In a celebrated 2003 case published in the Archives of Neurology, for example, a 40-year old Virginia school teacher with no history of pedophilia developed a sudden interest in child pornography and began making sexual overtures to his stepdaughter. His wife reported his behavior and he was arrested and assigned to a 12-step program for sex offenders. He flunked out of the course—he couldn’t stop propositioning staff members—and was sentenced to prison. Only a day before he was set to surrender, however, he appeared in a local emergency room with an explosive headache and a range of other neurological symptoms. Doctors scanned his brain and found a tumor the size of an egg in the right orbitofrontal cortex, the region that processes decision-making and other so-called executive functions. The tumor was removed and the compulsive sexuality vanished along with it. Less than a year later, the tumor returned—and so, almost in lockstep, did his urges.

“There’s no one spot in the brain for pedophilia,” says Stephen J. Morse, professor of both law and psychiatry at the University Of Pennsylvania. ”but damage to the orbitofrontal region is known to be associated with disinhibition. We know that various forms of brain damage can contribute to difficulties in being guided by reason.”

Other, more recent studies, are finding roots of criminality in other parts of the brain. As’s Maia Szalavitz reported in April, a team of researchers led by Kent Kiehl, associate professor of psychology at the University of New Mexico, published a study in the Proceedings of the National Academy of Sciences in which the brains of 96 male felons sentenced to at least a year in jail for crimes including robbery, drug dealing and assault, were scanned in a functional magnetic resonance imager (fMRI). While they were in the fMRI, the men performed a task that required them to hit a key on a computer when they saw the letter X on a screen, but refrain when they saw the letter K. Since the X appeared 84% of the time and since the two letters look awfully similar to begin with, it was easy to get into the habit of over-clicking.