It’s unnerving when someone with no criminal record commits a disturbingly violent crime.  Perhaps he stabs his girlfriend 40 times and dumps her body in the desert. Perhaps he climbs to the top of a clock tower and guns down innocent passers-by. Or perhaps he climbs out of a car at a stoplight and nearly decapitates an unsuspecting police officer with 26 rounds from an assault rifle. Perhaps he even drowns his own children. Or shoots the President of the United States.

The shock is palpable (NB: those are all actual cases). The very notion that someone—our neighbor, the guy ahead of us in the check-out line, we (!)—could do something so terrible rubs at our minds. We wonder, “What happened? What in this guy snapped?”

After all, for the last 20 years, the accused went home to his family after work—why did he go rob that liquor store? What made him pull that trigger?

The subject hit home for me this week when I was called to jury duty. As I made my way to the county courthouse, I wondered whether I would be asked to decide a capital murder case like the ones above. As a young neuroscientist, the prospect made me uneasy.

At the trial, the accused’s lawyers would probably argue that, at the time of the crime, he had diminished capacity to make decisions, that somehow he wasn’t entirely free to choose whether or not to commit the crime. They might cite some form of neuroscientific evidence to argue that, at the time of the crime, his brain wasn’t functioning normally. And the jury and judge have to decide what to make of it.

Between 2005 and 2012, according to a recent study by Nita Farahany at Duke University, some 1600 criminal cases presented neuroscientific evidence in defense of the accused. In terms of whether the defense wins, Farahany told Greg Miller of the Atlantic, “percentage-wise, people seem to do a little better with neuroscience than they do without.” Theoretically, neuroscience’s influence on legal proceedings is entirely justified. Neuroscience can and should inform how the legal system considers behavior.

I agree—up to a point. In a paper for a forensic psychiatry textbook (“Forensic Psychiatry and Psychology Practice”), Alexander Westphal, Spencer Higgins, and I agree that neuroscience has much to offer. But we also discuss a series of flaws and limitations in how courts use neuroscientific evidence—specifically neuroimaging—to understand behavior.

While I waited in line to walk through the metal detectors, I reflected on our paper, specifically on how the legal system and neuroscience differ in their approach to human behavior and the unreasonable burden this difference places on a jury.

Our “Introduction to Being a Juror” video instructed us to look for reasonable doubt to some aspect of the case.  If we found any reasonable doubt, we should find the accused not guilty.

Juries evaluate two aspects of a case for reasonable doubt: whether the accused actually committed the crime (legally known as actus reus) and whether at the moment of the crime the accused understood that he or she was doing wrong* (legally known as mens rea).  Even with perfect evidence that the accused actually did commit the crime, if he didn’t intend to or knowingly commit the crime he’s not fully guilty.

The absence of mens rea mitigates the accused’s criminal responsibility, entitling him to a lesser sentence: for example, life imprisonment instead of the death penalty. A particularly convincing lack of mens rea or mental defect could result in a jury deciding the accused was “guilty but insane or mentally ill.

Sitting in the “coffee only”area I thought that if actus reus was proven—if the accused actually committed a shockingly violent crime like the ones I cite above—then the presence of some brain abnormality almost goes without saying. Normal people do not stab their girlfriend or mother-in-law. They do not gun down innocents or take shots at the President. Abnormal behavior reflects abnormal brain function, so an appeal to neuroscience is justified to explain criminal behavior.

But jurors are instructed to evaluate, not abnormal brain function per se, but the capacity for free will, which the courts argue is necessary for mens rea.  This is a big deal.

According to the Supreme Court (Morisette v. United States (1952)), “belief in freedom of the human will and a consequent ability and duty of the normal individual to choose between good and evil” is a “universal” and “persistent” and “instinctive” foundation stone in the U.S. legal system’s approach to punishment, sentencing, and incarceration.  Such language affirms free will but does not explicitly deny free will’s philosophical opposite: determinism.

As if to resolve any ambiguity, the Supreme Court (U.S. v. Grayson (1978)) flatly declared that “a deterministic view of human inconsistent with the underlying precepts of our criminal justice system.” In short, legal accountability requires the existence of free will and if something—whether a brain abnormality or drug or circumstance—hinders or obstructs free will, legal accountability is diminished.

Sitting in the New Haven County courthouse, these Supreme Court declarations became real for me. If I was a juror, I’d be asked whether there was reasonable doubt that an individual possessed complete freedom to choose whether to commit a crime. I might have to evaluate neuroimaging evidence that supports or denies this claim.

The irony is that the court’s approach to free will is incompatible with the “universal”—yet un-instinctive—tenet of neuroscience that behavior is the end-product of brain function.

Brain function is produced by biochemical reactions that are sculpted by an interplay of genetic and environmental factors. So the brain is not free, and therefore, human behavior is not free.

In fact, neuroscience demonstrates that, despite our intuition, free will may not exist at all. Neuroscience cannot conclusively demonstrate that free will does not exist—but every study that reports some correlation between genes, environment, and behavior adds to the burgeoning reasonable doubt that each of us captains a sovereign mind, that we are independent and equal before the law.

In our paper, we acknowledge the impasse between neuroscience and the law and we present a framework that sidesteps the free will and determinism debate.

Consider whether my sister’s Chihuahua, Penny, has moral capacity, or better yet criminal responsibility. Notwithstanding Penny’s distinct personality, intelligence, and so on, it’s hard to think of Penny as morally or criminally responsible.  In neuroscience terms, Penny doesn’t have the brain areas that produce that behavior.

Penny is not morally responsible—and for no fault of her own: her genes didn’t produce a brain capable of moral subtlety so we don’t fret much about whether Penny is “free” to be morally responsible.

If Penny lacks the brain areas necessary for moral responsibility, it seems reasonable that a member of our own species may also lack part of this neural system through some developmental process like a genetic or environmental abnormality or through an acquired process like a stroke or tumor.

Without fully functioning moral centers (e.g. Klüver–Bucy Syndrome), a full expression of moral reasoning is impossible in the same way that without fully functioning language centers (e.g. aphasia), a full expression of speech is impossible.

When we consider that neural centers are necessary for moral behavior, asking whether brain function is free or determined misses the point: without the brain centers that produce moral judgment, moral judgment simply can’t take place.

There is ample reasonable doubt that anyone has free will. The question in criminal responsibility should be whether the accused had the brain centers necessary to produce moral behavior and, if not, whether society can increase or restore those abilities. But that is not the jury’s mandate.

Whether these neural systems are determined or free is irrelevant, but whether someone possesses the required neural systems and whether they are functioning properly is of great importance.

After I'd gone through a couple cups of coffee and much expectant hand wringing, the judge (unfortunately?) dismissed me from jury duty. I left with the conviction that for neuroscience and the legal system to meaningfully communicate, one of them needs to change their “universal” approach to behavior. I wonder which it will be.

*Editor's note: the description of "mens rea" in the original verison of this post was incorrect.