He says his brain was coded to kill.

Anthony Blas Yepez didn’t deny beating an elderly man to death in Santa Fe, New Mexico, six years ago in a fit of rage. But after learning that he had a rare genetic abnormality linked to sudden violent outbursts, he argued for leniency, saying he wasn’t fully in control of himself when he committed the crime.

The claim seemed like a stretch to the judge, prosecutors and some scientists. But Yepez took it to the New Mexico Supreme Court, which agreed to consider it.

The court’s decision — still months away — could accelerate a trend in the criminal justice system: the use of behavioral genetics and other neuroscience research, including the analysis of tumors and chemical imbalances, to explain why criminals break the law. The rapidly developing field is forcing officials to confront new questions about how changes in the brain influence behavior — leading some to rethink notions about guilt and punishment.

This cutting-edge evidence, collected through brain scans, psychological exams and genetic sequencing, has been deployed in a range of ways: to challenge whether a defendant was capable of premeditated murder, whether a defendant was competent to stand trial, whether a defendant should be put to death. Most of those attempts to use neuroscience as a defense have failed, researchers say. But some — about 20 percent, according to one study — have worked, winning defendants new hearings or reversals.

That is troubling to researchers who fear some of the tactics push the boundaries of science.

“The law at the moment exists in this gray zone where everyone acknowledges that both genetic and environmental factors could affect culpability,” said Owen Jones, a Vanderbilt University law professor who directs the Research Network on Law and Neuroscience. “But how do you know when, and how much?”

A test for brain science

Yepez’s journey to this frontier began with a mail-order DNA test.

In October 2012, he was charged with first-degree murder for beating to death the boyfriend of his girlfriend's mother. He admitted to attacking the 75-year-old victim during a drunken dispute. But he said he could not remember much of it, and could not explain why he’d reacted so violently. He seemed bewildered at what he had done, according to Ian Loyd, a public defender who was assigned to represent him.

A few months later, as they prepared for trial, Loyd attended a conference in Washington. One of the speakers was forensic psychiatrist William Bernet. Bernet told the story of a Tennessee murder defendant, facing the possibility of the death penalty, who persuaded a jury in 2009 that he deserved a less severe punishment. The defendant had argued that a genetic deficiency — a variant of a gene named MAO-A, which regulates aggressive behavior in men — along with abuse he had suffered as a child were partly to blame for his crime.

The mutation’s impact on criminal behavior was first documented in 1993 in members of a Dutch family with a severe version that has since been found in a handful of families worldwide. There are less extreme, and less rare, versions that have been linked to an increased risk of criminal convictions — but only among men who also suffered from abuse as children. Some researchers began dubbing MAO-A the “warrior gene,” a term that was picked up by documentary filmmakers, talk show hosts and consumer-DNA testing companies.

Loyd frantically scribbled notes, thinking of Yepez. “Maybe he’s got this gene too,” Loyd recalled thinking.

Loyd went online and found a commercial genetic testing company, FamilyTreeDNA, that charges $99 to determine if someone has the MAO-A deficiency. He had one of his investigators visit Yepez at the Santa Fe County jail, where he swabbed Yepez’s cheek for cells. A few weeks later, the results came back positive.

“This is the defense I want to pursue,” Loyd told Yepez.

Anthony Yepez New Mexico Corrections Dept.

The consumer test wouldn’t hold up in court, so Loyd called Bernet, a professor emeritus at Vanderbilt University School of Medicine, who suggested getting a geneticist to perform a more comprehensive test. The geneticist, David Lightfoot, concluded that there was “no doubt” that Yepez had the MAO-A mutation, according to court filings. A psychologist also administered a series of tests on Yepez, who said he’d been mistreated as a child, including beatings with a belt buckle, according to Loyd.

A judge held a pretrial hearing in January 2015 to decide whether those findings could be used as evidence. A doctor testifying for the defense said the MAO-A deficiency and claims of abuse made Yepez “predisposed to committing violent behavior.” But prosecutors argued that the science was not reliable, and that the connection with the murder was weak. The judge refused to allow it.

Four months later, a jury ─ unaware of Yepez’s genetic mutation ─ convicted him of second-degree murder. The judge sentenced him to 22 years in prison.

Yepez appealed. A higher court said the judge should have allowed the genetic evidence, but did not overturn the verdict, saying it wouldn’t have made a difference because a second-degree murder conviction didn’t require showing Yepez had “specific intent” to kill his victim. Yepez appealed to the New Mexico Supreme Court, which agreed last fall to hear the case.

Yepez, who remains in Lea County Correctional Facility, could not be reached for an interview. But his lawyers said they hope the state Supreme Court will grant him a new trial, this time using the genetic evidence to help explain the killing.

Helen Bennett, the lawyer representing Yepez before the state Supreme Court, said the case will test how neuroscience is complicating determinations of whether someone intended to commit a crime.

“These genetic markers and the way we’re learning how they operate in the brain makes the determination of intent much more nuanced,” Bennett said.

A growing strategy

The growth of neuroscience evidence — typically in the form of brain scans and psychological tests — dates back about three decades. It has most often been used to seek leniency for juveniles or against the death penalty for killers. But the strategy has expanded to a wider set of cases.

A few years ago, Nita Farahany, a law and philosophy professor at Duke University, set out to measure neuroscience’s growing influence in the criminal justice system. She found more than 2,800 judicial decisions from 2005 to 2015 in which defendants had cited neuroscience in their arguments. The crimes ranged from fraud and kidnapping to drugs and murder. About 20 percent of those defendants had a favorable outcome of some kind, such as a new hearing to consider whether to include the evidence at trial or the reversal of a prior ruling.

“Year after year, more and more criminal defendants are using neuroscience to bolster their claims of decreased responsibility for their criminal conduct and decreased moral culpability relevant to their sentencing,” she wrote in a study published in the January issue of the Annual Review of Criminology.

She has cited as an example a California man whose lawyers persuaded a judge in 2008 that he was incompetent to stand trial on charges of murdering his mother by showing he’d been diagnosed with developmental disorders. In another case, in Arizona, a man charged with possession of a load of marijuana gave a confession and then tried to recant, saying a brain injury and severe migraines prevented him from understanding that he had waived his right to remain silent. The judge didn’t believe him. In a third case, a woman who was convicted of first-degree murder in 2011 in the killing of her friend during an argument said her mental illness and a brain tumor played a role, and a Massachusetts court agreed, overturning the conviction.

Many such claims overstate the science, Farahany said — neuroscience may be able to help understand someone’s predispositions and proclivities, but can’t say what they were thinking or feeling at the time of the crime. That is why many scientists object to its use in the courtroom.

Behavior is determined by a multitude of forces within the brain, with genes only providing a starting point, researchers say. A person’s experiences or environment play a large role. And it’s difficult to show a direct cause and effect involving a particular condition.

Henry Greely, director of the Center for Law and the Biosciences at Stanford Law School, who collaborated with Farahany on her most recent study, said that if he were a judge in Yepez’s case, he probably would not allow the MAO-A evidence.

“It’s not going as far as junk science, but it’s close,” he said.

Even so, he added, attempts by criminal defendants to use neuroscience “will increase, even if legislatures and courts try to stop them, because there are so many different ways you can try to bring it up.”

Many scientists and researchers point out that prosecutors, too, might one day seize on neuroscientific evidence, using it to argue that a defendant is dangerous and should be punished harshly.

Bennett, Yepez’s lawyer, said it was important to push the criminal justice system to embrace neuroscience and think differently about why people do bad things.

“We have a duty as citizens to listen to that science and make informed decisions before we take away another person’s freedom,” she said.