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The last prosecution witness to testify against the man who held Elizabeth Smart captive for nearly a year was a psychiatrist who said Brian David Mitchell met 17 of 20 criteria for diagnosis as a psychopath.

Evidence of psychopathy, characterized by a lack of empathy and ability to control impulses, is often seen as a "double-edged sword" in the criminal justice system, because judges could use it to justify shortening or lengthening a felon's time behind bars. In Mitchell's case, a federal judge ordered him to prison for the rest of his life for kidnapping and raping the Salt Lake City teenager.

A new survey of judges, conducted by an interdisciplinary team of University of Utah scholars, shows this sword mostly cuts toward harsher punishment, but a surprise finding suggests it depends on how much biology enters the courtroom.

Responses to Utah's questionnaire show judges are inclined to give shorter sentences when they are provided the genetic and neurological basis for a psychopathic defendant's behavior, even though such evidence demonstrates a strong likelihood he will re-offend, according to findings to be published Friday in Science.

The judges' rationales and hypothetical sentences were given in response to a courtroom scenario, based on an actual Georgia case, where a man was convicted in a vicious and unprovoked beating. Absent the diagnosis, they would punish such a crime with a sentence of 10.7 years, on average.

"Judges who got the diagnosis jumped it up to 14 years. That's a significant increase, but when they got the additional biological story about the causes of psychopathy they dropped it down to 13," said co-author James Tabery, an assistant professor of philosophy. "In some sense the sword cuts both ways."

Tabery's co-authors are social psychologist Lisa Aspinwall and legal scholar Teneille Brown. With the help of undergraduate Gabi Cash, the U. team recruited 181 state court judges from 19 states, including Utah, to participate in the anonymous on-line survey.

"We are discovering a lot more information about the biological cause of various mental illnesses," said Brown, a professor of law specializing in bioethics. "As we begin to see criminal offenders more as broken machines rather than moral monsters, the question is, is that going to affect the way judges sentence them? Our study answered a resounding yes."

But how sentences are affected appears to be "a mixed bag," and just two states — Maryland and Utah — accounted for much of the downward departures in sentencing, observers noted.

In punishing criminals, judges and prosecutors must balance the intangible values of justice and public safety. Scientific evidence can help achieve the balance, Salt Lake County District Attorney Sim Gill said.

"We will always struggle to find what that is as a society. I'm a fan of smart sentencing, a data-driven model. We should get as much information as we can," said Gill, who had no role in the U. study.

The U.S. legal system protects offenders whose mental illness prevents them from appreciating the wrongness of their actions. Psychopaths, on the other hand, do know the difference between right and wrong, but are they to blame for their disorder? There is evidence that psychopathy can result in part from parental failings, according to Vanderbilt University law professor Christopher Slobogin.

But many endured abusive or neglectful parenting and grew up to lead crime-free lives. Slobogin believes the Utah study illustrates the legal system's ambivalence toward psychopaths.

"Many have argued that a diagnosis should be the basis for an insanity defense and a finding of not guilty. Most judges resist that conclusion and think of psychopaths as the epitome of evil," said Slobogin, director of the school's criminal justice program and an affiliate professor of psychiatry. "Mental illness is often associated with dangerousness, and dangerousness drives a lot of sentencing."

In the fictional scenario conveyed to the judges in a 1,200-word vignette, 24-year-old Jonathan Donahue pistol-whipped a Burger King night manager in a botched robbery. The suspect was caught and bragged about his conduct to fellow jail inmates. The victim remained in a coma for 20 days and suffers a permanent neurological deficit that gives him trouble remembering words and controlling fine-motor movements. A jury convicted Donahue of aggravated battery, but acquitted him of armed robbery.

Psychologists diagnosed the felon with psychopathy; psychopaths are pathologically irresponsible, self-centered, dishonest and shallow.

At sentencing, the judge was informed of this diagnosis by either the prosecution, bent on showing that Donahue poses a future threat, or by defense attorneys, who want to demonstrate that Donahue isn't fully responsible for his actions. One half of surveyed judges received a biological explanation for Donahue's disorder from expert witnesses, while the other half were not given the explanation.

If the judge heard the biological story from the defense, they were 2.5 times more likely to cite mitigating factors when explaining their sentencing rationale, a finding that Brown found particularly intriguing.

"If you can see a reduction in sentences for the psychopath, who is cold, callous, likely to re-offend, you could anticipate reductions for people with other mental illnesses that carry less stigma, like bipolar, developmental disability or post-traumatic stress disorder," she said.

The question is likely to get more acute as an expanding scientific understanding of mental illness increasingly filters into the criminal courts.