On July 1, 2013, Amos Joseph Wells III went to his pregnant girlfriend's home in Fort Worth, Texas, and shot her multiple times in the head and stomach. He then killed her mother and her 10-year-old brother. Wells surrendered voluntarily within hours, and in a tearful jailhouse interview told reporters, "There's no explanation that I could give anyone, or anybody could give anyone, to try to make it seem right, or make it seem rational, to make everybody understand."

Heinous crimes tend to defy comprehension, but some researchers believe neuroscience and genetics could help explain why certain people commit such atrocities. Meanwhile, lawyers are introducing so-called neurobiological evidence into court more than ever.

Take Wells, for instance. His lawyers called on Pietro Pietrini—director of the IMT School for Advanced Studies in Lucca, Italy, and an expert on the neurobiological correlates of antisocial behavior—to testify at their client's trial last year. “Wells had several abnormalities in the frontal regions of his brain, plus a very bad genetic profile," Pietrini says. Scans of the defendant's brain showed abnormally low neuronal activity in his frontal lobe, a condition associated with increased risk of reactive, aggressive, and violent behavior. In Pietrini's estimation, that "bad genetic profile" consisted of low MAOA gene activity—a trait long associated with aggression in people raised in abusive environments—and five other notable genetic variations. To differing degrees, they're linked with a susceptibility to violent behavior, impulsivity, risk-taking, and impaired decision-making.

"What we tried to sustain was that he had some evidence of a neurobiological impairment that would affect his brain function, decision making, and impulse control," Pietrini says. "And this, we hoped, would spare him from the death penalty."

It did not. On November 3, 2016, a Tarrant County jury found Wells guilty of capital murder. Two weeks later, the same jury deliberated Wells' fate for just four hours before sentencing him to die. The decision, as mandated by Texas law, was unanimous.

In front of a different judge or another jury, Wells might have avoided the death penalty. In 2010, lawyers used a brain-mapping technology called quantitative electroencephalography to try to convince a Dade City, Florida, jury that defendant Grady Nelson was predisposed to impulsiveness and violence when he stabbed his wife 61 times before raping and stabbing her 11-year-old daughter. The evidence's sway over at least two jurors locked the jury in a 6-6 split over whether Nelson should be executed, resulting in a recommendation of life without parole.

Nelson's was one of nearly 1,600 court cases examined in a recent analysis of neurobiological evidence in the US criminal justice system. The study, by Duke University bioethicist Nita Farahany, found that the number of judicial opinions mentioning neuroscience or behavioral genetics more than doubled between 2005 and 2012, and that roughly 25 percent of death penalty trials employ neurobiological data in pursuit of a lighter sentence.

Farahany's findings also suggest defense attorneys are applying neuroscientific findings to more than capital murder cases; lawyers are increasingly introducing neuroscientific evidence in cases ranging from burglary and robbery to kidnapping and rape.

"Neuro cases without a doubt are increasing, and they're likely to continue increasing over time" says Farahany, who adds that people appear to be particularly enamored of brain-based explanations. "It’s a much simpler sell to jurors. They seem to believe that it’s much more individualized than population genetics. Also, they can see it, right? You can show somebody a brain scan and say: There. See that? That big thing, in this person’s brain? You don’t have that. I don’t have that. And it affects how this person behaves.”