Many personality disorders have been associated with criminal behavior and serial murder.

The simple explanation of serial murders is not always the correct one.

When the topic of serial murder comes up, almost reflexively, the diagnosis of psychopathic personality is given as an explanation for the offender’s behavior. Question: "Why did he kill all these people?" Answer: "He’s a psychopath." It seems that once it is proclaimed that the serial killer is a psychopath, everything is understood. This assertion has gained such widespread acceptance that its validity is never questioned. But are all serial killers really psychopaths? The answer is not simple.

Psychopathic personality has never been an official diagnosis in any of the editions of the Diagnostic and Statistical Manual of Mental Disorders (1). As a result, there has never been a generally agreed-upon definition of this disorder. In fact, Schlesinger and Ben Ari (2) have documented at least 23 different definitions of psychopathy.

Perhaps the work with the most impact in understanding psychopathic personality is that of Cleckley (3), in his iconic text The Mask of Sanity. Cleckley believed that psychopaths wear a mask (or façade) of hyper-normality to cover up their deep disturbance, which is primarily a lack of interpersonal attachment. About 50 years later, Hare (4) developed a tool (the Psychopathy Checklist) designed to assess the presence of psychopathy in individuals. Here, the evaluator rates subjects on various traits and behaviors and derives a number or a score that indicates how psychopathic the person is. This might appear to be an objective measure of psychopathy; however, the ratings are nothing more than a series of subjective impressions (with some guidelines) that make objectivity more apparent than real (5).

To add to the confusion, other personality disorders have been associated with criminal behavior and serial murder. Sociopathic personality—a term that is often used interchangeably with psychopathic personality—is the disorder ascribed to individuals whose behavior does not conform to societal or cultural standards.

Antisocial personality disorder has been the DSM diagnosis (since 1968) for individuals whose behavior brings them repeatedly into conflict with society; they display traits such as selfishness, callousness, irresponsibility, impulsivity, and inability to feel guilt or to learn from experience. Individuals with narcissistic personality disorder have a strong need for admiration, a grandiose sense of self-importance, a preoccupation with attaining unlimited success, and a belief that they are unique in some special way—this disorder is also sometimes associated with criminals and murderers. The diagnosis of unspecified personality disorder is given to individuals with traits and characteristics associated with several different personality disorders.

What does the empirical research show with respect to all these different personality disorders and their connection to serial murderers? According to a 2026 search of the previous 20 years, using the Medline Complete database of the National Library of Medicine, the most common diagnosis of serial killers, according to the research, is that of antisocial personality disorder, not psychopathic personality. The second most common personality disorder is psychopathic personality; however, many of the research studies did not provide an operational definition for that term. They used different methods to assess psychopathy, including the Psychopathy Checklist, or they assumed individuals were psychopaths because they “lacked empathy and remorse, had charisma, and displayed deceptiveness and egocentricity.”

Two empirical studies of serial murderers cited a diagnosis of sociopathic personality, with one study arriving at the diagnosis because the offenders could not differentiate good or bad at “an emotional level.” Several researchers found narcissistic personality among their sample of serial killers and relied on DSM diagnostic criteria, on prior court documents, or on general observations. And a few research studies listed a personality disorder, but did not note any specific traits or characteristics.

“Are all serial killers psychopaths?” The answer is no—not all of them. Serial murder is a rare and extraordinarily complex criminal behavior, with many subtypes encompassing different behavioral patterns, motives, backgrounds, and crime scene behaviors. The desire to make things seem simple is understandable; in many cases, such as this one, the simple explanation is not always correct.

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, published periodically from 1952 through 2022. The DSM is a standard reference classifying mental disorders and associated criteria that are generally accepted in the mental health field. Its purpose is to facilitate reliable diagnoses.

Schlesinger, L.B. & Ben Ari, T. (2023). Personality disorders and criminal behavior. In M.M. Houck (Ed.). Encyclopedia of Forensic Sciences (pp. 113-143). New York: Elsevier.

Cleckley, H. (1976). The Mask of Sanity. St. Louis: Mosby, (Work originally published in 1941).

Hare, R.D. (1991). Manual for the Revised Psychopathy Checklist. Toronto: Multi-Health Systems.

Litwack, T.R. & Schlesinger, L.B. (1999). Dangerousness risk assessments: Research, legal, and clinical considerations. In A.K. Hess & I.B. Weiner (Eds.). Handbook of Forensic Psychology (2nd ed., pp. 171-217). New York: Wiley.

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