A justice system hardwired into the brain ensures that wrongdoers get the punishment they deserve, research has shown.

It tempers an emotional response to gruesome details of a crime with a cool assessment of intent, brain scan studies revealed.

Only if an act is premeditated are emotions allowed to sway the severity of a ‘sentence’, scientists found.

The research, which involved volunteers reading graphic or factual crime scenarios, confirmed what judges already know – that emotional responses have a big impact on our desire to punish.

For that reason gratuitously graphic photos or other evidence may be excluded from a trial if it is not relevant and potentially prejudicial.

US lead researcher Rene Marois, from Vanderbilt University in Nashville, said: “A fundamental aspect of the human experience is the desire to punish harmful acts, even when the victim is a perfect stranger. Equally important, however, is our ability to put the brakes on this impulse when we realise the harm was done unintentionally.

“This study helps us begin to elucidate the neural circuitry that permits this type of regulation.”

In the tests, functional magnetic resonance imaging (fMRI) was used to scan the brains of 20 male and 10 female volunteers while they read a series of brief stories depicting the actions of a character called John.

John caused four different levels of harm to two other protagonists, Steve or Mary – death, maiming, physical assault or property damage.

In half, the harm was identified as deliberate and in the other half unintentional.

Two versions of each scenario were created, one with a straightforward factual description and the other spiced up with graphic details.

When the harm is not intended, we don’t simply shunt aside the emotional impulse to punish

For instance, in one mountain climbing scenario John cuts Steve’s rope. The factual description relates how Steve falls 100 feet and dies from his injuries shortly after impact.

The graphic scenario tells how Steve ‘plummets’ down the rock face and breaks nearly every bone in his body. His screams are muffled by “thick, foamy blood flowing from his mouth”.

After reading each scenario the volunteers were asked to rate the punishment deserved.

The study, reported in the journal Nature Neuroscience, showed that when harm was described in a graphic or lurid fashion, people set the punishment level higher.

This only happened when the harm was intentional. When it was unintentional, the type of description made no difference.

“What we’ve shown is that manipulations of gruesome language leads to harsher punishment, but only in cases where the harm was intentional; language had no effect when the harm was caused unintentionally,” said co-author Michael Treadway, from Harvard Medical School in Cambridge, the US.

The fact that graphic language alone caused people to ratchet up the severity of punishments suggests that real-life photos, videos and other crime scene material might have an even stronger impact, said the researchers.

“Although the underlying scientific basis of this effect wasn’t known until now, the legal system recognised it a long time ago and made provisions to counteract it,” Treadway added.

“Judges are permitted to exclude relevant evidence from a trial if they decide that its probative value is substantially outweighed by its prejudicial nature.”

The scans showed that the amygdala, an almond-shaped region of the brain that plays a key role in emotion and fear processing, responded most strongly to the graphic scenarios.

But again, this effect was only seen when harm was done intentionally. When actions were deliberate the amygdala also communicated strongly with the dorsolateral pre-frontal cortex, a brain region critical to punishment decision-making.

Unintentional harm triggered a different network involving in decoding the mental states of other people. It became more active and suppressed the amygdala’s responses to graphic language.

“This is basically a reassuring finding,” said Marois. “It indicates that, when the harm is not intended, we don’t simply shunt aside the emotional impulse to punish.

“Instead, it appears that the brain down-regulates the impulse so we don’t feel it as strongly. That is preferable because the urge to punish is less likely to resurface at a future date.”