Admissibility of Expert Testimony regarding Genetic Predisposition to Violence

Facts of the Case

Mr. Yepez lived with his girlfriend Jeannie Sandoval and her adoptive mother's boyfriend, George Ortiz. On October 29, 2012, Mr. Yepez killed Mr. Ortiz during an argument. Mr. Yepez and Ms. Sandoval subsequently set Mr. Ortiz's body on fire. Mr. Yepez was charged with first-degree murder, conspiracy to commit first-degree murder, tampering with evidence, and unlawful taking of a motor vehicle.

Prior to the trial, Mr. Yepez filed a motion in limine to admit expert testimony about his predisposition to violence, on the basis of his having a genotype that results in low monoamine oxidase A (MAOA) activity and his childhood abuse. This expert testimony was based on research that evaluated the relationship between childhood abuse, MAOA activity, and antisocial and violent behavior. In one prominent study, Brunner et al. had identified an association between MAOA complete deficiency and impulsive aggression among males in a family (Brunner HG, Nelen M, Breakenfield XO, et al. Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A. Science. 1993 October; 262[5133]:578–580). Another study by Caspi et al. was noted to find that male victims of childhood maltreatment who had a genotype with low MAOA levels had a predisposition toward adult antisocial behavior (Caspi A, McClay J, Moffitt TE, et al. Role of genotype in the cycle of violence in maltreated children. Science. 2002 September; 297[5582]: 851–854). Mr. Yepez had a low-activity MAOA genotype and a history of childhood maltreatment; therefore, experts posited that he had a predisposition toward impulsive violence.

The state filed a motion in limine to exclude this testimony, on the basis that this testimony did not meet Daubert criteria, with data lacking both reliability and relevance to the case at hand, because of the poorly understood science and not clearly established relationship. The state further argued that his self-reported abuse history was not corroborated, and the expert testimony may mislead and confuse the jury. After a Daubert/Alberico hearing at which he was not successful, Mr. Yepez moved for reconsideration.

The district court ultimately determined that this expert testimony was inadmissible, by standards established in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993). Although the district court found the research on childhood maltreatment, low MAOA activity, and a predisposition to violence to be reasonably reliable, the court did not admit the testimony because the experts did not clearly explain how the science led to their conclusions that Mr. Yepez had a predisposition to impulsive violence. Though the court found that the broad findings of the studies met Daubert criteria, the expert testimony did not and misstated the results. Mr. Yepez was ultimately found guilty of second-degree murder, tampering with evidence, and unlawful taking of a motor vehicle. He was sentenced to 22.5 years in prison.

Mr. Yepez appealed his conviction, arguing that the district court abused discretion by improperly excluding the expert testimony. The Court of Appeals found the district court's exclusion of the testimony to be in error, because the jury should have been permitted to evaluate the merit of the experts' opinions. The Court of Appeals further ruled that this error was harmless because it only served to prove that Mr. Yepez was prone to impulsive, rather than deliberate, violence, and Mr. Yepez was convicted of general intent crimes that did not require proof of deliberation. Mr. Yepez appealed this holding, on the basis that this error was prejudicial and harmful, because it also affected jury deliberation between second-degree murder and lesser offenses such as manslaughter. The state also appealed this holding and sought to vacate the Court of Appeals' opinion regarding the admissibility of the expert evidence. The Supreme Court of New Mexico granted certiorari to review whether or not the district court's exclusion of this expert evidence was an abuse of discretion.

Ruling and Reasoning

The Supreme Court of New Mexico ultimately determined the testimony to be inadmissible, based on its lack of reliability. The Court of Appeals' judgment was reversed and Mr. Yepez's conviction was affirmed.

This lack of reliability was based on several factors, including the complex, multifactorial relationship between genetic and environmental factors and acts of violence. The court noted that research has not clearly established MAOA and childhood abuse as definitively involved in antisocial behavior or in impulse control and, in fact, there are many biological and other etiologies that influence impulsivity and antisocial behavior. The court also noted that the experts failed to clearly explain the methodology behind their conclusions that a low-activity MAOA genotype leads to a higher risk of impulsive violence.

The court also noted that the content of the expert testimony did not fit appropriately with the facts of the case. For example, the Brunner study identified a relationship between complete MAOA deficiency and aggression in males, not low MAOA activity as was the case with Mr. Yepez. Expert testimony did not establish a causal link between Mr. Yepez's genotype and his actions on the night of the offense. One expert testified that he could not establish a “clear connection” between Mr. Yepez's genetic predisposition and his actions on the night of the offense because genetics do not “directly” cause individuals' actions (Yepez, p 587).

The court acknowledged the complex relationship between genetics, environment, and behavior. The court also noted the distinction between a person having a predisposition to develop a behavior and a person actually developing this behavior. In the Caspi study, the authors noted that childhood maltreatment alone increases an individual's risk of adult criminal behavior. The court examined psychological research that noted that genetic assessments are not used as predictive measures of violent behaviors because of a lack of a clear relationship between a genetic finding and violent behaviors. One expert testified that low MAOA activity, combined with childhood maltreatment, increased an individual's risk of violence to 85 percent, but the court noted that his source for this information was unclear.