A study published in the journal Science found that if judges were told a criminal was a psychopath, they considered it an aggravating factor. But if they also heard biological explanations for the disorder, they gave shorter sentences.

Researchers from the University of Utah who conducted the study said the findings were surprising and worrying, and external experts said they had problematic implications for how brain science might affect criminal justice in future.

"In the coming years, we are likely to find out about all kinds of biological causes of criminal behavior, so the question is, why does the law care if most behavior is biologically caused?" said Teneille Brown, an associate professor at the university's college of law.

Seena Fazel, a clinical senior lecturer in forensic psychiatry at Britain's Oxford University, noted there are already known biological bases for many disorders criminals suffer from, including drug abuse, alcoholism and antisocial personality disorder.

"So if psychopathy reduces your sentence because it has a biological basis, why shouldn't these other more common conditions also result in reduced sentences?" he said.

MITIGATING FACTOR

The Utah team carried out a survey of 181 judges in 19 U.S. states who were given a hypothetical case of aggravated battery to consider. They found that when judges were given a "biomechanical" explanation for a criminal's psychopathy, they saw this as a mitigating factor and reduced the sentence, on average by about a year.

"Judges that heard information about the genetic and developmental causes of psychopathy gave significantly shorter sentences," said Lisa Aspinwall, a psychologist who worked with Brown on the study.

Brown said what was "so striking" about these results in psychopaths was that it was likely there may be even sharper reductions in sentences for defendants with more sympathetic diagnoses such as mental retardation or schizophrenia.

Several studies in recent years have found that psychopaths who have committed serious crimes like murder and rape have faulty connections in their brains which show up on functional magnetic resonance imaging (fMRI) scans.

These and other advances in neuroscience have led some to worry that such scientific evidence may be used increasingly in court to explain criminal actions or argue mitigating circumstances.

BREIVIK'S SENTENCE

In a report earlier this year by Britain's national academy of science, the Royal Society, leading scientists and lawyers advised extreme caution on the use of brain scans and genetic data in court.

Interest in the minds of violent criminals has also been sharpened by the trial of Anders Behring Breivik, who massacred 77 people in Norway in July 2011, in which the prosecution argued Breivik was insane. If judges agree, he will go to a mental institution instead of a prison.

The verdict in his trial is due to be announced on Aug 24.

Brown and her colleagues said their study raised ethical questions: Whether it was right to reduce a criminal's sentence because defective genes or brain function meant he had less self-control and ability to tell right from wrong. Or whether such evidence should be an argument for a harsher sentence because the criminal may be more likely to reoffend.

The study's results showed that judges who were given a biological explanation for a convict's psychopathy imposed sentences averaging 12.83 years - about a year less than the 13.93-year average imposed by judges who were only told the defendant was a psychopath.

Even though a year is a relatively small reduction, Brown said she was "amazed the sentence was reduced at all given that we're dealing with psychopaths, who are very unsympathetic".

Fazel said he would be very concerned about the reliability in legal terms of the idea of psychopathy. The condition is not currently in diagnostic classification systems, he said, and two separate experts may come to very different conclusions about whether or a not a criminal is psychopathic.

"If a condition or disorder is going to be used to mitigate sentence length, it should have high levels of reliability, as found with severe mental illnesses like schizophrenia," he said.

(Editing by Ben Hirschler and Pravin Char)

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