What’s more, our brains are highly susceptible to the power of suggestion. Imagining past or future events can affect how we feel right now. During an fMRI scan parts of the brain may light up in response to thoughts as well as action or recalled actions, which means that simply remembering a bout of severe pain could elicit the same effect as feeling the pain. We’re also highly capable of self-deception. “A parent who has shaken and killed their baby may be so horrified by their actions that they convince themselves they were innocent,” says Nicholas Mackintosh, at the department of experimental psychology at the University of Cambridge and chair of a 2011 UK Royal Society report on neuroscience and the law. This can skew fMRI-type lie-detection techniques, which can’t distinguish between people who are telling the truth and those who are guilty but believe they are telling the truth.
Though some are eager to embrace the new techniques in court, others in the legal system are highly skeptical. “The error rates are not very well known,” says Owen Jones, director of the MacArthur Foundation Research Network on Law and Neuroscience, at Vanderbilt University Law School, in Nashville.
Joshua Greene, a psychologist at Harvard University in Boston, is also troubled by the use of fMRI in court to prove guilt. “fMRI differentiates a group who deceived a lot from a group who didn’t, but it’s terrible at identifying whether a person is lying on a single question,” he says. Crucially, says Jones, lab studies of neuroscientific techniques don’t account for real-world situations. “We don’t have good data on how these techniques would operate on people under high-stakes stress, being accused of something that could put them in jail or even executed,” he says.
Neuroscience may have helped Grady escape lethal injection, but most attempts to influence the outcome of a case with neuroscientific evidence have been unsuccessful. “It becomes difficult to pinpoint the relationship between the exact act and the physiological condition,” says Jones. “After all, we don’t know the base rate in society for people who walk around with impairments in their brain who don’t throw their wives out of the window of a tall building.”
But, studies are beginning to highlight potential differences in the brain structures of some criminals. For example, research shows that psychopaths and murderers have physical abnormalities in the amygdala, a part of the brain that mediates feelings like fear and anxiety, and in the prefrontal cortex, which regulates emotions like empathy and guilt. In one small study, Adrian Raine, a criminal psychologist at University of Pennsylvania in the US, showed that the amygdalae of psychopaths are 18% smaller, on average, than non-psychopaths. It is a finding backed up by several other studies.
Research also seems to suggest that some of these differences may start in early childhood. In another study, Raine and his colleagues investigated if children who weren’t afraid of punishment would be predisposed to criminal activity as adults. Other studies have shown that people with underperforming amygdalae don’t fear punishment as much as the average person. To look for answers, Raine used data from a large study of fear conditioning in three year olds born in 1969 and 1970. When the group reached the age of 23, the scientists searched the court records for serious offences, and found that 137 of them (out of 1,795 in total) had been listed as criminal offenders. When Raine compared the offenders with a group of non-offenders, he found that the kids with poor fear conditioning were much more likely to become criminals as adults.