Fallon says nobody in his family has real problems with those behaviors. But he wanted to be sure. Conveniently, he had everything he needed: Previously, he had persuaded 10 of his close relatives to submit to a PET brain scan and give a blood sample as part of a project to see if his family had a risk for developing Alzheimer’s disease.

After learning his violent family history, he examined the images and compared them with the brains of psychopaths. His wife’s scan was normal. His mother: normal. His siblings: normal. His children: normal.

“And I took a look a look at my own PET scan and saw something disturbing that I did not talk about,” he says.

What he didn’t want to reveal was that his orbital cortex looks inactive.

“If you look at the PET scan, I look just like one of those killers.”