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This is MORNING EDITION, from NPR News. I'm Renee Montagne.

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And I'm David Greene. Scientific explanations frequently play a role in courtrooms. Lawyers trot out all kinds of science to make their case. Some researchers were curious about the power of those arguments. A new study, published in this week's issue of the journal "Science," tried to measure how one kind of scientific explanation affects the actual sentences criminals receive. NPR's Alix Spiegel has the story.

ALIX SPIEGEL, BYLINE: In 1991, a man named Stephen Mobley robbed a Domino's Pizza in Hall County, Georgia, and shot the restaurant manager dead. Now, crimes like this obviously happen all the time, but this particular case became a national story.

JIM TABERY: A couple things made the Mobley case really, really famous.

SPIEGEL: This is Jim Tabery, a professor of philosophy at the University of Utah. Tabery's studied the case, and he says one reason the crime became so well-known was because Mobley just seemed so cold-hearted.

TABERY: You know, he seemed to really revel in this - just abhorrent crime that he committed.

SPIEGEL: From the crime scene, it appeared that the manager had been on his knees, begging for his life, when Mobley shot him execution style, in the head. Then afterwards...

TABERY: He bragged about killing the individual, and he had the word "Domino" tattooed on his back.

SPIEGEL: But there was another reason Mobley's case became famous. Right around the time Mobley went to trial, a study was published about this extremely interesting gene.

HAN BRUNNER: MAOA - Monoamine oxidase A.

SPIEGEL: A scientist named Han Brunner had been looking into MAOA, what happens when there's a mutation in it. And to do that, he'd studied this very unusual Dutch family.

BRUNNER: Where a number of the males in the family essentially completely lacked the MAOA product - the product of the MAOA gene. And the individuals that had this complete lack of MAOA, engaged in all sorts of just awful behavior - exhibitionism, rape, assault, attempted murder. I mean, you know, these are not the kind of people you want to bump into, in a dark alley.

SPIEGEL: So one theory that got a lot of attention, at the time, was that a defect in this gene might, in a sense, cause criminality.

BRUNNER: And when this came out, Mobley thought, hey, I should be genetically screened for my MAOA status.

SPIEGEL: After all, Mobley reasoned, if his crime was the product of biology and not his own choices, judges would have to be more forgiving.

BRUNNER: More lenient when it came time to dole out a punishment.

SPIEGEL: But here's the question: Would the biological explanation actually reduce Mobley's sentence, or could it increase it? After all, there is more than one way to think about what this biological explanation means.

LISA ASPINWALL: My name is Lisa Aspinwall, and I study how people think about causes.

SPIEGEL: Lisa Aspinwall is a psychologist at the University of Utah. With Tabery, she did the study on how biological explanations affect the sentences of psychopaths. And as Aspinwall points out, while Mobley is right - studies show biological explanations do tend to make people more forgiving - the idea that aggression is written into the very cells of a person, can also push people in the opposite direction. You know -

ASPINWALL: They might also conclude that there's nothing to stop him from continuing to be violent in the future.

SPIEGEL: So to figure out which one of these ideas is more powerful in our current criminal-justice system, Aspinwall and Tabery gave 181 judges a description of a convicted criminal - which was actually based on Mobley. All the judges were then told...

BRUNNER: This individual is a psychopath, diagnosed psychopath. Half the judges then got an additional story, from a neurobiologist. And the neurobiologist then added to that diagnostic story: Here's where psychopathy comes from.

SPIEGEL: Specifically...

ASPINWALL: (Reading) Psychopaths recruit less oxygen to the amygdala during tasks that involve emotional learning, relative to healthy controls.

SPIEGEL: This is Aspinwall, reading from the biological explanation that they provided to the judges. It is very compelling stuff, filled with all kinds of scientific jargon. And the message is really clear: It is beyond their control.

ASPINWALL: Psychopaths simply do not have the biological resources to experience anxiety, in the face of others suffering.

SPIEGEL: Exactly this kind of testimony happens in courts around the country every day. So how does it affect things? Well, simply using the term "psychopath" adds an average of five years to criminal sentences, according to this study. But once you start talking biology...

ASPINWALL: It did create a significant reduction in sentencing; from 14 years, on average, without the biological mechanism, to just about 13 years.

SPIEGEL: In other words, Mobley was right. Our sympathy for the idea that biology might be responsible for criminal behavior, is powerful. And you can see that in his own case as well. Mobley originally got the death penalty for his crime, but appealed on the grounds that his counsel was bad because he'd never been tested for an MAOA defect.

One appeals court agreed with him and actually reduced his sentence from death, to life in prison. But that was overturned, ultimately, by another court, which said MAOA testimony shouldn't be considered. And Mobley was put to death in 2005. Essentially, for Mobley, access to a biological explanation was the difference between life and death - a lot of power for a simple idea.

Alix Spiegel, NPR News, Washington.

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