Imagine you are a juror for a horrific murder case. Harry is the defendant. You sit down with 11 of your peers--people who may not be up on the latest scientific understanding about human behavior. Most of the jurors have never heard the word "neuroscience" nor given a moment's thought to the concept of "free will." And you know that most jurors have little patience for criminal-defense arguments based on such notions as "temporary insanity." The jurors are there to determine whether Harry committed the crime, and if they decide he did, they will deliver their verdict without regret. But have they considered whether Harry acted freely or as an inevitable consequence of his brain and his past experiences?

Although advances in neuroscience continue at a rapid pace, their ethical and legal implications are only beginning to be taken into account. The link between the brain and behavior is much closer than the link between genes and behavior, yet the public debate about the legal implications of genetic findings far outweighs that given to brain research.

Progress in neuroscience and technology raises numerous issues with respect to the core constructs of law, such as competency to stand trial, the genesis of violent behavior and the determination of whether witnesses are lying [see "The New Lie Detectors," by Laurence R. Tancredi, on page 46]. For example, knowing that a brain deficiency predisposes certain people to violence would present a host of controversial questions, including whether we might "mark" these people for surveillance by authorities; whether preemptive treatment of these people is desirable; whether juries are likely to discriminate against them; and whether society might change how it punishes and rehabilitates such people who are convicted of crimes. How far along are we, today, in being able to make such determinations?

Free Will vs. Free Wont
Perhaps the most fundamental implication of 21st-century brain science is that a way may exist to evaluate free will. The logic goes like this: The brain determines the mind, and the brain is subject to all the rules of the physical world. The physical world is determined, so our brains must also be determined. If so, then we must ask: Are the thoughts that arise from the brain also determined? Is the free will we seem to experience just an illusion? And if free will is an illusion, must we revise our conception of what it means to be personally responsible for our actions?

This conjecture has haunted philosophers for decades. But with new imaging tools that show the human brain in action, these questions are being reexamined by neuroscientists and, increasingly, the legal world. Defense lawyers are looking for that one pixel in their client's brain scan that shows an abnormality--some sort of malfunction that would allow them to argue: "Harry didnt do it. His brain did it. Harry is not responsible for his actions." [For more on the relative accuracy of such scans, see "Fact or Phrenology?" by David Dobbs, on page 24.]

At the same time, we must realize that even if the causation of an act (criminal or otherwise) is explainable in terms of brain function, that does not mean that the person who carries out the act is exculpable. Although brains can be viewed as more or less automatic devices, like clocks, we as people seem free to choose our own destiny. Is there a way to settle this dilemma?

A first step was taken in the 1980s by Benjamin Libet, now emeritus professor of physiology at the University of California at San Francisco. If the brain carries out its work before one becomes consciously aware of a thought, as most neuroscientists now accept as true, it would appear that the brain enables the mind. This idea underlies the neuroscience of determinism. Libet measured brain activity during voluntary hand movements. He found that between 500 and 1,000 milliseconds before we actually move our hand there is a wave of brain activity, called the readiness potential. Libet set out to determine the moment, somewhere in that 500 to 1,000 milliseconds, when we make the actual conscious decision to move our hand.

Libet found that the time between the onset of the readiness potential and the moment of conscious decision making was about 300 milliseconds. If the readiness potential of the brain is initiated before we are aware of making the decision to move our hand, then it would appear that our brains know our decisions before we become conscious of them.

This kind of evidence seems to indicate that free will is an illusion. But Libet argued that because the time from the onset of the readiness potential to the actual hand movement is about 500 milliseconds, and it takes 50 to 100 milliseconds for the neural signal to travel from the brain to the hand to actually make it move, then there are 100 milliseconds left for the conscious self to either act on the unconscious decision or veto it. That, he said, is where free will arises--in the vetoing power. Neuroscientist Vilayanur S. Ramachandran of the University of California at San Diego, in an argument similar to 17th-century English philosopher John Locke's theory of free will, suggests that our conscious minds may not have free will but do have "free wont."

Resisting Violent Tendencies
Many other experiments show that our brain gets things done before we know about them. But what does this mean for real-life problems of free will, such as violent behavior? Is there a way to use current scientific knowledge to argue for reduced culpability under the law?

