Ideas 2011 July/August 2011

The Brain on Trial

Advances in brain science are calling into question the volition behind many criminal acts. A leading neuroscientist describes how the foundations of our criminal-justice system are beginning to crumble, and proposes a new way forward for law and order.

Changes in the balance of brain chemistry, even small ones, can also cause large and unexpected changes in behavior. Victims of Parkinson’s disease offer an example. In 2001, families and caretakers of Parkinson’s patients began to notice something strange. When patients were given a drug called pramipexole, some of them turned into gamblers. And not just casual gamblers, but pathological gamblers. These were people who had never gambled much before, and now they were flying off to Vegas. One 68-year-old man amassed losses of more than $200,000 in six months at a series of casinos. Some patients became consumed with Internet poker, racking up unpayable credit-card bills. For several, the new addiction reached beyond gambling, to compulsive eating, excessive alcohol consumption, and hypersexuality.

What was going on? Parkinson’s involves the loss of brain cells that produce a neurotransmitter known as dopamine. Pramipexole works by impersonating dopamine. But it turns out that dopamine is a chemical doing double duty in the brain. Along with its role in motor commands, it also mediates the reward systems, guiding a person toward food, drink, mates, and other things useful for survival. Because of dopamine’s role in weighing the costs and benefits of decisions, imbalances in its levels can trigger gambling, overeating, and drug addiction—behaviors that result from a reward system gone awry. Physicians now watch for these behavioral changes as a possible side effect of drugs like pramipexole. Luckily, the negative effects of the drug are reversible—the physician simply lowers the dosage, and the compulsive gambling goes away.

The lesson from all these stories is the same: human behavior cannot be separated from human biology. If we like to believe that people make free choices about their behavior (as in, “I don’t gamble, because I’m strong-willed”), cases like Alex the pedophile, the frontotemporal shoplifters, and the gambling Parkinson’s patients may encourage us to examine our views more carefully. Perhaps not everyone is equally “free” to make socially appropriate choices.

Does the discovery of Charles Whitman’s brain tumor modify your feelings about the senseless murders he committed? Does it affect the sentence you would find appropriate for him, had he survived that day? Does the tumor change the degree to which you consider the killings “his fault”? Couldn’t you just as easily be unlucky enough to develop a tumor and lose control of your behavior?

On the other hand, wouldn’t it be dangerous to conclude that people with a tumor are free of guilt, and that they should be let off the hook for their crimes?

As our understanding of the human brain improves, juries are increasingly challenged with these sorts of questions. When a criminal stands in front of the judge’s bench today, the legal system wants to know whether he is blameworthy. Was it his fault, or his biology’s fault?

I submit that this is the wrong question to be asking. The choices we make are inseparably yoked to our neural circuitry, and therefore we have no meaningful way to tease the two apart. The more we learn, the more the seemingly simple concept of blameworthiness becomes complicated, and the more the foundations of our legal system are strained.

If I seem to be heading in an uncomfortable direction—toward letting criminals off the hook—please read on, because I’m going to show the logic of a new argument, piece by piece. The upshot is that we can build a legal system more deeply informed by science, in which we will continue to take criminals off the streets, but we will customize sentencing, leverage new opportunities for rehabilitation, and structure better incentives for good behavior. Discoveries in neuroscience suggest a new way forward for law and order—one that will lead to a more cost-effective, humane, and flexible system than the one we have today. When modern brain science is laid out clearly, it is difficult to justify how our legal system can continue to function without taking what we’ve learned into account.

Many of us like to believe that all adults possess the same capacity to make sound choices. It’s a charitable idea, but demonstrably wrong. People’s brains are vastly different.

Who you even have the possibility to be starts at conception. If you think genes don’t affect how people behave, consider this fact: if you are a carrier of a particular set of genes, the probability that you will commit a violent crime is four times as high as it would be if you lacked those genes. You’re three times as likely to commit robbery, five times as likely to commit aggravated assault, eight times as likely to be arrested for murder, and 13 times as likely to be arrested for a sexual offense. The overwhelming majority of prisoners carry these genes; 98.1 percent of death-row inmates do. These statistics alone indicate that we cannot presume that everyone is coming to the table equally equipped in terms of drives and behaviors.

And this feeds into a larger lesson of biology: we are not the ones steering the boat of our behavior, at least not nearly as much as we believe. Who we are runs well below the surface of our conscious access, and the details reach back in time to before our birth, when the meeting of a sperm and an egg granted us certain attributes and not others. Who we can be starts with our molecular blueprints—a series of alien codes written in invisibly small strings of acids—well before we have anything to do with it. Each of us is, in part, a product of our inaccessible, microscopic history. By the way, as regards that dangerous set of genes, you’ve probably heard of them. They are summarized as the Y chromosome. If you’re a carrier, we call you a male.

Genes are part of the story, but they’re not the whole story. We are likewise influenced by the environments in which we grow up. Substance abuse by a mother during pregnancy, maternal stress, and low birth weight all can influence how a baby will turn out as an adult. As a child grows, neglect, physical abuse, and head injury can impede mental development, as can the physical environment. (For example, the major public-health movement to eliminate lead-based paint grew out of an understanding that ingesting lead can cause brain damage, making children less intelligent and, in some cases, more impulsive and aggressive.) And every experience throughout our lives can modify genetic expression—activating certain genes or switching others off—which in turn can inaugurate new behaviors. In this way, genes and environments intertwine.

