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Do All Serial Killers Have a Genetic Predisposition to Kill?

Exploring a complex question.

Wikimedia Commons
Source: Wikimedia Commons

The question "Are serial killers born or made?" inevitably comes up in almost any discussion of the most notorious and most barbaric serial killers in human history (that is, serial killers who not only satisfy the FBI’s definition of a serial killer but who instinctively make us think of them as sadistic monsters—the killers who, in Emily Dickinson’s words, trespass the forbidden boundary between fiction/fantasy and reality.).*

Not unsurprisingly, no one really knows the answer to this question. The most likely answer, though, is that the majority of the most prolific and dangerous serial killers were genetically disposed to behave anti-socially and furthermore grew up in an environment that cultivated a disregard for the lives of others.

*[According to the FBI, in order for a killer to be a serial killer he or she must commit at least 3 murders over a time period of at least 3 years at 3 different locations with an emotional cooling off period in between.]

Antisocial Personality Disorder

Although we do not know whether or to what extent you can be “born” a serial killer of the monstrous kind, we do know that many barbarous serial killers have antisocial personality disorder (not infrequently combined with an inflated ego, or narcissism). Antisocial personality disorder is the clinical term for what colloquially is known as psychopathy, or sociopathy. The disorder is characterized by:

  • A disregard for morals, social norms, and the rights and feelings of others.
  • Exploitation of others in harmful ways for their own gain or pleasure (sadistic tendencies).
  • Manipulation or deceit of others using superficial charm, feigning innocence or disability, or pretending to be working for an admirable cause.
  • A lack of empathy for others and a lack of guilt or remorse about harming others.
  • Explicit or hidden hostility, irritability, agitation, aggression or violence.
  • A lack of fear of dangerous situations and behaviors that often leads to unnecessary risk-taking.
  • A failure to learn from the consequences of their good or bad actions.
  • A history of unstable relationships (including romantic relationships, friendships and relationship with parents).
  • A failure to fulfill work and financial obligations.
  • Recurrent fall outs with authority figures, including law enforcement, and sometimes sanctions, arrest and conviction.

The heritability of antisocial personality disorder is estimated to be 0.38 (or 38 percent). Heritability is the proportion of differences in traits in a population that are due to genetic differences as opposed to differences in the environment. A heritability of 0.38 tells us that, on average, about 38 percent of the individual differences that we observe in degree of “sociability” (or “anti-sociability") are in some way attributable to individual genetic differences. It does not mean that 38 percent of any person's sociability (or anti-sociability) is due to their genes and that the other 62 percent is due to their environment.

So, even if we discover that a notorious serial killer has antisocial personality disorder, this does not shed much light on whether he or she was born or made.

What further complicates giving a reasonably plausible answer to the question of heritability is that most people with antisocial personality disorder are not monstrous serial killers or even criminals. Many people with this personality disorder simply fail as functional human beings—without thereby committing any crimes. Many others thrive in leadership positions in companies, non-profit organizations, universities and even governments that give them control over other people’s lives.

So, even if you have antisocial personality disorder, the chance that you become a serial killer is extremely low, which is to say that the heritability of antisocial personality disorder gives us nearly no insight into whether—or to what extent—serial killers are born or made.

Genetic Factors Unrelated to Antisocial Personality Disorder

What further complicates finding out whether some of the most barbaric serial killers in history were born or made is that antisocial personality disorder (sometimes with a hint of narcissism) is just one potentially inherited characteristic that commonly appears among serial killers. It seems that other, potentially unrelated, genetic factors might also be contributing to a person’s chances of becoming a serial killer.

Studies have shown that a greater percentage of male serial killers have extremely high levels of testosterone in their bodies compared to the average male (Scott, 2000).

Although there may be a connection between high levels of testosterone and antisocial personality disorder, it is likely that the two conditions can exist independently. It is also likely that genetic differences can partially explain differences in testosterone levels. So, if high testosterone can trigger tendencies to become violent, then we have another factor that is partially genetic and that may be a factor in triggering the kind of violence seen in serial killers.

Abnormal levels of the brain chemical dopamine—which is responsible for pleasure and motivation—or the receptors (binding sites) for dopamine, may also be a contributing factor. People who naturally have low levels of dopamine or a low number of dopamine receptors need greater excitement to feel stimulated or motivated. Sometimes only the ultimate thrill can move them (this is a likely scenario for the Zodiac killer).

A common characteristic of serial killers is that they seek the ultimate thrill (as described in letters by The Zodiac killer). Some are motivated by the adrenaline rush of the hunt, the stalking and catching of their victims, and the perfection of their skills; killing for them is a sport (The Zodiac killer, Robert Hansen "Butcher Baker”, Israel Keyes). Others are motivated by the extreme pleasure they get from being in complete control of another person’s life, whether they live or die, when they die, how they die, how they feel when they die, what happens to them after they die, including eating them dead bodies (fx. Jeffrey Dahmer "Milwaukee Cannibal”).

