Despite the recent publication of excellent histories of criminology, our understanding of the origins and evolution of criminological knowledge remains fragmentary. We know little about the development of the concept of psychopathy, for example, even though it is one of the oldest and most enduring of all criminological ideas; nor do we have much sense of how psychopathy theory affected the development of criminology as a field of study. This article analyzes the meanings of `psychopathy' in the first US texts that used the term, works published about 1915 to 1925. Locating the evolution of criminology in the context of the history of the professions, I explain why the concept of psychopathy attracted early 20th-century criminologists, how they used it and how it affected their work.