The first gene to be linked to antisocial behavior was identified in 1993 in a large Dutch family that was notorious for violence. Fifteen years of painstaking research revealed that they all lacked the same gene. This gene produces an enzyme called MAOA, which regulates the levels of neurotransmitters involved in impulse control. If you lack the MAOA gene or have the low-activity variant you are predisposed to violence. This variant became known as the warrior gene.
Researchers believe that about 30% of all men have this so-called warrior gene, but whether the gene is activated or not depends crucially on what happens to a person during childhood. The “warrior gene” is most likely to be triggered in people who were mistreated or abused in childhood.
The so-called warrior gene theory has been used only twice as a defense to murder in the United States. In seven other cases, the theory was introduced as a mitigating factor in sentencing.
Jurors had convicted Anthony Blas Yepez in 2015 for the death of the 75-year-old man who raised Yepez’s girlfriend. Evidence showed that Yepez killed the man, George Ortiz, after Ortiz hit Yepez’s girlfriend during a night of drinking. Yepez and Ortiz had struggled during the fight, and Ortiz’s recliner fell over. When Ortiz appeared to be dead, Yepez set fire to his body.
Yepez had the “warrior gene,” which lowers activity of the MAOA enzyme that breaks down serotonin and other neurotransmitters. A normal level is 5; Yepez’s level was about 1.
A jury convicted Yepez of second-degree murder and sentenced him to 22 years in prison. He appealed his conviction on the grounds the trial court mistakenly excluded evidence that the warrior gene predisposed him to violence. The New Mexico Supreme Court is now considering whether evidence of the genetic variation should have been allowed. If he wins his appeal, Yepez would get a new trial.
In another case, Bradley Waldroup shot his wife’s friend eight times, killing her. Then he attacked his wife with a machete, although the wife survived. His defense team discovered that Waldroup had the low-activity variant of the monoamine oxidase A (MAOA) gene. At trial a forensic scientist testified that Waldroup’s genetic makeup, combined with the abuse he had experienced as a child, left him at greater risk of violent behavior. In a shock verdict the jury had convicted him of voluntary manslaughter, and not first-degree murder as most expected.