Jared Lee Loughner (2011 Tucson mass shooting), James Holmes (2012 Aurora mass shooting), Adam Lanza: Scientists have studied the brain and genome of Lanza, who carried out a massacre at Sandy Hook Elementary School in Connecticut in 2012. (Getty Images/AFP/AP)

Genetic counseling was not commonplace in 1978, but the Dutch woman who came to University Hospital in Nijmegen, the Netherlands, hoped geneticists could help her learn what was wrong with so many of the men in her family, including her own son. They had mild developmental disabilities and, more disturbingly, they were repeatedly violent — responsible for acts of arson, assault, attempted rape and attempted murder. It was such an obvious problem that 16 years earlier, an uncle had drawn up a family tree of male brutality that went back to the 1870s.

That family’s case launched studies that didn’t bear fruit until the early 1990s, when researchers found a specific gene mutation on all of the X chromosomes of all the violent men. Some women in the family had the mutation, too, but since women have two X chromosomes, it appeared that the other one functioned normally and made up for the defect.

In other words, the men appeared to be genetically predisposed to violence.

As Lois Parshley writes in the May/June issue of Popular Science, that was the beginning of modern research into the link between heredity and sociopathic behavior — a pursuit that raises disturbing questions of Nazi-like eugenics as well as hopeful ideas of screening and treatment.

Parshley’s story includes a visit to Kent Kiehl, a psychologist who has been interviewing and brain-scanning imprisoned psychopaths for decades, and recounts how scientists have studied the brain and genome of Adam Lanza, who fatally shot his mother and killed 26 people at Sandy Hook Elementary School in Connecticut in 2012 before committing suicide. She also describes one of the 80 U.S. criminal court cases since 1994 in which lawyers cited genetics as a defense: Bradley Waldroup, who admitted to a shotgun-and-machete bloodbath in which he nearly killed his ex-wife and brutally murdered her friend, got a prison sentence instead of the death penalty after he presented evidence that he had the X-chromosome defect the Dutch had found decades earlier.

One thing Parshley doesn’t do is what is promised on the magazine’s cover: “Can Genes Make You Kill? We Settle This Question Once and For All,” the headline reads. Instead, she makes clear that this is an ongoing and controversial question. Environmental factors are overwhelmingly important , she writes, and quotes Deborah Denno, director of the Fordham University Neuroscience and Law Center: “While genes influence behavior, they do not govern nor determine it.”