Was Adolf Hitler a psychopath? Would he meet the criteria established by modern psychiatry? These were the questions invariably raised by audiences in Germany when I would give talks there about my 2012 book The Wisdom of Psychopaths. Fortunately, I was in a position to answer them with data. In an ongoing study, I had been asking the official biographers of prominent historical figures to fill out, on their subject's behalf, an abbreviated version of the Psychopathic Personality Inventory–Revised (PPI-R). This short psychometric test uses 56 questions to quantify a person's psychopathic personality traits.