Robert D. Hare, an eminent researcher in the field of criminal psychology and developer of the Hare Psychopathy Checklist used to rate a person’s psychopathic or antisocial predisposition, was responsible for some of the most foundational understandings of psychopathy. The term [Page 1413]psychopathy dates back to the 19th century, but its use between that time and World War II was quite rare. Psychopathy as a concept evolved slowly over the following decades, with mental health professionals coming to understand and even slowly accept that the term encompasses a wide range of features. As professional understandings of the term and condition developed over time, the terms psychopath and sociopath were substituted for one another.