Recipe for Murder: What makes a serial killer?

Ted Bundy. Ed Gein. John Wayne Gacy. Jeffrey Dahmer.

Do these names ring a bell? You guessed it—these are some notorious serial killers. To some extent, we could say that serial killer stories fascinate us. The cold-blooded, mad genius of a criminal who goes into unimaginable scrutiny to take lives has been the focus of a whole genre. 

In reality, serial killers are ruthless people—they challenge the concept of humanity. Cruel beyond measure, they seldom show any empathy or remorse. This article attempts to do what has escaped forensic psychologists for decades: we aim to understand the rationale behind these monsters. We visit clinical, developmental, and forensic psychology to answer the ultimate question: what makes a serial killer?

Series like “Mindhunter” exploit the fascination we have with serial killers. Taken from Deadline.

What, exactly, is a ‘serial killer’?

Strictly speaking, a serial killer is a criminal that has committed at least three murders with a cooldown period in-between. It’s not only about the number of victims; they kill as a habit. Generally, the victims share common features or, at the very least, motifs. Although there are cases of random killings, it is more likely to see patterns of a modus operandi in the way a serial killer acts. 

As methods differ, so do motivations. There are multiple classification methods for serial killers, but four common types stand out:[1]

  • Sexual sadists: driven by the pleasure of power, domination, humiliation, torture, and sexual needs (e.g. John Wayne Gacy).
  • Delusional killers: harbour psychotic or delusional thoughts to rid the world of ‘undesirable’ people (e.g. David Berkowitz).
  • Custodial killers: also called “angels of death”, typically healthcare members who kill vulnerable people under their care (e.g. Charles Cullen).
  • Utilitarian killers: driven by financial or material gain, although anger or revenge may be involved (e.g. Dr H. H. Holmes)

This list is not exhaustive for two reasons. First, serial killers are so few and scarce that it is very difficult to study them. Second, each case is unique. Seeking common elements ignores the differences that make them dangerous. What is consistent across all of them, however, are the risk factors associated with serial killing. 

Raising a Murderer

Research has found that childhood trauma, like abuse, relates to serial killers.[2] A study compared murderers with the general public. They found that the former were up to 25 times more likely to have experienced some form of abuse.[3] This does not imply that every abused child will grow to become a real-life Hannibal Lecter. Rather, it means that criminals will likely have a history of violence behind them.

Children subjected to verbal, psychological or physical violence can grow up to become violent adults. Taken from Mental Help.

Why are childhood experiences so important? The ‘cycle of violence’ hypothesis states that aggression begets aggression.[4] When a child suffers abuse or neglect, they are more likely to replicate that behaviour as adults.[5,6]  This has a cognitive and social explanation.

Cognitive psychology says that our knowledge about the world is (mostly) shaped by experience. This includes our self-schema (what we think of ourselves) and our social schema (what we think of others).[7] When a child experiences violence, their social schema ‘accommodates’ violence as the norm. They might learn to suppress empathy, which can atrophy the brain areas involving emotional impulses. Children might learn that violence is the right way to face problems. The result? An angry adult who resorts to aggression when things don’t go their way. 

Social psychology suggests that human beings (particularly children) learn through observation.[8] We adopt conducts by looking at other people and imitating them. So, when a child sees their parents acting violently on the regular, they equate violence to problem-solving. They’ll repeat those behaviours, even unconsciously. In extreme cases, that aggressiveness can turn into harmful actions done to others, like murder.

The Killer Gene

If you watch Riverdale, you might be familiar with the term ‘killer gene’. The real-life version, monoamine oxidase A (MAOA), may predispose people to engage in violent behaviour if the gene is defective.[9,10] The research on this relationship is so extensive that an Italian court reduced the sentence of a man accused of murder because he had a mutated MAOA gene

Bradley Waldroup was another case of a murderer benefiting from the “faulty genes” defence. Taken from Times Free Press.

The explanation involves a bit of chemistry. The MAOA gene impacts the processing of dopamine and serotonin, neurotransmitters linked to pleasure and happiness. A mutation in the gene or a decrease in its expression hinders the effect of these substances in our brain. Moreover, people with low-functioning MAOA genes are hypersensitive—they are more affected by negative experiences and prone to overreact violently.[11] 

Some ethically dubious experiments have shown that MAOA-knockout mice (i.e. deactivated gene) showed more aggressive behaviour than healthy rodents.[12] Another study found that a mutation in the MAOA gene across different generations in a Dutch family resulted in a history of recurrent violent, criminal behaviour.[13]

Nature and Nurture

So, between nature (gene) and nurture (childhood abuse), which one truly paves the way to a life of murder? The answer is… both. Psychology coins this a gene-environment interaction.[14] Think of it this way: your genes lay out a blueprint of yourself. Your environment, actions, and experiences determine how that blueprint translates into your actual self. For example, the MAOA gene could be harmless but, coupled with a violent childhood, it could prompt you to a more aggressive lifestyle. Perhaps, a killer one. 

