Serial killers commit heinous acts that are unimaginable to the average human being.
Identifying the motive behind acts of violence such as murder is something doctors have researched for decades. Motives aren't always clear, and often, victims have no prior relationship with the killer.
Serial killers are defined as people who have murdered at least three people, with a period of time in between, according to the Federal Bureau of Investigation (FBI). The FBI also notes that with these crimes, there is usually an indication that the killer felt dominance over their victim.
The unidentified Jack the Ripper is often considered the first modern serial killer. Jeffrey Dahmer and Ted Bundy are others that have become the subject of many true-crime podcasts and conversations, books and movies.
John Wayne Gacy, The Zodiac Killer, H.H. Holmes, Samuel Little and Dennis Rader are also among some of the most notorious serial killers in history.
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Often, serial killers have a history of trauma or neglect, according to Psychology Today.
Engagement in delinquent behavior is common, specifically having a fascination with setting fires and harming or torturing animals.
While having an anti-social personality can be a trait serial killers display, a common myth is that all serial killers are loners. However, this is not always the case.
Historically, there have been serial killers who maintain relationships with their friends, have families of their own, own a home, hold a job and are active members of society, according to the FBI.
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Serial killers, as a group, have been diagnosed with a variety of different personality disorders, according to the FBI. Some serial killers have displayed symptoms of psychosis, according to Psychology Today.
"Psychosis refers to a collection of symptoms that affect the mind, where there has been some loss of contact with reality," according to the National Institute of Mental Health. "During an episode of psychosis, a person's thoughts and perceptions are disrupted and they may have difficulty recognizing what is real and what is not."
Others are diagnosed with severe bipolar disorder. The disorder is "a mental illness that causes unusual shifts in a person's mood, energy, activity levels, and concentration," according to the source.
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Some serial killers display traits that are consistent with psychopathy, which is defined as "a neuropsychiatric disorder marked by deficient emotional responses, lack of empathy, and poor behavioral controls, commonly resulting in persistent antisocial deviance and criminal behavior" by the National Institutes of Health. The FBI notes that serial killers commonly feel a lack of remorse or guilt, impulsivity, the need for control and predatory behavior, which are consistent with the disorder.
Even though there are cases of mental illness present in certain cases, most serial killers are not considered insane under the law, according to the FBI.
