Criminal defendants still cite a ‘gene for violence.’ It doesn’t exist.

The New Mexico Supreme Court ruled last month on an extraordinarily important question: Should a criminal defendant be allowed to argue that a specific gene rendered him unable to control his violent behavior? The court concluded the answer was no, in this instance: It upheld the conviction for second-degree murder of Anthony Blas Yepez, who killed a man in 2012. Yepez had sought at trial to introduce evidence that he had what’s been called the “warrior gene” — a version of a gene known as MAOA, which has been linked to violence in some studies. The district court of Sante Fe County excluded expert testimony on that subject; later, the New Mexico appellate court ruled that the exclusion was a mistake but that it did not affect the outcome of the trial. The New Mexico Supreme Court has now found that the district court’s original rejection of the evidence was reasonable.

The state Supreme Court, however, missed an important opportunity: It did not go nearly far enough in batting down the scientifically suspect claim that there is a gene...