In the 1870s, something very strange happened to the French-Canadians working as lumberjacks in Northern Maine. When startled, they would jump in the air, imitate those around them, and even obey random commands. This was the Jumping Frenchmen of Maine disorder.
There's no neurological disorder quite as infamous as psychopathy, and yet figuring out exactly what goes on in a psychopath's brain is extraordinarily difficult. We've now got an answer...and all it took was scanning the brains of forty medium-security prisoners.