Evidence from patients with brain lesions confirms that the prefrontal cortex plays a critical role in social behavior. And psychological exams indicate that people who repeatedly commit violent crimes often have antisocial personality disorder (APD). It would therefore be interesting to know if criminals with APD, who demonstrate abnormal social behavior similar to that of patients with prefrontal lobe damage, also have abnormalities in the prefrontal areas of the brain. To address this question, Adrian Raine, a psychology professor at the University of Southern California, and his colleagues imaged the brains of 21 people with APD and compared them with the brains of healthy subjects and other controls. They found that people with APD had a reduced volume of gray matter and a reduced amount of neural activity in the prefrontal areas as compared with the controls. This finding indicates that there is a structural difference between the brains of criminals with APD and the brains of the normal population. The outcome also suggests that a volume difference in gray matter in that area of the brain may lead to a functional difference in social behavior.

In 2002 Antonia S. New, associate professor of psychiatry at the Mount Sinai School of Medicine, looked at a specific characteristic of APD--impulsive aggression. Using positron emission tomography, her team monitored the metabolic activity of the brain in response to an excitatory chemical called m-CPP in people with impulsive aggression and in healthy, nonaggressive controls. M-CPP normally activates the anterior cingulate (a frontal area of the brain known to be involved in inhibition) and deactivates the posterior cingulate. The opposite was found to be true for people with impulsive aggression: the anterior cingulate was deactivated, and the posterior cingulate was activated. The investigators concluded that people with impulsive aggression have less activation of inhibitory regions and that this may contribute to their difficulty in modulating aggression.

If findings such as these are true, it is still possible that certain violent people do not inhibit their impulses even though they could inhibit them--and therefore should be held responsible for their actions. Future research will be needed to determine how much prefrontal damage is necessary, or to what degree the gray matter is reduced, for the cessation of inhibitory function and thus perhaps for the mitigation of responsibility.

Neuroscientists must realize, however, that for any given brain state, the correlation of nonviolent behavior could be just as high as the correlation of violent behavior. For example, most patients who suffer from lesions involving the inferior orbital frontal lobe (in the prefrontal cortex) do not exhibit antisocial behavior of the sort that would be noticed by the law. Even though a patient's wife, say, might sense changes in her husband's behavior, the man is still constrained by all the other forces in society, and the frequency of his abnormal behavior is no different than would be seen in the normal population.

The same view is true for people with schizophrenia, a disease marked by disassociation between intellect and emotions and by difficulty controlling moods and actions. The rate of aggressive criminal behavior is not greater among schizophrenics than it is among the normal population. Because people with lesions in the inferior orbital frontal lobe or with schizophrenia are no more likely to commit violent crimes than unaffected people, it seems that merely having one of these brain disorders is not enough to remove responsibility.

Automatic Brains, Interpretive Minds
Although mechanistic descriptions of how the physical brain carries out behavior have added fuel to the general idea of determinism, experts have argued that the concept of free will can coexist with determinism.

In 1954 noted English scientist and philosopher Alfred J. Ayer put forth a theory of "soft determinism." He argued, as Scottish moral philosopher David Hume had two centuries earlier, that even in a deterministic world, a person can still act freely. Ayer distinguished between free actions and constrained actions. Free actions are those that are caused by internal sources--by one's own will (unless one is suffering from a disorder). Constrained actions are those that are caused by external sources--for example, by someone or something forcing you physically or mentally to perform an action, as in hypnosis or in disorders such as kleptomania. When someone performs a free action to do A, he or she could have done B. When someone makes a constrained action to do A, he or she could have done only A.

Ayer argued that actions are free as long as they are not constrained. Free actions are not dependent on the existence of a cause but on the source of the cause. Although Ayer did not explicitly discuss the brain's role, one could make the analogy that those actions--and indeed those wills--that are caused by a disease-free brain are not constrained, even though they may be determined. In this way, the brain is determined, but the person is free.

With each passing decade, the world knows more about the mechanistic action of the nervous system and how it produces perceptual, attentional, and mnemonic functions and decisions. Yet there is still much to learn about how the brain enables the mind.

