How teenage brains are different?

DZHOKHAR TSARNAEV was 19, legally an adult, at the time of the Boston Marathon bombing in 2013. He now faces 30 federal counts related to the attacks, 17 of which could carry the death penalty. Yet the trial, which began earlier this month, is not really about establishing Mr Tsarnaev’s guilt. Judy Clarke, the lead defence lawyer, has readily agreed with federal prosecutors that her client, along with his older brother, placed and detonated home-made bombs near the finishing line of Boston’s annual marathon in April 2013. Instead, the case seems to be about culpability. Many believe Ms Clarke’s main work will be at the sentencing phase, when she is expected to cite recent neurological research to support the theory that her client should not be punished as an adult, as his brain had not yet reached the level of maturity typical of most adults. But is there such a thing as a “teenage brain”, and does it help to explain the high rates of recklessness among teenagers?

The past decade has yielded a wealth of research that shows that adolescent brains behave differently from adult brains. Neuroimaging studies reveal that the brain continues to mature as people enter their early 20s, and the regions responsible for controlling impulses and making plans—ie, the frontal lobes, home of the prefrontal cortex—are the last to develop. Specifically, the brain blooms with neural connections until a child reaches the age of 11 or 12, and then it selectively prunes away the underused ones, or “grey matter”, throughout adolescence. As the brain grows more streamlined, it becomes better at processing information. The remaining connections are then made more efficient by a process called myelination, which essentially insulates neuronal axons with a sheath of fatty cell material, or “white matter”. The process of replacing grey matter with white matter does not reach the prefrontal cortex until people are in their early 20s. Studies show a relationship between increased myelination and an improved ability to make decisions and control impulses.

Some now argue that this science helps to explain why teenagers across cultures are not only moody and compulsive, but also especially likely to seek novelty, take risks and follow peers. Indeed, although teenagers tend to be at the peak of physical health, rates of death by injury jump between ages 15 and 19. Crime rates also peak in the late teenage years, and then decline by the early 20s. Policymakers who once blamed this puzzling behaviour on raging hormones now point to neuroscience. Evidence that adolescent brains behave differently from adult ones is believed to have influenced the Supreme Court’s view that juveniles should be treated differently from adults by the courts. In three landmark cases over the past decade, the justices ruled that because adolescents are marked by a “lack of maturity” and an “underdeveloped sense of responsibility”, anyone who commits a crime while under the age of 18 is no longer a candidate for execution or a mandatory life sentence without parole. “[D]evelopments in psychology and brain science continue to show fundamental differences between juvenile and adult minds,” the justices explained in a majority opinion in 2010. Some now say that because the brain continues to mature while people are in their early 20s, the presumption that offenders should be treated as adults at 18 is perhaps arbitrary and misguided.

But although it is always tempting to use hard science to explain otherwise perplexing behaviour, neuroimaging research is still in its infancy. There is much about the brain that no one understands yet, and there is rarely a clear relationship between a particular brain region and a discrete function, so any links between brain structure and behaviour remain speculative. Indeed, while researchers often take care to show that the relationship between how a brain looks and how someone behaves is correlative, often this link is misinterpreted as a sign of causation. Scientists also point out that brain science cannot be understood in a vacuum. All behaviour is a function of many influences, including parenting, socioeconomic status, nutrition, culture and so on. So the policy implications of these discoveries remain unclear. Still, there are plenty of sound reasons for treating younger offenders differently from older ones. Most fundamentally, they are not afforded the same rights as adults, and they are not legally allowed to control their own environments or associations, so punishing them in the same way as adults makes little sense. As to whether a 19-year-old offender deserves the death penalty, however, that will be left to Mr Tsarnaev’s jury to decide.