State v. Yepez: Admissibility and Relevance of Behavioral Genetic Evidence in a Criminal Trial

For the past quarter century, criminal courts in the United States and other countries have struggled with decisions about the role of behavioral genetic evidence (1). Impelled by studies linking violent behavior to changes in the function of the gene that produces monoamine oxidase A (MAOA)—an enzyme that degrades the monoamine neurotransmitters—defense attorneys have attempted to introduce data about their clients’ MAOA activity to reduce the charges against them or mitigate their punishment. Although MAOA evidence has never had the impact that its advocates predicted, it has not been abandoned by the criminal defense bar (2). The latest, ultimately unsuccessful effort to introduce testimony about a defendant’s MAOA gene activity highlights both the challenges defense attorneys face in getting such evidence admitted and the difficulties courts have in thinking clearly about the impact of genetics on criminal behavior.

Litigating the Admissibility of Genetic Evidence

In 2012, Anthony Blas Yepez and his girlfriend, Jeannie Sandoval, were living with George Ortiz, the 75-year-old boyfriend of Sandoval’s adoptive mother. Yepez got into a “tussle” with Ortiz, who died after the altercation. Yepez subsequently doused the body in cooking oil and set it ablaze. He and Sandoval then took the victim’s car keys and left to buy alcohol. Yepez was arrested and charged with first-degree murder, which in New Mexico is defined as a “willful, deliberate, and premeditated killing.”

The evidence linking Yepez to the crime was sufficiently strong that he could not simply claim innocence. Instead, Yepez sought to reduce the charge he faced from first-degree murder to a lesser offense by arguing that he could not have formed a deliberate intent to kill Ortiz. To support this claim, at a pretrial hearing, Yepez sought permission to introduce expert testimony that he had a low-activity MAOA gene, which, coupled with maltreatment he had experienced in childhood, had predisposed him to “maladaptive or violent behavior.” This predisposition, he alleged, rendered him incapable of forming the specific intent required by the first-degree murder charge (sometimes referred to as the “warrior gene defense”). The state sought to exclude this testimony, and a hearing was held to determine whether the genetic evidence was sufficiently reliable to be admitted.

Yepez introduced the testimonies of a forensic neuropsychologist and two academic psychologists in support of his motion to admit evidence of his MAOA status. Two scientific studies were discussed extensively by the academic experts. The first was a 1993 study by Brunner et al. (3), which found that a complete deficiency of MAOA activity in a Dutch kindred was correlated with impulsive aggressive behavior, in what has become known as “Brunner syndrome.” The second study, by Caspi and colleagues (4) in 2002, found that childhood maltreatment coupled with low levels of MAOA activity was correlated with antisocial behavior. Moreover, the experts noted, these findings had been replicated in many (although not all) later studies and confirmed by meta-analyses. The neuropsychologist testified that Yepez had “an extremely low function of the [MAOA] gene” and that the “low MAOA activity gene made him exceptionally predisposed to committing violent behavior.”

Notwithstanding this testimony, the trial judge issued a ruling excluding the MAOA evidence. Not only did the genetic findings not meet New Mexico’s standard for a mental disorder that could render a person incapable of forming an intent to kill, the judge ruled, but the neuropsychologist’s testimony did not correspond with the scientific evidence cited in support. The underlying studies, the court determined, examined “violent” rather than “impulsive” actions, and thus the data did not support the purported relationship between reduced activity of Yepez’s MAOA gene and increased impulsivity—the basis on which lack of a deliberate intent to kill was premised. In the end, though, even without the genetic evidence, Yepez avoided a first-degree murder verdict, being convicted instead of second-degree murder, tampering with evidence, and unlawful taking of a motor vehicle.

Yepez appealed his conviction, arguing that the trial court improperly excluded the MAOA evidence, which he now asserted was germane to the charge of second-degree murder as well. The court of appeals agreed with Yepez, holding that it was an error to exclude the MAOA evidence at trial, because the jurors should have been able to weigh for themselves the expert testimony regarding the MAOA science and its implications for Yepez’s defense. Nevertheless, the court of appeals also held that this error was harmless, because evidence on the MAOA gene was offered to negate the “willful, deliberate, and premediated” elements of first-degree murder and it “had no relevance to the remaining charged offenses . . . ,” including his conviction for second-degree murder (5).

Struggling With MAOA Testimony

Both Yepez and the state appealed the appellate court’s ruling. The state argued that the appellate court inappropriately determined that the trial court erred in excluding the MAOA evidence, and Yepez argued that the error was not “harmless” and affected his constitutional rights. The New Mexico Supreme Court rejected both claims (6). The crux of the court’s analysis hinged on two factors: the reliability (or validity) of the science, and the “fit” between the science and its application to this case.

