Although highly controversial, Hervey Cleckley's Mask of Sanity provides one of the most influential clinical descriptions of psychopathy in the 20th century. At the crux of his argument, Cleckley claims that many psychopathic personalities go undiagnosed because they maintain a social mask that conceals their mental disorder and enables them to blend in with society. Furthermore, many of these affected individuals appear to function normally in accordance with standard psychiatric criteria.