Dzhokhar Tsarnaev was convicted in April in federal court in Boston on multiple charges in the 2013 Boston Marathon bombings. (AP Photo/FBI, 2013 file photo)

Dzhokhar Tsarnaev, convicted earlier this month in the 2013 Boston Marathon bombing, faces the death penalty on 17 separate counts. Later this week, according to the Boston Herald, his lawyers will call a neuroscientist — perhaps the first of several — as a witness. Almost certainly this will be part of a strategy to show that Tsarnaev, who was 19 when he committed the crime, possessed an “immature teen brain,” which made him especially susceptible to the influence of his deceased radicalized brother, Tamerlan.

By saying that Tsarnaev possessed an “immature teen brain,” the defense is citing a well-established neuroscientific finding that the killer’s brain, like all teenage brains, was still in a formative stage. Indeed, researchers have shown that the human brain is not fully developed until the mid-20s.

The “immature brain” and its implications for reduced culpability has become a staple of the juvenile justice movement.  The National Juvenile Justice Network has asserted that brain science “gives advocates and lawyers working on behalf of juveniles scientific proof for their claims.”

At least five percent of all murder cases that go to trial feature the introduction of have neuroscience evidence, according to Nina Farahany, professor of law at Duke University. Ten years ago, that percentage was less than 0.01 percent. Fully 24 percent of capital cases invoke neuroscience as part of a mitigation strategy.

Here are two classic examples of functional imaging used in a legal context:

One is the case of Herbert Weinstein, a retired executive. In 1991, he killed his wife in a pique and threw her body out the window of their Upper East Side Manhattan apartment. In a positron emission tomography (PET) scan, Weinstein’s left frontal lobe showed a gaping black void the size of a quail egg. It was a fluid-filled cyst that had formed within the weblike tissues surrounding the brain. Over many years, the cyst had expanded slowly into the underside of the frontal lobe, displacing and compressing brain tissue that in the scan glowed red and green, colors representing regions of “hypometabolism,” or decreased energy use.

According to the defense, this compression produced a severe impairment in Weinstein’s ability to appreciate the difference between right and wrong.  In contrast, most radiologists who examined the evidence concluded that Weinstein’s cyst exerted little effect on the functioning of his brain. When surgeons offered to remove the cyst, Weinstein declined. Nonetheless, the judge decided to admit some of the neurological evidence at trial. Prosecutors then agreed to let Weinstein plead guilty to manslaughter; he was sentenced  to seven to 14 years. During his time in prison, Weinstein was a model prisoner. Some scholars theorize that the prosecutors allowed a plea because they worried that the scans would be unduly persuasive to a jury.

The second is the case of a 41-year-old school teacher who developed a strong interest in child pornography and uncontrollable sexual urges (including impulses toward his step-daughter) following the appearance of a tumor in his right orbitofrontal cortex. Doctors excised the growth, and the patient’s pedophilic urges disappeared completely. Yet a year later, his sexual appetite returned. Sure enough, a brain scan demonstrated that the tumor had grown back. The tumor was almost certainly a cause of the teacher’s intense sexual urges; at the very least, it probably released a brake on a preexisting desire. After the tumor was found, the teacher was not held responsible for violating the terms of a pre-scan conviction for child molestation.

In both of these cases a single defendant’s criminal behavior was  attributed to an organic defect that led, the defense argued, to uncontrollable deficiencies in the person’s capacity to make choices or exercise his self-control. The immature brain defense, however, is another matter altogether. This defense is applied not to a given defendant’s brain but rather to an entire class of brain — the adolescent brain.

The first high-profile application of the immature teen brain arose in a landmark 2005 U.S. Supreme Court case, Roper v Simmons. At age 17, Christopher Simmons broke into a home and surprised the resident, Shirley Crook, whom he killed by taking her to a wooded area and shoving her, bound and gagged, into a river.

Simmons’s defenders traced these actions to the neurobiology of the typical adolescent brain. In a joint amicus brief, the American Medical Association, the American Psychiatric Association and other organizations explained that the adolescent reward system is more sensitive than that of an adult. As a result, teens find sensation-boosting and pleasurable activities, including the approval of peers, extremely compelling.

In Simmons, the Court ruled five to four that executing people for a crime they had committed as a minor violates the constitutional ban on cruel and unusual punishment.  Subsequently, in Graham v. Florida in 2010, the Court banned sentences of life without parole for juveniles convicted of nonlethal crimes. In a 2012 majority ruling against mandatory sentencing of life without parole for juvenile killers, the majority cited neuroscience.

These decisions for those under 18 make sense. After all, adolescents’ personalities are less formed than adults’, less predictive of their life-long character and often more amenable to treatment. Moreover, depriving teens who murder of the possibility of parole not only denies them education and training in some prisons but also robs them of an incentive to improve themselves.

These facts are compelling, but they have one thing in common: They are the products of social and psychological investigation and experience, not brain science.

So, why invoke neuroscience when it adds little and arguably nothing to the scientific picture? For rhetorical effect: Numerous studies reveal that claiming “my client’s brain made him do it” weakens ascriptions of responsibility in a way that “his lousy childhood made him do it” does not. To many laypersons, neurological terms suggest largely fixed internal processes that lead inexorably to only one behavior; some data suggest that vivid brain scans may bolster the thrust of such mechanistic explanations.

In the case of the adolescent brain, the mechanisms revolve around several well-established observations about the rates at which certain neural tracts and brain regions develop. In short, the neural communication between the prefrontal cortex (a brain region needed for decision-making and impulse control) to the amygdala (an area that helps to mediate fear) is still being refined. And the nucleus accumbens, which is involved in motivation and reward, is more sensitive in teens than in people in their mid-20s.

In all likelihood, the jury will be shown impressive-looking Technicolor brain scans. But what they will see is merely a biological retelling of a well-established behavioral story. Volumes of data on teen behavior and cognition already tell us that when adolescents consider their options for action under calm circumstances, they make decisions that are comparable with those of adults, but when peer pressure enters the picture, they are more likely to choose rashly.

What’s more, the defense will not be talking about a particular neural or mental condition of Tsarnaev as a young man, but of teens as a class. How many teens are murderously violent? Extremely few. And although they might skateboard down railings and not plan well for their SATs, deliberately causing bloody mayhem in the name of a cause requires a moral mindset that transcends the impulsive acts of your average teen.

To be sure, there are rare occasions when a defendant who is severely mentally ill possesses a defective brain that impedes his or her ability to exercise control or reason, or to form intent or be deterred –the legal requirements for criminal responsibility.

Critically, however, today’s brain images alone can’t tell us who these teens are. If brain scans are to play a scientifically legitimate role in determining criminal responsibility of a defendant or in reducing a defendant’s sentence, they need to be able to assist us in answering legal questions. That means, at bottom, that these scans must be amenable to being deciphered in such a way that they bear narrowly on potentially excusing or mitigating mental states, such as damaged capacity for reason or an impaired ability to form intent or exert self-control.

Thus far, no brain scans have been able to do this.

In the next posts, we will discuss brain scans and the courtroom more broadly and how they have been used in other cases. We’ll also comment on expert witness testimony regarding the adolescent brain.

Sally Satel is a psychiatrist and a resident scholar at the American Enterprise Institute. Scott O. Lilienfeld is a professor of psychology at Emory University. They are the authors of “Brainwashed — The Seductive Appeal of Mindless Neuroscience” (Basic, 2013).