The court critically analyzed the underlying scientific studies. It noted that the Caspi study and the subsequent meta-analyses examined “antisocial behavioral generally,” not impulsive violence. The court noted that, “While some studies speculate that MAOA may be involved in regulating impulse control, others hypothesize that impulse control is a separate variable dependent largely on frontal lobe brain function. . . . These differences illustrate that it is not yet understood how MAOA activity and childhood maltreatment increase the likelihood of antisocial behavior.” Moreover, the court observed that the defense experts conceded that the mechanism underlying the relationship between the MAOA gene and antisocial behavior is not known, that it may not involve impulsivity, and that no expert had furnished scientific studies finding a link between MAOA and impulsive violence. Therefore, in contrast to the ruling of the appellate court, the supreme court held unanimously that the trial court was within its discretion to exclude the MAOA evidence.

Although the supreme court could have stopped at that point, the judges moved to consider the issue of “fit.” The court assumed that the MAOA evidence was being offered to “make the existence of deliberation [by Yepez, in deciding to kill Ortiz] less likely.” It noted that “[t]he finding of the Brunner study was that those with a no-activity MAOA genotype are impulsively aggressive. If Yepez had this characteristic of impulsive aggression, it would tend to negate the element of deliberate intent.” However, Yepez had a low-activity MAOA gene, not a no-activity MAOA gene. The court criticized the experts who “seemingly imported the findings scientifically attributable to Brunner syndrome and concluded, on the basis of Yepez’s low-activity MAOA genotype, that Yepez was predisposed to impulsive aggression. We agree with the district court that this apparently unfounded reliance on the Brunner study renders the proffered expert opinions merely speculative and therefore irrelevant.”

Even assuming that the linkage between the low-activity MAOA gene and childhood maltreatment included a “statistically significant predisposition to violent behavior,” the court concluded that the MAOA evidence was not relevant to whether Yepez had formed a specific intent to kill. “The question, it seems to us, is whether a person has the phenotype, or trait, of impulsive antisocial behavior, not whether that person is at risk for developing the phenotype.” Unlike neurological deficits, impulse-control disorder[s], or polysubstance abuse, which have “known effects” that can be weighed to evaluate the likelihood of a defendant having deliberated or planned a killing, the link between the MAOA gene, child maltreatment, and antisocial behavior is too “ambigu[ous],” “multivariable,” and “uncert[ain.]” In summary, the court held that “evidence of mere genetic susceptibility to a given mental condition is not relevant on the issue of deliberate intent, at least in the absence of evidence that such susceptibility is so well understood and has such strong predictive value as to be clinically validated as an indicator of the mental condition.”

The End of Behavioral Genetic Evidence in Criminal Court?

The Yepez odyssey illustrates how courts are confused by the science and use of behavioral genetic evidence. While the trial court accepted as reliable the science of the underlying genetic evidence studies, the appellate court did not address the underlying science, and the New Mexico Supreme Court, which spent considerable effort reviewing the scientific studies, concluded that they do not speak to impulsive violence and are therefore not germane. One group of scholars—which submitted an amicus brief that the supreme court declined to review—went even further in arguing that “the general scientific consensus has emerged that the earlier MAOA studies (on which the defense experts relied) are now an outdated method of studying psychiatric genetics” (7). Do these developments signal the demise of behavioral genetic evidence in court? We think not—at least in jurisdictions outside New Mexico.

Observant defense attorneys will no doubt learn important lessons from the Yepez case. In particular, attorneys will need to tighten the nexus between the outcome measure in scientific studies and the particular element in the criminal statute that they are addressing with the genetic evidence. The failure of Yepez’s experts to link his MAOA test results and alleged history of maltreatment as a child to impulsive behavior per se doomed his appeal to the New Mexico Supreme Court. Moreover, given the court’s resistance to considering “evidence of mere genetic susceptibility” to impulsivity, as opposed to evidence demonstrating actual impulsive behavior by the defendant, courts may be looking more closely in the future at the real-life impact of the purported genetic condition on the defendant.

Attorneys will also need to rely on different, perhaps updated studies of behavioral genetic evidence—although many of these studies are considerably more complex than the simple “candidate gene”–type studies of the past and importantly often fail to detect a link among genetics, adverse childhood experiences, and violent behavior (7). Nevertheless, in high-stakes criminal trials, defense attorneys will likely pursue any approach that could mitigate culpability, and thus they are likely to continue to introduce complex genetic evidence that could have such an effect. This is especially likely to be true in capital sentencing, where the rules governing the admission of scientific evidence are relaxed and defense attorneys might believe they have “nothing to lose” by proffering such evidence.

The “nothing to lose” argument has at least two flaws. First, considerable commentary has discussed the possibility of a double-edged-sword effect in introducing behavioral genetic evidence. According to this theory, behavioral genetic evidence could have both a mitigating effect—because the behavior is the partial product of a genetic disposition over which the individual has no control—as well as an aggravating effect—because genes are immutable, and therefore the individual will continue to present an elevated risk for danger in perpetuity. Simply put, although the debate continues on whether the double-edged-sword effect is a real phenomenon (8), introducing behavioral genetic evidence is not necessarily a risk-free proposition to criminal defendants. Second, proffering this type of evidence requires an incredible amount of resources. The Yepez case involved three experts conducting analyses, writing reports, and participating in multiple evidentiary hearings. In our view, limited defense resources could likely be used better, especially given the lessons of a case such as Yepez where, in the end, none of the experts were permitted to testify at trial.