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‘Your Honor, My Genes Made Me Do It’

There have been many theories to explain violent behavior. The latest involves a defective ‘warrior gene.’

Recent high-profile cases of mass shootings have renewed a vigorous debate about the causes of violent behavior. Predicting violence, whether by sentencing judges, parole boards or mental health professionals, has been a perplexing issue as we try to unravel the personal and social forces behind criminal behavior.

Every decade brings a new “Eureka!” In the 1970s, we became convinced that urban poverty caused violence. In the ’80s, it was all due to mental illness, and in the ’90s substance abuse was definitely the culprit. Now it is malfunctioning brains that are to blame.

Thousands of defendants each year now present brain scans and other test results as part of their defense. So far there is little reliable science to support most claims. Yet an article in the August 17 issue of Science magazine suggests that this may not necessarily matter in court.

Experts at the University of Utah, led by psychologist Lisa Aspinwall, conducted an experiment using a hypothetical defendant. It suggested that if a judge is presented with testimony that the defendant’s violent or criminal behavior is in part caused by a genetic predisposition to aggression or criminality—and if he is told that research shows that the brains of psychopaths look different from those of nonpsychopaths—the judge is likely to impose a lighter sentence. The study has generated widespread attention. It implies that judges may be more lenient if they are persuaded that a person didn’t have free will because he was, in effect, born to be bad, and his brain and genes “made him do it.”

It has long been understood that if a mental illness or brain injury causes a bad behavior, it might make a defendant less culpable than an offender whose brain theoretically allowed a wider range of lawful choices. But what about someone without an identified injury or illness of the brain? What about someone who claims that because he has a certain genetic and neurological structure he was predisposed to behave badly?

In the hypothetical presented in the Science paper, a make-believe scientific expert testified that psychopathy has a neurobiologic cause—a gene variation (low monoamine oxidase, or MAO-A, activity), atypical brain functioning (in the amygdala) and other neurodevelopmental factors. While this argument is now becoming common in real criminal cases throughout the country, it represents an unfounded exercise in biologic and neurological determinism.

Here are just a few simple errors in this line of reasoning:

• You can’t isolate a single gene from an individual and claim that it causes a complex human behavior such as violence. Genes don’t operate in isolation but interact with social environments and other genes, and we often don’t understand how gene variants operate in the working brain. Yet if we have learned anything over the past 100 years of violence research, it is that human violence is a complicated and multifactorial behavior and is not now—and may never be—reduced to a series of genetic variations or mutations.

• It is now abundantly clear that the specific genetic argument presented in these cases is incorrect. Although it became all the rage over the last decade to claim that a defendant with the low-activity variant of the MAO-A gene (dubbed the “warrior gene”) was predisposed to violent or impulsive behavior, we now know that this link lacks proof.

The early and inflated claims for it were based on studies of a single family that didn’t have just a variant of the gene and the enzyme it makes, but was missing it altogether. Such claims were also based on studies investigating an interaction between the gene and an abusive environment—not on genetic makeup in isolation.

Subsequent studies of violence and MAO-A have had inconsistent results, and it is not even clear that having the low-activity variant of the “warrior gene” actually results in low enzyme activity in the brain. More than one in three men carry this genetic variant, but the vast majority don’t commit violent crimes. At most, this variant may produce a small increase in the risk of antisocial behavior among men with a history of abuse.

• The same controversies have emerged regarding brain-scan evidence. Defendants argue that findings on the new functional neuroimages (mostly from fMRIs) show that their brains operate in abnormal ways, causing them to behave impulsively or violently.

The problem lies in the ambiguity of this causation. Although a small number of brain injuries and illnesses have well-researched impacts on behaviors, many don’t. In most cases we are not sure how variants in brain structure and function affect behavior, or whether other regions of the brain can compensate for affected circuits. Structural differences seen on a brain scan don’t necessarily result in functional changes in individual behavior.

We are making remarkable strides in identifying specific functional brain networks and the genetic and environmental causes for disruptions in these networks. However, until we can make well-founded, scientifically sound and legally relevant links between genes, brains and behaviors, judges, juries and the public should be wary of neuroscience in the courtroom. The criminal-justice system can’t function the way it is supposed to if the evidence presented by the defense or the prosecution inside the courtroom brings us no closer to the truth of what happened outside the courtroom.

Drs. Edersheim and Price are co-directors of the Center for Law, Brain and Behavior at Massachusetts General Hospital and Harvard Medical School. Dr. Smoller is on the center’s scientific faculty.

Source: Wall Street Journal, Oct. 22, 2012. By Judith G. Edersheim, Bruce H. Price and Jordan W. Smoller.
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