The Center for Law, Brain & Behavior puts the most accurate and actionable neuroscience in the hands of judges, lawyers, policymakers and journalists—people who shape the standards and practices of our legal system and affect its impact on people’s lives. We work to make the legal system more effective and more just for all those affected by the law.

Dzokhar Tsarnaev: Adolescent or Adult?

By Laurence Steinberg | The Boston Globe | March 30, 2015

As the trial of Dzhokhar Tsarnaev, the accused Boston Marathon bomber, moves into its defense phase, his attorneys likely will lean heavily on the science of adolescent development to argue that their client should be spared the death penalty. Judy Clarke, Tsarnaev’s lead defense attorney, has already made several references about Tsarnaev’s youthfulness and susceptibility to the influence of his older brother.

Technically, the proceeding is about determining the appropriate punishment for Tsarnaev, who has admitted his involvement in the attack. But the trial is also a referendum on how we view and define adolescence.

Tsarnaev was 19 at the time of the bombing, an adult under every state’s criminal law. States often have departed from the presumptive age of majority, 18, in prosecuting and punishing juvenile offenders, but the departures always have been toward a younger, not older, dividing line. Arguing that a 19-year-old is still an adolescent is a step in a new direction.

Proffering developmental immaturity as a mitigating factor in a trial involving individuals who are Tsarnaev’s age is consistent with recent discoveries in the study of brain development, which show that substantial maturation continues well beyond 18. We can’t point to a specific chronological age at which the adolescent brain becomes an adult brain, because different brain regions mature along different timetables, but important developments, some of which are relevant to sentencing decisions, are still ongoing during the early 20s.

There is considerable misunderstanding among the public about why developmental immaturity is relevant to determinations of criminal culpability. The issue is not whether adolescent immaturity excuses criminal conduct — it doesn’t — but whether it diminishes someone’s responsibility for his actions, in much the same way that coercion or unusually strong emotions might. In general, if a “reasonable person” might respond under the same circumstances in the way the defendant did (the textbook example is committing an act of violence after discovering your spouse in bed with someone else), then the defendant would likely be viewed as less than completely responsible for his or her behavior, and punished less harshly as a result.

The defense team will probably point to several US Supreme Court decisions issued during the past decade, in which the Court’s majority concluded that adolescents’ immaturity rendered them less culpable than fully mature adults. Writing for the court’s majority in Roper v. Simmons, the case that abolished the juvenile death penalty, Justice Anthony Kennedy identified three features of adolescence that mitigate juveniles’ culpability: impetuous decision-making, heightened susceptibility to coercion, and an unformed personality.

In subsequent decisions that built on Roper, the court connected adolescent impulsivity to research on brain development, arguing that adolescents’ neurobiological immaturity meant that their crimes are often due to factors they can’t control. More recently, research also has identified the neural bases of adolescents’ intensified susceptibility to peer pressure and is revealing the period to be one of heightened neuroplasticity, or capacity for the brain to change.

Arguing that a 19-year-old is still an adolescent is a step in a new direction.

It is unlikely that a jury can be persuaded that the bombing was an impulsive act. Whether Tsarnaev’s character is still amenable to change is not likely to be of interest, because if he is convicted but not sentenced to death, he will certainly be sentenced to life without parole. But in much the same way that Lee Malvo, the 17-year-old Washington, D.C., sniper, was spared the death penalty because his immaturity was seen as having left him susceptible to the influence of his 41-year-old accomplice, Tsarnaev’s attorneys will argue that he, too, was immature enough to be coerced by his brother. What makes this strategy particularly interesting is that research on susceptibility to peer influence shows that this period — roughly between 17 and 20 — is precisely the age when individuals develop adult-like capacity to resist the coercive influence of others.

During oral arguments in Roper, Seth Waxman, who represented the defendant, posited that raising the age boundary for capital punishment from 16 to 18 was justified on the basis of new scientific evidence that had emerged since the last time the Court had considered this matter. Justice Antonin Scalia asked Waxman whether the age boundary used to determine who is eligible for capital punishment should automatically be revised whenever new science about maturity emerges.

If neurobiological immaturity makes adolescents inherently less responsible for their crimes, and if science now demonstrates that the brain is still maturing well into the early 20s, should we rethink where we draw the boundary between adolescence and adulthood under the law? The Boston Marathon bombing trial is important not only because the crime was so horrific, but because it forces us to ask hard questions about how best to judge the behavior of those who are legal adults, but in many respects neurobiological adolescents.

Laurence Steinberg, professor of psychology at Temple University, is author of “Age of Opportunity: Lessons From the New Science of Adolescence.“​ Read the original piece in The Boston Globe.