The Hare Checklist turned a vague label into a scored diagnostic tool

Ronson centers much of the book on the Hare Psychopathy Checklist, created by criminal psychologist Robert Hare, which scores individuals across twenty traits — including glib superficial charm, grandiose self-worth, pathological lying, lack of remorse, and impulsivity — each rated on a 0-to-2 scale, with a total score above a set threshold (commonly cited around 30 out of 40) indicating likely psychopathy.

Before Hare's checklist, psychopathy diagnosis was largely impressionistic, varying wildly between clinicians. The checklist gave the field something it lacked: a standardized, trainable, replicable way to assess a specific cluster of traits, which is a real scientific advance and has become widely used in forensic and correctional settings.

But Ronson quickly complicates any comfort in this precision: the checklist measures a pattern of traits rather than detecting some single underlying essence, and applying it well requires real clinical judgment — judgment that, he shows throughout the book, is far more fallible and subjective in practice than the checklist's clean numeric score would suggest.

Takeaway: a scored checklist feels objective, but scoring a person still requires a fallible human judgment call at every item.