January marked the start of what promises to be a four-month public reckoning: the trial of alleged Boston Marathon bomber Dzhokhar Tsarnaev. If the press reports about the evidence against him are accurate, most of the trial will not be about guilt or innocence; it will be about sentencing. Not a who-done-it, but a why-done-it.

If Tsarnaev is found guilty, the death penalty will be on the table, and the proceedings will turn to a grave question, part jurisprudence, and part moral philosophy: Is this defendant the most evil and culpable of all? A human being who deserves the most severe of all punishments?

One thing, I believe, is certain: If this case proceeds to the sentencing phase, the black box everyone will be talking about will be the cranium, and how the brain drives behavior will be the central story.

In these protracted sentencing hearings, the scales of justice balance lists of aggravators and mitigators, all outlined by law.

Aggravating factors in this case might include the political motive for the bombings, the risk posed to others during the course of the Tsarnaev brothers’ dramatic attempt to flee, the “heinous,” “cruel,” or “depraved” manner of the crime’s execution, and the substantial planning and premeditation that might have preceded the bombings.

Mitigating factors — factors that weigh in favor of life in prison rather than a death sentence — cast the broadest net. Any aspect of a defendant’s background, record, character or circumstance is fair game for the defense team. It could try to demonstrate that Tsarnaev had some kind of impaired capacity to appreciate that his acts were wrong or illegal, or that he was under some kind of demonstrable duress. It could also bring to light hardships during his upbringing that limited his opportunities or narrowed his ability to choose wisely.

The defense team has already given public hints as to the central themes of its mitigators. They will feature life within the Tsarnaev family, including Dzhokhar’s relationship with his parents, his brother Tamerlan, and his sisters. Will anything in these family dynamics rise to the level of psychological duress or impaired capacity? There will likely be plenty of traditional testimony from forensic psychologists and psychiatrists regarding whether or not Tsarnaev was under the sway of his radicalized and perhaps dominant older brother, particularly after the Tsarnaev parents left the country. The prosecution will likely counter with a line of evidence regarding Dzohokar’s relative independence and his network of friends and activities outside of the family structure.

Then comes the brain.

Judy Clarke, lead defense attorney, and one of the nation’s premier death penalty litigators, will surely not overlook the new body of neuroscientific evidence regarding the immaturity of adolescent brains. In a recent trilogy of cases (known as Roper, Graham, and Miller ) the U.S. Supreme Court was influenced by neuroscientific evidence about the juvenile brain when making sweeping changes in how adolescents are tried and sentenced. The court concluded that adolescent brains were less mature than those of adults in ways that warranted differential treatment under our criminal laws.

Although Tsarnaev was 19 at the time of the bombings, his lawyers might argue that much of this brain research applies, as it outlines a period of relative immaturity that stretches from mid-adolescence all the way into the early 20s. Generally speaking, this research shows that adolescents are less mature, and they are more likely to make ill-considered decisions. They bow to peer influences and respond excessively to thrill seeking and immediate rewards. Think money, sex, drugs and friends.

Beginning in the teens, there are major changes in brain architecture and function that temper these qualities — among them, synaptic pruning of the prefrontal cortex, improved connectivity and changes in dopamine receptors — all of which support self-control, delayed gratification and the development of a moral compass.

Here’s the rub. What the research doesn’t show makes it problematic for defense attorneys. The research does not show that adolescents are incapable of making well-considered choices. Quite the contrary.

A great many adolescents are capable of making grown-up decisions, but they are more likely to do so when getting guidance and feedback from a caring and responsible adult. They don’t engage their most mature faculties in high-stakes situations, especially when influenced by peers or family members or embroiled in intensely emotional situations or relationships. It’s not just the brain that matters, it’s the brain in context.

Another hurdle? These generalized group averages about neuronal immaturity are not directly applicable to any single individual. Group averages predict little with regard to individual brain function. Brains are not created equally. They mature at different rates, and the brain of any particular 16-year-old might function at the maturity level of a 20-year-old, and vice versa.

In the end, behavior trumps brain scans. If the prosecution has evidence that Tsarnaev was making controlled, considered and strategic choices, this will belie claims that his brain was too immature to allow better decisions.

Finally, the news media have widely reported that Dzhokhar Tsarnaev was a well-known campus marijuana dealer and smoked pot frequently. If this is true, his defense team might argue that his frequent use of cannabis led to impaired decision-making.

Certainly, daily marijuana use by teenagers correlates with lowered academic performance, failure to graduate from high school and college, high rates of attempted suicide and high rates of other illicit drug use. Acute intoxication distorts perception and impairs attention and judgment and a host of other neurologic functions. And withdrawal comes with its own symptoms — insomnia, irritability, inattention, anxiety, and depression, to name only a few.

Recent studies have shown quantifiable brain changes in deep brain structures (nucleus accumbens, amygdala) for moderate users, with marijuana dependency associated with decreased IQ, increased impulsivity and impairments in working memory.

Very important general findings, but less important in the individual case. For this to carry decisive weight, the defense team would have to make a precise link between these findings and this defendant.

Is there any evidence that the alleged bomber was intoxicated on Marathon Monday? In the throes of withdrawal? If he was, is there evidence that these symptoms were so severe that they compromised basic functions like the intention to act or the capacity to plan?

If not acutely intoxicated, what about a fixed deficit from years of drug use? If this were true, it would be present across almost all functional domains. Did he falter in every facet of his life? Certainly his school work was suffering, as the press has widely reported that he was flunking out of college. This, however, might simply be the usual consequence of skipping class and smoking weed instead of doing your homework.

To show a significant drug-related impairment, he would have to show impaired memory, disinhibition or lack of self-control in many avenues of his life — in his relationships and his daily activities. These deficits should show up on common neuropsychological tests. If evidence of deficits is absent on these tests or upon inquiry into multiple life domains (such as family, peers, school, community), then this absence would point to the conclusion that Tsarnaev was someone who was capable of refraining from participation in this tragic and violent act but simply chose to do wrong.

Judith G. Edersheim is a psychiatrist and co-director, Massachusetts General Hospital Center for Law, Brain & Behavior. This article originally appeared in WBUR’s CommonHealth.