A 26-year-long study successfully captured the essence of this gene-environment interaction. From 1972 until 1998, researchers observed 255 children. Those who had experienced abuse and had the killer gene were most likely to develop antisocial and criminal behaviour in their adulthood. This was compared to those who only had experienced childhood abuse. Additionally, they were predicted to be almost 10 times more likely than non-abused children with the killer gene to be convicted of violent crime by the age of 26. The killer-gene-and-abusive-family group accounted for 44% of the cohort’s convictions, despite being only 21% of the total sample.[15] 

Leatherface from “Texas Chainsaw Massacre” had a violent childhood. We don’t have the results of his DNA tests, sadly. Taken from SyFy.

The gene-environment perspective has essentially garnered the most support in the nature vs. nurture debate, as this isn’t the only study to highlight the interaction between gene and environment.[16]


Serial killers are unique by nature. Most of them have a history of abuse during their childhood. Others have a defective or absent version of the MAOA gene. Most, however, seem to fall in both categories simultaneously. 

In reality, there isn’t one defining factor that makes someone a serial killer. We can talk about genes and social learning as much as we want, but as we said earlier, this is a question that has dodged forensic research for a long time. While some individuals may be more likely to turn into murderers than others, a completely sane person could become a monster under the right circumstances. It is far-fetched, but not impossible.

In the end—as disheartening as it sounds—turning into a Ted Bundy is within the average person’s reach. In the words of Heath Ledger in The Dark Knight: “All it takes is a little push”.

Academic References

  1. Miller, L. (2014). Serial killers: I. Subtypes, patterns, and motives. Aggression and Violent Behavior, 19(1), 1-11.
  2. Marono, A. J., Reid, S., Yaksic, E., & Keatley, D. A. (2020). A behaviour sequence analysis of serial killers’ lives: from childhood abuse to methods of murder. Psychiatry, psychology and law, 27(1), 126-137.
  3. Mitchell, H., & Aamodt, M. G. (2005). The incidence of child abuse in serial killers. Journal of Police and Criminal Psychology, 20(1), 40-47.
  4. Widom, C. S. (1989a). The cycle of violence. Science, 244(4901), 160-166.
  5. Curtis, G. C. (1963). Violence breeds violence—perhaps?. American Journal of Psychiatry, 120(4), 386-387.
  6. Dutton, D. G., & Hart, S. D. (1992). Evidence for long-term, specific effects of childhood abuse and neglect on criminal behavior in men. International Journal of Offender therapy and comparative criminology, 36(2), 129-137.
  7. Dodge, K. A. (1993). Social-cognitive mechanisms in the development of conduct disorder and depression. Annual review of psychology, 44, 559.
  8. Bandura, A., & Walters, R. H. (1977). Social learning theory (Vol. 1). Englewood Cliffs, NJ: Prentice-hall.
  9. McDermott, R., Tingley, D., Cowden, J., Frazzetto, G., & Johnson, D. D. (2009). Monoamine oxidase A gene (MAOA) predicts behavioral aggression following provocation. Proceedings of the National Academy of Sciences, 106(7), 2118-2123.
  10. Widom, C. S., & Brzustowicz, L. M. (2006). MAOA and the “cycle of violence:” childhood abuse and neglect, MAOA genotype, and risk for violent and antisocial behavior. Biological psychiatry, 60(7), 684-689.
  11. Sohrabi, S. (2015, January). The criminal gene: The link between MAOA and aggression. In BMC proceedings (Vol. 9, No. 1, pp. 1-1). BioMed Central.
  12. Shih, J. C., Chen, K., & Ridd, M. J. (1999). Monoamine oxidase: from genes to behavior. Annual review of neuroscience, 22(1), 197-217.
  13. Brunner, H. G., Nelen, M., Breakefield, X. O., Ropers, H. H., & Van Oost, B. A. (1993). Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A. Science, 262(5133), 578-580.
  14. Ottman, R. (1996). Gene–environment interaction: definitions and study designs. Preventive medicine, 25(6), 764.
  15. Caspi, A., McClay, J., Moffitt, T. E., Mill, J., Martin, J., Craig, I. W., … & Poulton, R. (2002). Role of genotype in the cycle of violence in maltreated children. Science, 297(5582), 851-854.
  16. Tiihonen, J., Rautiainen, M. R., Ollila, H. M., Repo-Tiihonen, E., Virkkunen, M., Palotie, A., … & Saarela, J. (2015). Genetic background of extreme violent behavior. Molecular psychiatry, 20(6), 786-792.

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