A key exception: The brains of people who exhibited anti-social behavior as teenagers but not as adults showed no such abnormalities. That’s good news for reformed bullies, but bad news for the lifers.

“Most people who exhibit antisocial behaviour primarily do so only in adolescence, likely as a result of navigating socially difficult years, and these individuals do not display structural brain differences,” Carlisi said. “It is also these individuals who are generally capable of reform and go on to become valuable members of society.”

What remains hazy is the question of causation — are the brains of bullies small because they’re bullies, or do they become bullies because of their small brains?