“The most disturbing facet of this ubiquitous childhood disorder is an utter lack of empathy,” Mateo said. “These people-if you can even call them that-deliberately violate every social norm without ever pausing to consider how their selfish behavior might affect others. It’s as if they have no concept of anyone but themselves.”

“The depths of depravity that these tiny psychopaths are capable of reaching are really quite chilling,” Mateo added.

For Truman Capote the outcome of his sojourn in west Kansas was decidedly mixed. In Cold Blood, which he immodestly heralded as a new form of non-fiction novel, was received with delirious approval; Norman Mailer dubbed Perry as one of the great characters in American literature. The book earned its author more than $2m, which he used to buy homes in Manhattan, the Hamptons, Palm Springs and Switzerland. But by all accounts such heavenly success also went to his head, and contributed to his downward spiral in a haze of lavish parties, drink and drugs. He failed to write another substantial work, and died in 1984.