The New Mexico Supreme Court ruled last month that Anthony Yepez, a criminal defendant convicted of second-degree murder, could not claim that a specific gene, known as MAOA, which has been linked to violence in some studies, rendered him unable to control his violent behavior, but failed to relay the view of contemporary experts in the field that the very idea of a “warrior gene” is based on obsolete science, reports the Washington Post. Such a ruling might have influenced other courts that either are, or soon will be, forced to confront this issue and stamp out an outmoded idea about criminality. A Dutch study published in 1993 examined the violent tendencies of one specific family with a history of aggressive outbursts and implicated MAOA, which encodes an enzyme that breaks down brain chemicals that had previously been associated with the regulation of aggression in animals and humans, and concluded that a low-acting version of the gene was associated with violence.
In 2002, another study concluded that only those with low-acting MAOA who suffered from childhood maltreatment were prone to violence. Lawyers latched on to both to defend clients who they now could say acted according to bad genetics instead of bad decisions. At least 11 instances have been identified where a defense team sought to introduce “warrior gene” evidence either to make a “not guilty” verdict more likely or to push for a conviction for a lesser crime to mitigate punishment. However, the end for this kind of work came with the introduction of whole-genome sequencing and analysis which revealed an entirely new picture about the relationship between genetic variation and behavior and showed that thousands of gene variants each contribute a very small amount to explain why one person differs from the next for a particular behavior. With its decision in State v. Yepez, the New Mexico Supreme Court failed to set the legal record straight and, as a result, puts more pressure on scientists and scientifically informed lawyers to spread the word that there is no gene — or combination of genes — yet identified that predisposes people to violence.