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Imagine you are serving on a jury: the defendant is charged with murder, but he also suffers from a brain tumor that causes erratic behavior. Is he to be held responsible for the crime? Now imagine you are the judge: What should the defendant’s sentence be? Does the tumor count as a mitigating circumstance?

The assignment of responsibility and the choice of an appropriate punishment lie at the heart of our justice system. At the same time, these are cognitive processes like many others—reasoning, remembering, decision-making—and as such must originate in the brain. These two facts lead to the intriguing question: How does the brain enable judges, juries, and you and me to perform these tasks? What are the neural mechanisms that let you decide whether someone is guilty or innocent?

A recent study published in the December 2008 issue of the journal Neuron, by Joshua Buckholtz and his colleagues at Vanderbilt University tackles exactly this question. Until recently, such topics would have been out of the reach of cognitive neuroscience for lack of methods; today, functional magnetic resonance imaging (fMRI) allows researchers to watch the brain “in action” as normal human participants make decisions about responsibility and punishment. In the new study, Buckholtz and colleagues asked participants to read vignettes describing hypothetical crimes that a fictitious agent, “John,” commits against another person. The stories were divided into three conditions: in the first, the “responsibility” (R) condition, the perpetrator was fully responsible for the negative consequences of his action against the victim; for instance, John might have intentionally pushed his fiancée’s lover off a cliff. In the “diminished responsibility” (DR) condition, mitigating circumstances were present that reduced John’s responsibility; imagine that John committed the same crime, but suffered from a brain tumor.

And finally, the “no crime” (NC) condition consisted of stories that did not describe crimes. The participants had to make judgments regarding the degree of punishment that John should receive, on a scale from one to nine.

The authors then analyzed the brain activation linked to these judgments. To identify neural correlates of responsibility, they contrasted activation in the R and DR conditions. Note that the stories in two conditions are identical, except for the degree to which John is responsible for his crime. This contrast thus aims to identify which regions of the brain are involved in assigning responsibility for a crime, holding constant the crime itself. Buckholtz and colleagues found a peak of activation in the right doroslateral prefrontal cortex (rDLPFC), a brain region on the top surface of the right frontal lobe that is known to be involved in high-level cognitive processes such as reasoning and decision-making. In addition, this same region was more active when subjects thought a diminished-responsibility crime deserved punishment compared with when it did not.

Thus, these findings suggest that rDLPFC might be involved in assigning responsibility for crimes, or making judgments about appropriate punishments. Based on this finding, one might have expected that activation in rDLPFC should be higher when participants decide that very severe punishments are appropriate. Buckholtz and colleagues found no correlation between neural activation and punishment magnitude in rDLPFC, however, suggesting that this brain region does not directly underlie the decision on the amount of punishment. In contrast, there was some evidence that activation in emotion-related areas, such as the amygdala, correlates with the degree of punishment subjects assign to John: higher punishment scores were associated with higher activation in these regions during the decision period.