Grady Nelson had his life spared by a brain scan. In January 2005, after he was released from a Florida prison, where he had served time for the rape of his step-daughter, he returned home to stab his wife 60 times, slash her throat and slam a butcher’s knife into her head. He also stabbed his two stepchildren. The kids survived the attack, but his wife did not. Five years later, a jury in Miami voted against the death sentence for Nelson’s crimes. Instead, they narrowly voted to sentence him to life in prison.

The defence attorneys argued that Nelson had major brain defects that could explain his behaviour. To show this, they submitted as evidence brain activity measurements from a method known as a quantitative EEG (Q-EEG). In a standard EEG, electrodes placed on the scalp measure the electrical activity of the brain. The Q-EEG is similar, except that a computer analyses the data and identifies brain regions of unusual activity. Lawyers had previously tried without success to present such data in court, but this was the first time in the US that a judge presiding over a major case had allowed it.

Clearly, it did the trick. In comments to the press, John Howard, an airport fleet services worker and member of the jury, said he had been about to vote for execution when the Q-EEG evidence had reversed his decision. “The technology really swayed me,” he told a Miami newspaper. “After seeing the brain scans, I was convinced this guy had some sort of brain problem.”

Neuroscientific evidence is increasingly being introduced into legal courts around the world, and novel brain-imaging techniques and interpretations are at the forefront. These approaches may be powerful new tools that help juries and judges determine the culpability of an accused and identify serial criminals. It’s also beginning to influence the way society thinks about sentencing and the treatment of criminals. But learning more about the neurobiological underpinnings of behaviour is also raising uncomfortable questions about free will. On top of all that there are scores of scientists who are critical of the use of such evidence in court.

‘High-stakes’

The tools of neuroscience offer tantalizing glimpses into the inner workings of the brain. For example, a common type of scan called functional magnetic resonance imaging (fMRI) can map brain activity by identifying regions of the brain that have higher levels of oxygen in the blood. Medical doctors use fMRI to determine the effects of head injuries or tumours for instance. But the technique also appeals to lawyers and legal scholars because it seems to show a shortcut to the truth. Humans may lie and cheat, but their brain scans reveal the facts, goes the reasoning.

For example, fMRI has been proposed as a way of telling whether someone seeking compensation for an injury actually experiences chronic pain. US lawyers have already tried to introduce this evidence into the courtroom, but it hasn’t managed to gain any traction yet. In 2008, Sean Mackey, a neurologist and director of Stanford University’s Division of Pain Management, dismissed one of the first attempts, in which fMRI evidence supposedly showed heightened activity in the “pain matrix” of a chemical burns victim seeking compensation from his employer.

These techniques hold promise, but it is dangerous to think neuroscience alone can convict a criminal right now. We like to think of the brain as a giant computer, wired like a well-defined circuit, with electrical signals zipping along neurons in a predictable way, but our brain’s software is complex and sometimes produces surprising results.

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.

Age gap

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.”

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