We recently attended a conference at which more than 80 leading scientists presented their findings on this very subject. It became obvious that the central question remains not only unanswered but unexamined. The brain scientists who are addressing issues of human cognition are illuminating which brain systems correlate with particular measurable human behaviors. For example, a series of studies might investigate which areas of the visual system become activated when a person attends to a particular visual stimulus. Although these correlations are of interest, the question of how the brain knows whether, when and how to increase the activity of a particular neuronal system remains unknown. Overall, modern studies always seem to leave room for the metaphorical ho-munculus, the little ghost in the machine that directs all brain traffic. It is common in neurology circles to hear the phrase "top-down versus bottom-up processes"--processes driven by feedback from "higher" areas of the brain rather than direct input from the sensory stimuli--but the fact is that no one knows anything about the "top" in "top-down." This is a major problem of cognitive neuroscience today, and we hope that it will soon become the subject of research.

Changing the Law
For now, we must operate with what we do know about the brain--and how that can influence the law. To address this, we must consider the current legal system's view of human decision making.

Under our legal system, a crime has two defining elements: the actus reus, or proscribed act, and the mens rea, or guilty mind. In order for Harry to go to prison for murder, both elements have to be proven beyond a reasonable doubt. The courts and the legal system typically work hard to determine the agency of the crime. Where they want help from neuroscience is on whether or not Harry should be held "personally responsible." Did Harry do it, or did his brain? This is where the slippery slope begins. Our argument is that neuroscience can offer very little to the understanding of responsibility. Responsibility is a human construct, and no pixel on a brain scan will ever be able to show culpability or not.

In practice, legal authorities have had great difficulty crafting standards to divide the responsible from the not responsible. For example, the rules for a finding of legal insanity that have existed in various forms for more than 150 years are all lacking. Experts for the defense and prosecution argue different points from the same data. What they would like, instead, is for neuroscience to come to the rescue.

But the crux of the problem is the legal system's view of human behavior. It assumes Harry is a "practical reasoner," a person who acts because he has freely chosen to act. This simple but powerful assumption drives the entire legal system. Even though we might all conceive of reasons to contravene the law, we can decide not to act on such thoughts because we have free will. If a defense lawyer can provide evidence that a defendant had a "defect in reasoning" that led to his inability to stop from committing the crime, then Harry can be deemed exculpable. The legal authorities want a brain image, a neurotransmitter assay or something to show beyond a reasonable doubt that Harry was not thinking clearly, indeed could not think clearly, and therefore could not stop his behavior.

The view of human behavior offered by neuroscience is at odds with this perspective. In some ways, it is a tougher view, in other ways more lenient. Fundamentally, however, it is different. Neuroscience is the business of describing the mechanistic actions of the nervous system. The brain is an evolved system, a decision-making device that interacts with its environment in a way that allows it to learn rules to govern how it responds. It is a rule-based device that, fortunately, works automatically.

Critics might raise the objection: "Aren't you saying that people are basically robots? That the brain is a clock, and you can't hold people responsible for criminal behavior any more than you can blame a clock for not working?" That is not the case. The comparison is inappropriate because the notion of responsibility has not emerged. It has not been denied; it is simply absent from the neuroscientific description of human behavior, as a direct result of treating the brain as an automatic machine. But just because responsibility cannot be assigned to clocks does not mean it cannot be ascribed to people. In this sense, human beings are special and different from robots.

This is a fundamental point. Neuroscience will never find the brain correlate of responsibility, because that is something we ascribe to people, not to brains. It is a moral value we demand of our fellow rule-following human beings. Brain scientists might be able to tell us what someone's mental state or brain condition is but cannot tell us when someone has too little control to be held responsible. The issue of responsibility is a social choice. According to neuroscience, no one person is more or less responsible than any other person for actions carried out. Responsibility is a social construct and exists in the rules of the society. It does not exist in the neuronal structures of the brain.

For now, that is all we can say. It would be rash to conclude on any other note than one of modesty about our current understanding of the brain and mind. Much more work is needed to clarify the complex issues raised by neuroscience and the law.

Still, we would like to offer the following axiom: brains are automatic, rule-governed, determined devices, whereas people are personally responsible agents free to make their own decisions. Just as traffic is what happens when physically determined cars interact, responsibility is what happens when people interact. Brains are determined; people are free.

This article and sidebar are adapted with permission from Neuroscience and the Law: Brain, Mind, and the Scales of Justice, edited by Brent Garland. 2004 Dana Press.