The Mask of Sanity is a book written by Hervey Cleckley, M.D., first published in 1941, describing the clinical interviews of Cleckley with incarcerated psychopaths. It is considered a seminal work and the most influential clinical description of psychopathy in the 20th century. The basic elements of psychopathy outlined by Cleckley are still relevant today. The title refers to the normal "mask" that conceals the mental disorder of the psychopathic person in Cleckley's conceptualization.
Cleckley describes the psychopathic person as outwardly a perfect mimic of a normally functioning person, able to mask or disguise the fundamental lack of internal personality structure, an internal chaos that results in repeatedly purposeful destructive behavior, often more self-destructive than destructive to others. Despite the seemingly sincere, intelligent, even charming external presentation, internally the psychopathic person does not have the ability to experience genuine emotions. Cleckley questions whether this mask of sanity is voluntarily assumed to intentionally hide the lack of internal structure, or if the mask hides a serious, but yet unidentified, psychiatric defect.
An expanded edition of the book was published in 1982, after the DSM, the manual used in the United States for categorizing psychiatric disorders, had changed the name and standards for the classification of psychopathy to antisocial personality disorder, incorporating most of Cleckley's 16 characteristics of a psychopath listed below. The original edition of the book is no longer available.
In the 1800s, Philippe Pinel first used the French term manie sans delire ("mania without delirium") to designate those individuals engaging in deviant behavior but exhibiting no signs of a cognitive disorder such as hallucinations or delusions. Although the meaning of the term has changed through numerous writings on the subject over time, the writing of Cleckley and his use of the label "psychopath" in The Mask of Sanity brought the term into popular usage.
The first edition of the book was based on Cleckley's clinical observations of adult male hospitalized "psychopaths". Additional experience with expanded populations led to subsequent editions of the book. The Mask of Sanity, fourth edition, presents the clinical data gathered on an expanded population of subjects, written in the form of dramatic, novelistic descriptions of 13 individuals, an amalgamation of the thousands he has interviewed.
In these vignettes, Cleckley presents a typical psychopath's behavior: for example, the psychopath's ability to tell vivid, lifelike, plausible stories that are completely fraudulent, without evincing any element of delusion. When confronted with a lie, the psychopath is unbothered and can often effortlessly pass it off as a joke. In another typical case history, the psychopath is psychiatrically hospitalized, but because of his constant trouble-making, leaving wards in an uproar, the hospital is finally forced to turn him over to the police. Eventually, the police become so sick of his repeated antics that they try to get him out of their way and hospitalize him again.
Cleckley introduced 16 behavioral characteristics of a psychopath: 
Some of the criteria have obvious psychodynamic implications, such as a lack of remorse, poor judgment, failure to learn from experience, pathological egocentricity, lack of capacity for love, a general poverty in major affective reactions, and lack of insight into his own condition. Starting in 1972, newer editions of the book reflected a closer alliance with Kernberg's (1984) borderline level of personality organization, specifically defining the structural criteria of the psychopath's identity integration, defensive operations and reality testing.
Cleckley also introduced the term "semantic dementia" (used today to refer to a medical disorder unconnected to Cleckley's meaning) to refer to the inability of this personality type to "understand the meaning of life as lived by ordinary people". Behind the mask of sanity, the psychopath's "emotional mechanism had collapsed".
In summary, Cleckley clearly distinguishes the psychopath from other disorders such as neurotic alcoholics, psychoneurotics, criminal sex offenders and typical criminals. The psychopath does not suffer from any obvious mental disorder. Cleckley characterizes the psychopath as, despite apparent intelligence, seeming to deliberately court failure and disaster for no obvious reason, what Cleckley calls a social and spiritual suicide, or semi-suicide. For example, the classic alcoholic drinks to avoid reality, while the psychopath drinks simply to get into trouble.
Cleckley is very clear that there are important distinctions between the psychopath and the average criminal:
Cleckley states that although a "considerable proportion" of inmates in penal institutions show indications of a psychopathic disorder, only a small proportion of typical psychopaths are likely to be found in an incarcerated environment.
As in his first edition, Cleckley makes no claim to offer an effective treatment for the condition in the 1982 edition.
One reviewer describes the author's viewpoint as presenting a paradox in that Cleckley's "keen clinical observations" are not integrated into a meaningful psychological model. Cleckley questions the usefulness of psychoanalytic approaches, while at the same time he uses some psychoanalytic explanatory concepts. The rich clinical detail is not developed into a systematic psychological theory.
The label "psychopath" as used by Cleckley had been embraced by popular culture, and is often applied to serial killers and other violent criminals, irrespective of whether they qualify (although most serial killers do); for this reason the imprecise popular use had been deplored. Therefore, although in popular culture the term is common, it had little relevance to criminology, forensic psychology or psychiatry.
Another drawback of the term is that there exists no international agreement on the diagnosis of psychopathy. It appears to be an "infinitely elastic, catch-all category".
Robert Hare developed a Psychopathy Checklist based on the psychopath construct developed by Cleckley. Later two items were removed from the checklist in order to more clearly represent the structure of a two-factor analysis.
The DSM-III committee, in attempting to develop a trait-oriented basis for the antisocial personality diagnosis, made efforts to combine the work of Lee Robbins's 1966 criteria of behavioral acts with trait items based on the work of Cleckley, as his list of core traits still remains relevant.