Dr. Hervey Milton Cleckley (1903 - January 28, 1984) was an American psychiatrist and pioneer in the field of psychopathy. His book, The Mask of Sanity, originally published in 1941, provided the most influential clinical description of psychopathy in the 20th Century. The term "mask of sanity" derived from Cleckley's observations that, unlike people with major mental disorders, a "psychopath" can appear to be normal and even engaging, while typically not suffering overtly from hallucinations or delusions. However, the "mask" covered a concealed psychosis.