In 2008, in El Cajon, California, 30-year-old John Nicholas Gunther bludgeoned his mother to death with a metal pipe, and then stole $1378 in cash, her credit cards, a DVD/VCR player, and some prescription painkillers. At trial, Gunther admitted to the killing, but argued that his conviction should be reduced to second-degree murder because he had not acted with premeditation. A clinical psychologist and neuropsychologist testified that two previous head traumas—one the result of an assault, the other from a drug overdose—had damaged his brain’s frontal lobes, potentially reducing Gunther’s ability to plan the murder, and causing him to act impulsively. The jury didn’t buy Gunther’s defense, however; based on other evidence, such as the fact that Gunther had previously talked about killing his mother with friends, the court concluded that he was guilty of first-degree murder, and gave him a 25-years-to-life prison sentence.
Gunther’s case represents a growing trend, a new analysis suggests. Between 2005 and 2012, more than 1585 U.S. published judicial opinions describe the use of neurobiological evidence by criminal defendants to shore up their defense, according to a study published last week in the Journal of Law and the Biosciences by legal scholar Nita Farahany of Duke University in Durham, North Carolina, and colleagues. In 2012 alone, for example, more than 250 opinions cited defendants’ arguments that their “brains made them do it”—more than double the number of similar claims made in 2007.
Conventional legal wisdom holds that neurobiological evidence, such as functional magnetic resonance imaging scans, evaluations of cognitive impairment, or tests for genetic variants that might affect mental function, is primarily used to argue against the use of the death penalty in capital cases, Farahany says. By combing through tens of thousands of published judicial opinions with the help of 17 law students and three undergraduates, using search terms such as “brain disorder,” “biological,” and “CT scan,” however, Farahany’s team found that use of such evidence is growing for a wide range of violations including robbery, fraud, and drug possession.
But bringing in neuroscientists doesn’t necessarily get defendants anywhere. Most judges and juries are not persuaded by neurobiological evidence used to argue that a defendant was not fully responsible for his or her actions, as in Gunther’s case, the analysis found. That’s partly because of legal standards for insanity—which, in many states, require that a defendant have a “complete lack of understanding” of the difference between right and wrong. Even if a person is mentally ill, that usually results in “some degree, rather than complete impairment of understanding,” Farahany and colleagues write. In general, neuroscientific insights about human behavior and decision-making often can’t say much about why a particular individual behaved “voluntarily” or with “intention” and is therefore legally guilty, Farahany says.
There are, however, some instances in which evidence from neurobiology is proving unexpectedly valuable, Farahany says. Her analysis found that the second most common use of neurobiological evidence in court is to challenge defendants’ competency to plead guilty, confess, stand trial, or be sentenced. In one such case, Farahany says, the defense established that a man was mentally incompetent to be sentenced after he had already been found guilty of fraud, she says. As a result, “he spent the rest of his life in a nursing home, rather than being sentenced.”
Overall, the new study suggests that neurobiological evidence has improved the U.S. criminal justice system “through better determinations of competence and considerations about the role of punishment,” says Judy Illes, a neuroscientist at the University of British Columbia, Vancouver, in Canada. That is not Farahany’s interpretation, however. With a few notable exceptions, use of neurobiological evidence in courtrooms “continues to be haphazard, ad hoc, and often ill conceived,” she and her colleagues write. Lawyers rarely heed scientists’ cautions “that the neurobiological evidence at issue is weak, particularly for making claims about individuals rather than studying between-group differences,” they add.
“What we are faced with is trying to ‘fit’ the round peg of neuroscientific techniques and information into the square hole of the law,” says James Giordano, a neuroethicist at Georgetown University in Washington, D.C. What is needed, Giordano says, is a body of "neuroscience for law" that meets the needs of the courts, “while recognizing the actual capabilities and limitations of current neuroscientific tools.” Illes agrees, saying that “better engagement of neuroscientists in the conversation on these matters should not merely be a call; it is an imperative.”