Yamada et al. / Nature Communications

Functional magnetic resonance imaging indicates regions of the brain in which activity correlated with increased sympathy toward a convicted criminal facing sentencing (green) and an inclination to reduce punishment (red). Common areas were found in a region known as the precuneus (yellow).

Sympathetic jurors show a characteristic pattern of brain activity when they decide to be lenient on a criminal, and the strength of that pattern can vary from juror to juror, researchers report. Such findings aren't of merely academic interest: Someday, this kind of neuroscience could well have an impact on the legal process itself.

The latest study, published today in the journal Nature Communications, is consistent with earlier research into the neurological roots of moral cognition. It's also in line with the well-supported view that mitigating circumstances can make a big difference when people consider how other people should be punished. This study, led by Makiko Yamada of Japan's National Institute of Radiological Sciences, bridges the gap by investigating how the brain turns information about mitigating circumstances into a legal outcome.

"Jurors are really like workers," Caltech neuroscientist Colin Camerer, one of the study's co-authors, told me. "They're chosen and instructed to do things with quite a bit of restraint. It's like you're 'hiring' these workers to do something that's literally life and death. But almost nothing is known about whether they're using their tools — brain activity — in an appropriate way."

To study that question, Yamada and her colleagues recruited 26 subjects to read 32 real-life stories about Japanese defendants facing sentences for murder. Half of the stories presented scenarios that were likely to elicit sympathy — for example, tales of life in poverty, or victimization by domestic violence, or a struggle with disease. The other half were "no-sympathy" scenarios.

After reading the stories, the subjects were put into MRI scanners and asked to modify a 20-year sentence for each defendant, either up or down. Then they rated themselves on how much sympathy they felt for the defendants, and how empathetic they considered themselves to be. Readings from three of the subjects were not included in the analysis because they moved excessively during the brain scans, and one subject fell asleep during the experiment. That left 22 people in the study.

The MRI results showed that brain areas known as the dorsomedial prefrontal cortex, the precuneus and temporo-parietal junction were activated during a sympathetic response, and that the precuneus and anterior cingulate cortex were activated during sentencing. These regions are generally associated with mental deliberation and moral conflict, as well as emotional pain.

The jurors who were more inclined to be lenient tended to show more activity in the right middle insula, an area known to be involved in the mental perception of visceral states. Camerer said the most sympathetic subject, as measured by brain activity, exhibited a 28-year range in sentencing: six years for the most forgivable murderer, and 34 years for the least forgivable. The jurors with the least sympathetic brains kept their sentences in a 10-year range — for example, from 15 to 25 years.

"This kind of variability is similar [but] probably much less than that seen in experimental studies of translating moral judgments to large dollar sums in punitive-damage tort cases," Camerer said.

The researchers said the variability in brain function could become a factor in future court cases. "Not every brain maps sympathy to prison sentences in the same numerical way. ... Differences in these brain circuits between individuals suggest that differential juror responses might need to be considered unequally," they wrote.

Camerer said another intriguing issue has to do with how individual jurors activate or deactivate their emotions during a criminal case. When they deliberate over a defendant's guilt or innocence, jurors are expected to hold their emotions in check. But when jurors consider the sentence of a convicted criminal, their emotional response to mitigating circumstances should become part of the  process.

"I could imagine where, on appeal, the argument would be that some of these jurors didn't override their emotions adequately," Camerer said. "If that's permitted as a legal issue, how do you know? We say, don't ask the person, ask the brain."

In the future, will jurors find themselves subjected to brain scans before or after a trial? If you were a defense lawyer, wouldn't you want to know you had 12 sympathetic brains on your side? If you were a prosecutor, wouldn't you want to make sure that jurors didn't let their right middle insula unduly influence their right temporo-parietal junction? Or does all this sound way too Orwellian? Feel free to weigh in with your verdict below.

More on your moral brain:

• Behaving badly? Blame your brain
• How monkeys handle moral outrage
• Judges are less lenient when they're hungry
• Study: Morality can be altered with magnets
• Brain's 'cheat sheet' makes moral decisions easier
• Flash interactive: Road map to the brain
• Mental health coverage on msnbc.com
• Cosmic Log archive on the brain

In addition to Yamada and Camerer, the authors of "Neural Circuits in the Brain That Are Activated When Mitigating Criminal Sentences" include Saori Fujie, Motoichiro Kato, Tetsuya Matsuda, Harumasa Takano, Hiroshi Ito, Tetsuya Suhara and Hidehiko Takahashi.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.