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New discoveries about the brain are raising the question: Can your genes make you kill? Already, neuroscience has been presented as evidence in more than 1,200 cases. It's being called neuro-law, and it played a role in a murder trial in Tennessee last year. That trial is one of the first where jurors heard evidence from neuroscience to help them decide guilt or innocence.

NPR's Barbara Bradley Hagerty has this final story in our series on the criminal brain. And a warning to our listeners: This report contains graphic descriptions of violence.

BARBARA BRADLEY HAGERTY: When the police arrived at Bradley Waldroup's trailer home in the mountains of Tennessee, they found a war zone. Assistant District Attorney Drew Robinson says there was blood on the walls, blood on the carpet, blood on the truck outside.

Mr. DREW ROBINSON (Assistant District Attorney): This is the defendant's hands as he's being handcuffed.

BRADLEY HAGERTY: They're bloody.

Mr. ROBINSON: Oh, they're just covered in blood.

BRADLEY HAGERTY: Waldroup shot his wife's friend, Leslie Bradshaw, eight times, and sliced her head open with a sharp object. Prosecutor Cynthia Lecroy-Schemel says when Waldroup was finished, he chased after his wife, Penny, with a machete, chopping off her finger and cutting her over and over.

Ms. CYNTHIA LECROY-SCHEMEL (Prosecutor): There are murders and then there are just hacking to death, just trails of blood. No, I've not seen one like this, and I've done a lot.

BRADLEY HAGERTY: Prosecutors charged Waldroup with the felony murder of Leslie Bradshaw, which carries the death penalty, and attempted first-degree murder of his wife, Penny. It seemed clear to them that Waldroup's actions were intentional and premeditated.

Ms. LECROY-SCHEMEL: One of them was, he told his children to come tell your mama goodbye, because he was going to kill her. And he had the gun, and he had the machete.

BRADLEY HAGERTY: It was a pretty straightforward case. Even Bradley Waldroup said so during his trial last year.

Mr. BRADLEY WALDROUP: I killed Leslie Bradshaw. I attacked my wife. I'm not proud of none of it.

Mr. WYLIE RICHARDSON (Defense Attorney): It wasn't a who done it; it was a why done it.

BRADLEY HAGERTY: Defense attorney Wylie Richardson.

Mr. RICHARDSON: And in this particular case, the testimony - we knew before we even, you know, well before we got to trial, was going to be very graphic. We had to do something to try to not dismantle, but to give a broader and fuller picture of what that was.

BRADLEY HAGERTY: How to do that? The answer, it turned out, lay in Bradley Waldroup's genes.

Richardson went to forensic psychiatrist William Bernet of Vanderbilt University, and asked him to give Waldroup a psychiatric evaluation. Bernet also took a blood sample and brought it to Vanderbilt's molecular genetics laboratory.

Since 2004, Bernet and lab director Cindy Vnencak-Jones have been analyzing the DNA of people like Bradley Waldroup.

Dr. WILLIAM BERNET (Forensic Psychiatrist, Vanderbilt University): We've tested about 30 criminal defendants since then, most of whom were charged with murder.

BRADLEY HAGERTY: They were looking for a particular variant of the MAO-A gene also known as the warrior gene because it has been associated with violence. Bernet says they found that Waldroup has the high-risk version of the gene.

Dr. BERNET: His genetic makeup, combined with his history of child abuse, created a vulnerability that he would be a violent adult.

BRADLEY HAGERTY: Over the fierce opposition of prosecutors, the judge allowed Bernet to testify in court that these two factors help explain why Waldroup snapped that murderous night.

Dr. BERNET: We didn't say that these things made him become violent, but they certainly constituted a risk factor or a vulnerability.

BRADLEY HAGERTY: Bernet cited scientific studies over the past decade that have found that the combination of the genes and child abuse increases one's chances of being convicted of a violent offense by more than 400 percent. Other studies have not found such a connection, but Bernet thought the jury should know about the gene.