When it comes to nature and nurture, the important point is that we choose neither one. We are each constructed from a genetic blueprint, and then born into a world of circumstances that we cannot control in our most-formative years. The complex interactions of genes and environment mean that all citizens—equal before the law—possess different perspectives, dissimilar personalities, and varied capacities for decision-making. The unique patterns of neurobiology inside each of our heads cannot qualify as choices; these are the cards we’re dealt.

Because we did not choose the factors that affected the formation and structure of our brain, the concepts of free will and personal responsibility begin to sprout question marks. Is it meaningful to say that Alex made bad choices, even though his brain tumor was not his fault? Is it justifiable to say that the patients with frontotemporal dementia or Parkinson’s should be punished for their bad behavior?

It is problematic to imagine yourself in the shoes of someone breaking the law and conclude, “Well, I wouldn’t have done that”—because if you weren’t exposed to in utero cocaine, lead poisoning, and physical abuse, and he was, then you and he are not directly comparable. You cannot walk a mile in his shoes.

The legal system rests on the assumption that we are “practical reasoners,” a term of art that presumes, at bottom, the existence of free will. The idea is that we use conscious deliberation when deciding how to act—that is, in the absence of external duress, we make free decisions. This concept of the practical reasoner is intuitive but problematic.

The existence of free will in human behavior is the subject of an ancient debate. Arguments in support of free will are typically based on direct subjective experience (“I feel like I made the decision to lift my finger just now”). But evaluating free will requires some nuance beyond our immediate intuitions.

Consider a decision to move or speak. It feels as though free will leads you to stick out your tongue, or scrunch up your face, or call someone a name. But free will is not required to play any role in these acts. People with Tourette’s syndrome, for instance, suffer from involuntary movements and vocalizations. A typical Touretter may stick out his tongue, scrunch up his face, or call someone a name—all without choosing to do so.

We immediately learn two things from the Tourette’s patient. First, actions can occur in the absence of free will. Second, the Tourette’s patient has no free won’t. He cannot use free will to override or control what subconscious parts of his brain have decided to do. What the lack of free will and the lack of free won’t have in common is the lack of “free.” Tourette’s syndrome provides a case in which the underlying neural machinery does its thing, and we all agree that the person is not responsible.

This same phenomenon arises in people with a condition known as chorea, for whom actions of the hands, arms, legs, and face are involuntary, even though they certainly look voluntary: ask such a patient why she is moving her fingers up and down, and she will explain that she has no control over her hand. She cannot not do it. Similarly, some split-brain patients (who have had the two hemispheres of the brain surgically disconnected) develop alien-hand syndrome: while one hand buttons up a shirt, the other hand works to unbutton it. When one hand reaches for a pencil, the other bats it away. No matter how hard the patient tries, he cannot make his alien hand not do what it’s doing. The movements are not “his” to freely start or stop.

Unconscious acts are not limited to unintended shouts or wayward hands; they can be surprisingly sophisticated. Consider Kenneth Parks, a 23-year-old Canadian with a wife, a five-month-old daughter, and a close relationship with his in-laws (his mother-in-law described him as a “gentle giant”). Suffering from financial difficulties, marital problems, and a gambling addiction, he made plans to go see his in-laws to talk about his troubles.

In the wee hours of May 23, 1987, Kenneth arose from the couch on which he had fallen asleep, but he did not awaken. Sleepwalking, he climbed into his car and drove the 14 miles to his in-laws’ home. He broke in, stabbed his mother-in-law to death, and assaulted his father-in-law, who survived. Afterward, he drove himself to the police station. Once there, he said, “I think I have killed some people … My hands,” realizing for the first time that his own hands were severely cut.

Over the next year, Kenneth’s testimony was remarkably consistent, even in the face of attempts to lead him astray: he remembered nothing of the incident. Moreover, while all parties agreed that Kenneth had undoubtedly committed the murder, they also agreed that he had no motive. His defense attorneys argued that this was a case of killing while sleepwalking, known as homicidal somnambulism.

Although critics cried “Faker!,” sleepwalking is a verifiable phenomenon. On May 25, 1988, after lengthy consideration of electrical recordings from Kenneth’s brain, the jury concluded that his actions had indeed been involuntary, and declared him not guilty.

As with Tourette’s sufferers, split-brain patients, and those with choreic movements, Kenneth’s case illustrates that high-level behaviors can take place in the absence of free will. Like your heartbeat, breathing, blinking, and swallowing, even your mental machinery can run on autopilot. The crux of the question is whether all of your actions are fundamentally on autopilot or whether some little bit of you is “free” to choose, independent of the rules of biology.

This has always been the sticking point for philosophers and scientists alike. After all, there is no spot in the brain that is not densely interconnected with—and driven by—other brain parts. And that suggests that no part is independent and therefore “free.” In modern science, it is difficult to find the gap into which to slip free will—the uncaused causer—because there seems to be no part of the machinery that does not follow in a causal relationship from the other parts.

Free will may exist (it may simply be beyond our current science), but one thing seems clear: if free will does exist, it has little room in which to operate. It can at best be a small factor riding on top of vast neural networks shaped by genes and environment. In fact, free will may end up being so small that we eventually think about bad decision-making in the same way we think about any physical process, such as diabetes or lung disease.

Presented by

David Eagleman is a neuroscientist at Baylor College of Medicine. This essay is adapted from his new book, Incognito: The Secret Lives of the Brain.

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