The Dutch researchers have pursued the gene since 1978, when one of the men’s sisters came to the University Hospital in Nijmegen to seek advice about having children. She described her family’s problem as mental retardation, not violent behavior. But when geneticist Han Brunner started studying the affected men, he found that low intelligence was not their main problem. Their IQs were nearly normal, but they shared a marked inability to control their impulses. Tracing the family back five generations, Brunner found 14 men who fit that profile, but not a single woman. Though no one yet knew whether the problem was hereditary, that pattern suggested a possible mode of transmission. Because men have only one X chromosome, they’re especially vulnerable to any defect it contains. (Women, with two X’s, can carry a defect on one of them without suffering any effects.) If this syndrome involved a bad gene, the X chromosome was the obvious place to look for it.
The researchers started analyzing blood samples, but because the X chromosome contains volumes of genetic material, it took them a decade to find the relevant mutation. According to the new study, the men’s problems stem from a tiny defect in the gene that enables the body to produce an enzyme called MAOA (monoamine oxidase A). MAOA breaks down the molecules that transmit signals within the brain. Because men with the mutation don’t produce the enzyme, their brains are presumably flooded with transmitters. No one can say with certainty that those chemicals provoke the men’s strange actions, but previous studies have linked transmitters called serotonin and noradrenaline to aggressive behavior. And there’s no question that the affected men fail to break such substances down. When the researchers studied 17-men from the cursed family, each of the five with behavioral problems exhibited an MAOA mutation–and each had unprocessed neurotransmitters in his urine.
However eye-popping, the new findings don’t imply that antisocial behavior is a simple matter of heredity. Obviously, a rare familial disorder can account for only a small fraction of the mayhem in the world. Even within the affected family, inheriting the gene doesn’t guarantee a life of violent crime. Though all the known carriers have had problems, some have fared better than others. At least one carrier has managed to keep a job and a family. As Ropers observes, “There are people who feel the urge to be violent, but are able to control it.” Still, as the new study makes clear, heredity can skew the odds against you.