Dr. BERNET: A person doesn't choose to have this particular gene or this particular genetic makeup. A person doesn't choose to be abused as a child. So I think that should be taken into consideration when we're talking about criminal responsibility.

BRADLEY HAGERTY: The genetic testing was only one piece of Waldroup's defense. His attorneys also argued that he was depressed, suffered from intermittent explosive disorder, and acted in the heat of passion. Still, defense attorney Shari Tayloe Young says the genetic evidence was critical.

Ms. SHARI TAYLOE YOUNG (Defense Attorney): I think that if that wasn't out there, then the jury, all they would have seen were all these horrible pictures where he took a machete and hacked at his wife. And all they would think is, yes, he's the worst of the worst, and that's what the death penalty is for the worst of the worst. But because they heard all the mental issues, they heard all that evidence, they understood what was going on in him, and understood why he did what he did.

Mr. ROBINSON: I would characterize it as smoke and mirrors.

BRADLEY HAGERTY: That's prosecutor Drew Robinson. He says the genetic evidence was just there to confuse the jury. So he called in his own expert, psychiatrist Terry Holmes of the Moccasin Bend Mental Health Institute in Chattanooga. Holmes urged the jury to ignore it.

Dr. TERRY HOLMES (Psychiatrist, Moccasin Bend Mental Health Institute): This was somebody who was intoxicated and mad, and was going to hurt somebody. And it had little or nothing to do with his genetic makeup.

BRADLEY HAGERTY: Holmes says it's way too early to use this research in a court of law, and he believes Bernet is spinning the data.

But jurors say they weren't spun. Sheri Lard, one of the 12, says it was just one piece of evidence that weighed heavily for some and for others, not at all. But, she says, it did figure into a major decision: whether to find Waldroup guilty of murder and impose the death penalty. They concluded that his actions were not premeditated, and agreed with the defense argument that Waldroup just exploded.

Ms. SHERI LARD (Juror): I remember when we were talking as a jury, the comment was brought up, you know, if I were in this situation, I would snap. But there was more to it. There was more to his whole life that led to that moment.

BRADLEY HAGERTY: Including his genes?

Ms. LARD: Oh, I'm sure. And his background, you know: nature vs. nurture.

BRADLEY HAGERTY: Another juror, Debbie Beaty, says the science helped persuade her that Waldroup was not entirely in control of his actions.

Ms. DEBBIE BEATY (Juror): A diagnosis is a diagnosis. You know, it's there. A bad gene is a bad gene.

BRADLEY HAGERTY: After 11 hours of deliberation, the jury convicted Waldroup of voluntary manslaughter in the death of Leslie Bradshaw, and attempted second-degree murder of his wife.

Prosecutor Drew Robinson was stunned.

Mr. ROBINSON: I was just flabbergasted. I did not know how to react to it.

BRADLEY HAGERTY: Nor did fellow prosecutor Cynthia Lecroy-Schemel. She worries that this sort of defense is the wave of the future.

Ms. LECROY-SCHEMEL: Anything that defense attorneys can have to latch onto, to try to save their clients' lives, or to try to lessen their culpability, they will do.

BRADLEY HAGERTY: Waldroup's attorney, Wylie Richardson, says she's right.

Mr. RICHARDSON: Yes, I would use it again.

BRADLEY HAGERTY: 'Cause it's a pretty potent weapon.

Mr. ROBINSON: Well, it seemed to work in this case.

BRADLEY HAGERTY: Bradley Waldroup was sentenced to 32 years in prison. At the hearing, Judge Carroll Ross told Waldroup that he should think twice about appealing. The state might not mind trying this again and asking for the death penalty, the judge said. You might not be as fortunate with a jury the next time.

Scientists and legal experts expect to see more cases like this as neuroscience makes inroads into the courtroom - and presents guilt and innocence not in terms of black and white, but in shades of gray.

Barbara Bradley Hagerty, NPR News.

MONTAGNE: Explore the other stories in our Criminal Brain series at our website, NPR.org.

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