Editorial: Can brain scanners judge age of criminal liability?
A FEW years ago, 17-year-old Christopher Simmons was convicted of breaking into Shirley Crook鈥檚 house in St Louis, Missouri, tying her up and throwing her off a bridge. The evidence was overwhelming and Simmons confessed to the murder. When the jury recommended a death sentence, Simmons鈥檚 defence referred to scientific papers that suggested a the brain of a typical 17-year-old was not yet fully mature. Not only did Simmons escape the death penalty, the US Supreme Court changed the law so that only those over 18 can face death row.
Now neuroscientists claim we are closer to being able to estimate brain maturity using brain scans, which might prompt lawyers to offer a defence of immaturity based on an accused individual鈥檚 own brain scan.
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Nico Dosenbach鈥檚 team at Washington University School of Medicine in St Louis reckon they can predict how old a person is using a 5-minute brain scan.
The idea is based on the fact that brain structure changes as we age. For example, the brain鈥檚 grey matter peaks in childhood and is then pruned throughout adolescence. White matter, which forms the connections between brain regions, steadily increases, eventually levelling off (see diagram).
As we age, parts of the brain that are further apart are better connected and better able to communicate. 鈥淭he short connections get weaker and the longer ones get stronger,鈥� says Dosenbach.
His team used functional MRI to measure brain activity in 195 people aged between 7 and 30. 鈥淚f you see the same regions of the brain are active in sync, it鈥檚 evidence that they鈥檙e connected.鈥�
The group tracked those changes, which levelled off at around age 20. After feeding the information into a computer, they were able to predict the chronological age of other individuals based on a 5-minute session in the scanner.
鈥淭he machine can predict with 92 per cent accuracy whether a brain belongs to an adult (over 25) or a child,鈥� says Dosenbach (Science, ).
鈥淭he findings are going to make a big splash,鈥� says at the National Institute of Mental Health in Bethesda, Maryland. He points out that as well as helping to diagnose developmental disorders, such a scan could, in theory, be used to get a defendant off the hook based on the immaturity of their brain. Other aspects of brain development, such as understanding how brain connectivity typically changes with age, would also need to be taken into account to determine whether the maturity of an individual鈥檚 brain matches their actual age.
The age of criminal responsibility is already a contentious issue. In the US alone the age at which a person can legally be tried as an adult ranges from 7 to 18, depending on the state. In Scotland the age is being raised from 8 to 12, and in England a group of neuroscientists, psychiatrists and legal professionals are campaigning for a rise in age of responsibility, saying 10 is too low.
鈥淲e know the brain of a 10-year-old is extremely immature compared to an adult, both structurally and functionally,鈥� says Sarah-Jayne Blakemore at University College London, a signatory on a letter being prepared for the Law Commission of England. 鈥淥ur argument is that when people make a legal ruling [for children of this age], they should take into account that the brain is very immature.鈥�
Others argue that by the age of 10 a child understands the difference between right and wrong. 鈥淎 child at the age of 2 knows not to bite someone,鈥� says at the University of California, Los Angeles.
鈥淚t鈥檚 not to do with knowing the difference between right and wrong,鈥� counters Blakemore. The group argue that because the prefrontal cortex (PFC) is the last part of the brain to mature, a child is unable to understand the long-term consequences of their actions. A child may be unable to suppress potentially dangerous behaviour, and make bad decisions. 鈥淚t鈥檚 the PFC that stops you doing 200 miles an hour down the motorway,鈥� says Blakemore.
鈥淭he ability to do these things changes a lot during adolescence, and courts need to take this into account when deciding punishments,鈥� agrees Giedd, who is not part of the campaign. A brain scan could aid this decision.
So how do you decide what the age of criminal responsibility should be? Although Blakemore鈥檚 co-signatories make the case for raising the age from 10 they say further research will be needed to determine what it should be.
鈥淭he idea that the brain suddenly becomes adult is misleading because the brain is plastic and continues to change throughout adulthood,鈥� says Blakemore. Dosenbach鈥檚 study suggests the major changes to the brain鈥檚 structure don鈥檛 start to level off until around age 20. But is this a realistic age of criminal responsibility?
鈥淚f the standard is 鈥榓s good as it鈥檚 going to get鈥� then the age of criminal responsibility should be set around 20,鈥� Giedd says. 鈥淏ut it鈥檚 a matter of whether we need the brain to be at its completely mature state.鈥�
鈥淚f brain maturity is set at 鈥榓s good as it鈥檚 going to get鈥� then the age of criminal responsibility should be 20鈥�
Another option is to have a sliding scale of responsibility for adolescents, decided on a case by case basis. In England and Wales, 10 to 14-year-olds used to be assessed this way, says barrister of 25 Bedford Row Chambers in London. The law changed after two 10-year-old boys were convicted of the torture and murder of 2-year-old James Bulger.
鈥淏ased only on my professional experience the sliding scale seemed to work best,鈥� says Mendelle. 鈥淪ome are able to appreciate the consequences of what they鈥檝e done, but children are often just ignorant of the consequences of their actions.鈥� In these assessments, a brain scan could come in handy, he adds.
Even if someone鈥檚 brain could be deemed physically 鈥渋mmature鈥�, it鈥檚 not yet clear how age-related changes in volume of grey and white matter tally with a person鈥檚 behaviour. 鈥淲e know that the brain is maturing physiologically,鈥� says , director of the Law and Neuroscience Project at Vanderbilt University in Nashville, Tennessee. 鈥淏ut that doesn鈥檛 necessarily come hand in hand with functional maturity, which is what the legal system is concerned with.鈥�
Toga agrees: 鈥淭here is no hard and fast rule of development, and there will always be some degree of variation depending on genetics, environment, experience, diet and stimulation.鈥�
Despite difficulties linking brain maturity to behaviour, it is likely lawyers will attempt to use brain maturity scans in the courtroom. at Vanderbilt University assessed 700 US legal cases between 2004 and 2009, including 45 juvenile cases. Of the adult defendants 16 per cent introduced brain scans which pointed towards mental illness and tumours, in an attempt to diminish responsibility for a crime.
In the US a neuroscientist can already be called to testify on behalf of a child and present peer-reviewed research to make the case that adolescent brains are immature.
鈥淚t isn鈥檛 always a winning claim, but in several cases it was successful in decreasing the sentence,鈥� says Farahany.
For better or worse, it is likely that individual brain scans for assessing brain maturity will be used in the courts in the next few years, says Farahany.
鈥淚ndividual brain scans for brain maturity will likely enter the courts in the next few years鈥�
