A neuroscientist at the UC Irvine School of Medicine, Fallon...first began studying the brains of murderers, rapists, and other criminals in the mid-1990s, when his university got its first PET brain-imaging machine...

Around 2005, Fallon started to notice a pattern in the scans of some of the criminals who were thought to be psychopaths, which led him to develop a theory: All of them appeared to have low levels of activity in a region of the brain located towards its center at the base of the frontal and temporal lobes. Scientists believe this region, called the orbital cortex, is involved in regulating our emotions and impulses and also plays a role in morality and aggression.

One day, Fallon's technician brought him a stack of brain scans from an unrelated Alzheimer's study. As he was going through the scans of healthy participants, they all looked normal — no surprises. But then he got to the last one.

It looked just like those of the murderers.

The identity of the brains in the scans had deliberately been masked so as not to bias the results. But Fallon couldn't leave it alone. "I said, we've got to check the [source] of that scan," Fallon recalled recently to Business Insider. "It's probably a psychopath... someone who could be a danger to society."

Turns out, the image wasn't a scan of just any random participant — it was a scan of his own brain.

...when he went home and told his wife what had happened — how his brain resembled that of a psychopath, at least according to his theory — she reacted very strangely.

It wasn't that surprising, she said.

...Even his daughter thought so — as a young child, she painted her dad as a "dark figure." Fallon's psychiatrist friends described things he had done in the past that they said showed a profound lack of empathy (one of the telltale signs of psychopathic tendencies), like skipping a friend's funeral because he thought it might be boring. His friends and family agreed. "I realized people had been telling me something for years, I just didn't put it together," Fallon said....