How far should neuroscience evidence go in court trials?

Those are some of the questions that were addressed during a presentation and mock trial Feb. 20 at the annual meeting of the American Association for the Advancement of Science (AAAS) in San Diego.

Hank Greely, a Stanford law professor and expert on the legal, ethical and social issues surrounding the biosciences, took on the role of prosecutor during a presentation titled "The Brain on Trial: Neuroscience Evidence in the Courtroom."

"The prosecutor's typical position is that brain scan evidence shouldn't be used because they say it's not scientifically useful," Greely said. "They say it will confuse the jury, that it's not relevant, that the technology isn't good enough yet. But most of all, they'll say that's fine that you found this person has an abnormal brain -- but how many other people have similar abnormalities and don't commit crimes? The answer will be: quite a few."

With no hard-and-fast rules on whether neuroscience evidence should be allowed in state and federal courts, Greely is studying criminal cases in California that have featured brain scan images to help prove guilt or maintain innocence.

He's so far found that defense attorneys are more likely than prosecutors to try using neuroscience evidence, but he cautions that the tool is a double-edged sword.

While an MRI result showing a deformed or malfunctioning brain could conjure empathy and a finding of innocence, it could also lead jurors and judges to opt for convictions and long sentences based on the assumption that such a damaged mind will only convince the person using it to offend again.

"Neuroscience evidence will probably mostly be used alongside behavioral evidence," Greely said. "There will always be behavioral evidence to show a defendant was crazy as a loon. Neuroscience will be able to further hammer home the idea that the person truly has a problem."