Sometimes the most intense thrill for the serial killer comes from knowing that they are the cause of the fear of the people in the towns where they operate. They get a deep satisfaction from the feeling of being in control of media, law enforcement and citizens (Dennis Rader “Bind, Torture, Kill,” David Berkowitz “Son of Sam").

Although many serial killers rape their victims before killing them (Ted Bundy, Gary Ridgway “The Green River Killer," John Gacy “The killer clown,” Michael Ross "The Roadside Strangler"), it is not usually sexual satisfaction by itself that motivates them but the control they feel when mercilessly taking what is not theirs.

As defects in the regulation of the dopamine system can be genetically determined, this characteristic is yet another potential genetic factor that may contribute to the chances of someone becoming a serial killer.

How to Make a Monster

But sometimes what people who raise the nature-or-nurture question really want to know is not the degree to which genes are responsible for making serial killers but, rather, whether a person who has no genetic predisposition could become a serial killer.

It is unlikely that we will ever have a definite answer to this question. But there are several reasons to think that some serial killers have no genetic predisposition to kill intentionally but are created purely by their environment.

One reason is that the heritability of the exact composition of traits that lead people to kill in malicious ways and on multiple occasions is bound to be extremely low. But if the genes play an insignificant role, this would mean that the wrong environment might suffice for a person to turn into a barbaric killer.

A second reason to think that it is plausible that serial killers are sometimes created by their environment without having the genetic makeup of a killer turns on what we know from interviews with serial killers caught alive.

Many serial killers report having had the fantasy of taking the life of another human being. Initially, they thought that they would never do it (Ted Bundy). They often describe their first strike as thrilling but also extremely nerve-wracking and so intense at the time that they thought they would never do it again.

However, the brain is capable of desensitizing. This is a phenomenon cognitive-behavioral therapists make use of in exposure theory—a technique commonly used to rid people of their phobias. The latter approach makes a person confront his or her fears gradually (e.g. a fear of spiders). The gradual exposure makes the fear neurons less prone to fire intensely when confronted with the fearful stimulus.

But people can become desensitized without going to a therapist. Repeated exposure to something that at first makes you extremely nervous or psychologically affected is likely to trigger you less and less. Notorious serial killers report that this kind of desensitizing happened to them. While the first strike was so intense that they thought it would be their last, each subsequent kill was less frightening to them. It became more and more normal.

For some monstrous serial killers, some degree of desensitizing happens long before their first murder. Ted Bundy was obsessed with sadomasochism/bondage long before he started killing and also experimented with theft. Jeffrey Dahmer "The Milwaukee Cannibal" experimented with dead and living non-human animals before taking up serial killing. Edmund Kemper "‎The Co-Ed Butcher” tortured cats from a young age. Albert DeSalvo, the Boston Strangler, also tortured animals and was known by the police for shoplifting. Charles Albright “The Eyeball Killer” shot small animals, which his mother helped him stuff. Michael Ross “The Roadside Strangler” was stalking women during his sophomore year of college and committed his first rape during his senior year. And, as one last example, Gary Leon Ridgway "The Green River Killer" (presumed to have killed more than 90 people) stabbed a six-year-old boy (who survived the attack) when he was only 16.

One possible contributing environmental factor, then, is desensitizing. But this requires that something motivates the person to start doing horrible things to begin with. We can imagine that a severely abused child starts acting out in extreme ways and then slowly gets desensitized, although this desensitizing is unlikely to be the predominant factor.

The following scenario could make someone a serial killer, even if the person has no genetic predisposition. As a child or young adult, our envisaged person is subjected to severe physical and emotional abuse, which causes him or her to act out in more and more severe ways (examples of killers who were severely abused during childhood include Donald "Pee Wee" Gaskins, Edmund Kemper "‎The Co-ed Butcher," John George Haigh "The Acid Murderer,” Albert DeSalvo "The Boston Strangler," John "Pogo" Gacy “The Killer Clown,” Anthony Sowell “The Cleveland Strangler,” and Colombian serial killer Pedro Lopez ). During the period of acting out or fighting back (perhaps by torturing animals or other children), abused children may experience a kind of control, importance and aim in life--feelings that have been sorely missed during early childhood. This kind of scenario plus desensitizing to more heinous crimes might suffice to make someone a serial killer. This is likely how messed up we humans can become without a genetic predisposition.

Berit “Brit” Brogaard is a co-author of The Superhuman Mind.


Schlecter, H. & Everitt, D. (1997). The A to Z encyclopedia of serial killers. Pocket Books. New York.

Scott, S.L. (2000). What makes a serial killer tick? Crime Library: Online.

Sears, D.J. (1991). To kill again: The motivation and development of serial murder. Scholarly Resources: Wilmington, Delaware

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