This is the second post in a series about the application of neuroscientific research about the adolescent brain to the defense of Dzohkar Tsarnaev, who was found guilty last month of bombing the Boston Marathon in April 2013.

Now, in the sentencing phase of the trial, the defense called Jay N. Giedd last week. Geidd, chief of brain imaging in the Child Psychiatry Branch, National Institute of Mental Health, is a prominent child and adolescent psychiatrist who specializes in brain imaging.

As we predicted in our earlier post, the expert witness described the development of the prefrontal cortex, the area instrumental in planning, impulse control, and judgment, relative to the limbic, or emotion-regulating, systems of the brain. Dr. Giedd indicated that the average teen’s prefrontal cortex — recall, Tsarnaev was 19 when he participated in the bombing — is only half as developed as it will be by the time its owner reaches his late twenties. As a result, the capacity of the prefrontal cortex to override the aggression or excitation mediated by the limbic portions of the brain tends to be weak during the adolescent years.

Dr. Giedd also spoke of teens’ predilection for choosing smaller, short-term rewards over long-term gains: They are “less worried about longer-term consequences.” The implication here is that Tsarnaev’s brain, like all teen brains, was especially sensitive to pressure by peers and loved ones. The approval of his domineering older brother Tamerlan, this narrative suggests, was made even more compelling for Dzohkar because of the way his brain functioned.

On cross-examination, though, Dr. Geidd acknowledged large variations in brain development across individuals. He further granted that “Even under age 10, they can do quite well with planning and consequences” and that there is much we don’t yet know about drawing strong connections between features of the brain and a given teen’s behavior. Trying to grasp the brain’s complexity, he noted, is “humbling business.”

It is also humbling to try to infer a killer’s state of mind — that is, his motives and understanding of right and wrong. Here, the immature brain strategy loses much of its force: still developing or not, teens’ basic moral schema are formed well before age 19. One does not need a fully formed brain to know that blasting nails and ball bearings into a crowd has lethal and personally tragic consequences. Any nine year-old who is not intellectually deficient grasps the finality of death and understands that killing innocent people is wrong. Although many teens might impulsively go on a joy ride or two with older friends or foolishly stay out late the night before a morning exam, virtually none seriously consider murdering others, let alone undertake the act of doing so.

How did the jury seem to respond to the information about the teen brain? We can’t tell from news reports whether the defense invoked vivid brain images (though we imagine that such exhibits would have been noted in the press coverage), but according to Jack Lepiarz, WBUR reporter, the testimony on the teen brain did not seem “sexy” relative to the gore-filled exhibits of jagged wounds and torn limbs presented by the prosecution during the guilt phase.

But how much influence would images wield if they had indeed been shown? During the earlier days of neurolaw, judges and prosecutors worried that brain scans would be too compelling, or “prejudicial,” to juries. That might have been the case a decade ago when multi-colored brain scans were a novelty in media coverage. Perhaps by now, however, many laypersons have become inured to them.

Recall the case of Herbert Weinstein discussed in the previous post. He was the executive who strangled his wife during an argument and then pushed her out the window of their Manhattan high-rise to make her death look like a suicide. Weinstein, as we noted, was discovered to have a large fluid-filled cyst in the right orbitofrontal cortex. Reportedly, the prosecutor was not impressed with the hypothesis that the cyst led him to commit murder, but he agreed to a plea deal because he thought the jury would be persuaded by the images.

In a 2009 case of an Illinois defendant named Brian Dugan, the judge, too, was concerned about the prejudicial power of brain scans. Dugan faced the death penalty for the kidnapping, rape, and murder of a 10-year-old girl. To vacate his death sentence, the defense team turned to fMRI to buttress the claim that he was a psychopath, a morally disabled man whose sickness was such that he could not feel right from wrong or that he did not care about the distinction.

As a psychopath, Dugan would have known that killing and raping an innocent person was against the law, but he would have been poor at empathizing emotionally and perhaps even regarded the misery he’d inflicted on others as being “their problem, not mine.” Under testing conditions, Dugan’s fMRI showed diminished activity in the “paralimbic system,” an interconnected set of emotion-processing structures in the brain.

At trial, however, the judge did not allow the psychologist called by the defense, Dr. Kent Kiehl of the University of New Mexico, to display scans of the defendant’s abnormal paralimbic activity; he worried that they might confuse or mislead the jury. As a compromise, he allowed Kiehl to show jurors a diagram of the findings and to explain their meaning. In the end, the jury was unmoved and sentenced Dugan to death.

In the courtroom there are, of course, so many variables that impinge on jurors’ views: the quality of the lawyering, the strength of witness testimony, the appeal of the defendant himself, and so on. In controlled lab studies, scientists have explored whether brain images can seduce participants into accepting logically flawed statements, but the findings have been mixed. Other work on prospective jurors suggests that scans may sometimes, but not always, lessen the amount of punishment participants mete out to presumed criminals. The scans, it seems, hold less persuasive power than neurobiological language to dampen observers’ intuition about blaming the perpetrator.

Some scientists argue that brain-based explanations of immoral or antisocial actions are effective antidotes against stigma. If people view mental illnesses as diseases of the brain, their reasoning goes, they may be less likely to blame offenders for their misbehavior. Research supports this possibility but there is a largely unappreciated flip side: attributing people’s bad behavior to bad brains (or genes) rather than poor character or a bad childhood leads others to believe that these individuals are more dangerous and resistant to treatment or rehabilitation. Biological explanations of mental illness and addiction further fuel pessimism about the likelihood of recovery and the effectiveness of treatment.

Thus far, then, it would appear that its influence on the jury may well have been minimal. If so, this is all for the good. No game-changing information was imparted by the neuroscientist’s testimony last week. For one thing, the testimony was about teens as a group, not about Tsarnaev himself; nor was it about a defect per se. Also, his crime has a compelling psychological explanation that no neuroscience story can readily explain away.

Nor did his testimony tell us much that psychologists — and many parents — have not already known for decades, namely, that many adolescents prioritize short-term rewards over long-term punishments. Tsarnaev was almost certainly influenced by his brother and swayed by his emotional appeals, but he repeatedly elected to follow his brother’s directives. His efforts were planned and protracted, and his actions sustained by a set of internally consistent, if profoundly hateful, beliefs. Tsarnaev’s story is a human tragedy, a tale of devastating personal choices about which developmental neurophysiology has precious little to add.