Should a diagnosis of psychopathy be reason to sentence a criminal lightly or extend time behind bars?

According to new research, judges are likely to add prison time to the sentences of psychopaths, who are known for a lack of empathy and poor impulse control. However, the tougher sentence is not quite as severe when the judges are given a biological explanation for the disorder.

In other words, the researchers write today (Aug. 16) in the journal Science, a psychopathy diagnosis is a "doubled-edged sword" — seen by the criminal justice system as both an aggravating and mitigating factor.

"It's interesting to us that the same piece of information could have very different implications for how we treat the defendant and judge what he would do in the future," study researcher Lisa Aspinwall, a social psychologist at the University of Utah, told LiveScience.

Psychopathy is a personality disorder that is resistant to treatment. Despite stereotypes, not all psychopaths are criminals or violent, though they do display a lack of emotion, empathy, guilt and impulse control. Psychopaths convicted of a crime are more likely than other convicts to become repeat offenders. [ Top 10 Controversial Psychiatric Disorders ]

The diagnosis, and the science behind it, is increasingly presented in courtrooms, mostly as a defense tactic to argue that the defendant is not as culpable for his or her crimes and should be spared the death penalty, said Teneille Brown, a law professor at the University of Utah and a co-author of the new study. But psychopathy could just as easily be used by the prosecution to suggest that the defendant is a callous monster who will strike again, Brown told LiveScience.

A number of recent trials have featured genetic testing for disruptions in Monoamine oxidase A, or MAO-A, in the defense's reasoning for a defendant's psychopathy. MAO-A is an enzyme associated with aggression in mice and, in some studies, humans. And defense attorneys used brain imaging in one high-profile case in 2009  to argue that their client, found guilty of multiple rapes and murders, should be spared the death penalty because he was a psychopath. (The man, Brian Dugan, was sentenced to death, but that sentence was commuted to life in prison after Illinois abolished the death penalty in 2011.)

So many factors are at play in criminal trials that it's difficult to gauge which ones influence the length of a convict's sentence. So Aspinwall, Brown and their University of Utah colleague James Tabery, a professor of philosophy, presented a fictionalized account of a crime by a remorseless criminal and recruited 181 state trial court judges to read it and pronounce sentence.

In the story, a man named Jonathan Donahue attempts to rob a fast-food restaurant. When the manager refuses to cooperate, Donahue hits him with his gun, causing moderate, permanent brain damage.

As presented in the vignette, the convict is highly unsympathetic, Aspinwall said.

"It's an unprovoked crime," she told LiveScience. "He brags about it. He gets a crown tattooed on his back because the crime was committed at a Burger King. There's nothing to like about this guy."

Donahue is convicted of aggravated battery and is now facing sentencing.

 (The case is based on a real one, Mobley v. State, in which the defendant sought to be tested for MAO-A as part of his defense.)

All of the judges were told Donahue is a psychopath. Half were also told about the biological and neurological basis of psychopathy. In half of those cases, the biological information comes from the defense as mitigating evidence. In the other half, the biological information comes from the prosecution and is used to argue that the man would surely reoffend. [ The History of Human Aggression ]

First, the researchers asked the judges their typical sentence for aggravated battery. The answer, on average, was nine years. Then the judges were asked to give their sentence in this particular case.

The judges who heard only that the convict was a psychopath opted to put him away for about 14 years on average. But those given a biological explanation for psychopathy handed down an average sentence of about 13 years.

"On the one hand, that 13 years is significantly higher than the nine they would normally sentence the individual to," Tabery told LiveScience. "On the other hand, it was significantly less than they sentenced the diagnosed psychopath to" without the biological explanation.

There was no significant difference in sentencing based on whether the prosecution or the defense made the biological case for the psychopathy diagnosis, though judges were twice as likely to mention biology as a mitigating factor in their sentencing when the defense brought it up.

Although brain disorders like psychopathy are, by definition, biologically based (as is any human behavior), people tend not to think of them that way spontaneously, Aspinwall said. Studies have found again and again that people are more willing to help others whose problems seem outside of their control, she said. Being reminded of the biological basis of psychopathy seemed to do that for judges.

"They judged the person as having reduced control over his impulses but also expressed concern that he would be more likely to reoffend," Aspinwall said. In open-ended explanations offered by the judges, they frequently said they were weighing these two opposing factors.

The researcher didn't get into the thorny moral question of whether judges and juries should see psychopathy as a warning of future danger or a reason for mercy. Tabery said considering the biological basis of a mental disorder as a mitigating factor could be  a slippery slope given that uncontrollable factors including genes and environment contribute to any given behavior.

The researchers hope to extend the research into other mental disorders to find out, for example, whether a convict with post-traumatic stress disorder would be seen differently than a convict with psychopathy would.

As more biological markers for mental disorders are discovered, these questions are likely to get more pressing, Brown said. 

"To the question of, 'Does this biological evidence influence judges' sentencing?' the answer is absolutely yes," Brown said. "Then that just leads to the interesting question of: Should it?"

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