to avoid punishments, show weak physiological responses to threats, and don't often recognize sadness or fear in others.

Newman takes a different approach. He views psychopathic personalities as the product of an attention deficit. Psychopaths focus well on their explicit goals but ignore incidental information that provides perspective and guides behavior, Newman holds. Most other people, as they take action, unconsciously consult such information, for instance, rules of conduct in social settings and nonverbal signs of discomfort in those around them.

Furthermore, because psychopaths ignore peripheral information that provides context and meaning to daily situations, Newman argues, they don't appreciate music, art, or other endeavors that require depth of feeling.

In one set of studies that Newman directed, psychopathic and nonpsychopathic prisoners viewed a series of mislabeled images, such as a drawing of a pig along with the word dog. Nonpsychopathic participants found these images confusing, taking considerable time to name the objects and read the labels. Psychopathic volunteers completed these tasks much more quickly and barely noticed the discrepancies between images and labels.

Newman suspects that this narrowing of attention in psychopaths jams their mental radar for discerning other people's emotional reactions. In a study slated to appear in the August sychological Science, he and his coworkers report examining people who had either a low or a high level of anxiety but weren't psychopaths. Study participants who exhibited an almost anxietyfree personality, which is one characteristic of psychopaths, showed no startle response-as measured by pronounced eye-blinks-to sudden noises that clearly surprised high-anxiety volunteers. In other words, the Wisconsin psychologist concludes, psychopaths and others who rarely or never feel anxious simply don't notice disturbing events or potential dangers in their surroundings and thus don't stop to consider them.

Unburdened by anxiety, "psychopaths respond to whims," Newman says. "This condition gets superimposed on a person's other characteristics, so a psychopath who is predisposed to violence will be violent on a whim."

Callous kids

Psychologist Paul J. Frick of the University of New Orleans recalls a boy who was recently referred to the mental health clinic where Frick works. The 10-year-old had trapped a cat and killed it by slowly slicing it with a knife. The youngster calmly explained to Frick that he wanted to see how much he could cut the animal before it died.

"He wasn't upset by the incident at all," Frick says. "He was a bit annoyed about being brought to me, though."

The boy might be a future surgeon, but it's more likely that he's headed for psychopathic pursuits, in Frick's view. The child's callousness and lack of emotion, seen in a small proportion of children and teenagers, probably foreshadow serious behavior problems, and perhaps even a psychopathic personality, in adulthood.

In such children, Frick finds a lack of guilt, an unemotional demeanor, llittle concern about others' feelings or about school, a refusal to keep promises, and difficulty forming lasting friendships.

Although about 1 in 100 kids displays such traits, nobody knows how many of them will grow up to become psychopaths. In a 2003 study, Frick's team tracked 98 children who began the study in grades 3, 4, 6, or 7. Children initially identified by their parents as callous and unemotional tended to continue to be regarded that way over the 4 years of observation.

Only a few children who started out as extremely callous and unfeeling became less so during the study. Environmental factors, such as high-quality parenting and living in a wealthy family, appeared to stimulate such improvement, Frick says.

A 2005 study of 3,682 identical and fraternal twin pairs at age 7, coauthored by the NIMH's Blair, identified a strong genetic contribution to callous, unemotional personality. Frick suspects that children who fit this description are born with a dispassionate temperament and tend not to notice or react to others' distress or to signs of danger.

"Having this temperament makes it difficult, but not impossible, to develop empathy and guilt," Frick proposes.

He and his coworkers find that for children as young as age 3 who display callousness and lack of emotion, parents and teachers report frequent fights and other serious behavior problems.

Researchers have had little success in devising effective treatment programs for kids who seem to be on the road to psychopathy. However, psychologists David J. Hawes and Mark R. Dadds, both of the University of New South Wales in Sydney, Australia, last year described one promising approach. Parents of 56 boys, ages 4 to 9, referred to a clinic for behavior problems received a 10-week training course. Hawes and Dadds found that the more callous and unemotional a boy was, the more likely he was to respond well to rewards and encouragement from parents for good behavior, but not to punishments for misbehavior.

Courting trouble

Psychologist John F. Edens of Southern Methodist University in Dallas has watched with concern over the past decade as many of his colleagues have testified in court with what he calls "overly zealous and empirically questionable conclusions" about psychopathy.

Because psychopaths are presumed to be irredeemably dangerous and untreatable, courts increasingly lean on psychological assessments to guide decisions about whether to confine offenders for indeterminate periods or even to execute them.

In the February Professional Psychology: Research and Practice, Edens outlined the limitations of current knowledge about psychopathy.

Consider that psychologists working for the prosecution and the defense in criminal cases often generate disparate psychopathy scores for the same defendants, Edens says. To make matters more confusing, the incriminating score of 30 or more on the PCL-R hasn't been rigorously linked to psychopathy.

Edens recommends that courtroom psychologists report a confidence range for each psychopathy score assigned to a defendant. Scores of individuals given the test under different conditions typically span 14 points, he says.

Moreover, although a high psychopathy score offers the strongest single indication of whether a prisoner will be violent in the near future, it doesn't doom an offender to a life of mayhem, Edens holds. For example, an inmate scoring above 30 on the PCL-R may resort to violence in the weeks after release from prison, he notes, but that score doesn't imply that he will be violent for decades and therefore requires indefinite imprisonment.

It might be best to stop using the term psychopath in court, Edens asserts, because the word carries a stigma that unduly sways juries. In studies with college students, he finds that they associate word psychopath with especially brutal, infamous murderers. In simulated court cases, participants acting as jurors assigned the death penalty to two-thirds of murderers portrayed in expert testimony as psychopaths, as opposed to roughly one-third of murderers described as either psychotic or free of mental disorders.

"Evil and psychopathy are overlapping ideas," Edens says. "Many people do evil things without being psychopathic."

Others, such as Derry Mainwaring-Knight, revel in psychopathy, doing evil by claiming to fight evil.