Is being a serial killer genetic?
Another request from a reader, who asked if I could look at the genetics of serial killers. I knew this would be an interesting one, not necessarily for the science but for the ethical questions that come with it. Does being genetically prone to violent acts mean ‘it’s not their fault’ and we should be lenient on them? Or conversely, does it mean they are more likely to reoffend and thus need to be locked away for the good of society? There was a famous case in Italy where a judge reduced the sentence of a murderer because he had a few gene variants associated with violent behaviour.1 On the other hand, some courts in the USA used genetic evidence to argue for longer sentences.2 My view is that both are wrong, and apart from legitimate serious mental illnesses, such as schizophrenia, people should be judged on their actions, not their genes. Plus, it turns out the science behind it not as strong as those judges obviously believe it is.
Googling ‘serial killer gene’ brings up lots of results for a gene called MAOA (monoamine oxidase A.) MAOA degrades neurotransmitters including noradrenalin, dopamine and serotonin. Very rare mutations in MAOA that effectively stop it from working, cause what is known as Brunner syndrome, named for the scientist who described the syndrome. 3 Brunner syndrome is associated with borderline intellectual disability and a tendency for impulsive actions and violent outbursts, with members of the family Brunner, et al. 3 studied arrested for attempted murder, rape, and arson. While Brunner syndrome is exceedingly rare, more common variants that reduce the activity of MAOA, known as MAOA-L variants, have been found to be associated with certain violent crimes, including gang related crimes, shootings and stabbings in multiple studies.4 So it seems like a good candidate for a ‘killer gene.’ But to put things in perspective, one study of adolescents found people with a rare variant (about 1% of the population in this study) that caused very low levels of MAOA were twice as likely to commit violent crimes as those without the variant.5 While that sounds like a large difference, it means if the average person has a say 5% chance of committing a violent crime, someone who carries this gene has a 10% chance. Most of the other studies included a more common MAOA-L variant, one present in about 40% of the population.4 So while MAOA may make someone more likely to be a killer, most people with MAOA-L variants are normal law-abiding citizens. In fact, one study that followed 1000 children to adulthood found that having a MAOA-L variant had no effect on ‘antisocial behaviour’ on its own, but had a large effect in people who were abused in childhood.6
Other genes associated with crime, especially violent crime, including CDH137 and HTR2B,8 are associated with poor impulse control, and indeed the majority of murders committed by participants in the studies were not premeditated.7 So while these may be (a small contribution to being a) ‘killer genes,’ are they ‘serial killer genes’? It may just be pop culture’s influence, but when I think of a serial killer, I don’t think of someone impulsive or with violent outbursts, but someone more cold and calculating who plans not only to do the crime but to get away with it. An extreme psychopath, in other words. So, what does the science say about the genetics of psychopathy?
Well, twin studies show that there is a significant genetic component. In a study of over 600 adolescent twins, Blonigen, et al. 9 found that identical twins were twice as likely to both have psychopathic traits compared to non-identical twins, and estimated the heritability of psychopathic traits to be about 50%. Interestingly, in a study of 7 year old twins, ‘callous-unemotional’ traits appeared to be strongly genetic, while more general ‘antisocial behaviour’ seemed to be more influenced by upbringing. Similarly to MAOA, most specific genes that have been linked to psychopathy are involved in neurotransmitter pathways, including COMT, which regulates a dopamine degrader, the dopamine receptor DRD4, the cannabinoid receptors CNR1 and FAAH, and OXT, the oxytocin gene.10 However, the effects of these genes were usually low, for example the study on DRD4 variants found having two copies of the ‘high risk’ variant increased psychopathy scores by less than 2%. So, it seems that psychopathy is highly polygenetic, with many different genes playing a small role. Of course, I must add that most psychopaths are not serial killers.
So, is being a serial killer genetic? I think it must be, because it seems to me that to do such things, they must be so significantly different in brain chemistry and wiring and so something must have gone wrong at the most fundamental level. But how much is genetic and how much is environmental? It would be very interesting if the genomes of the worst serial killers could be analysed to see if they have unique genetic mutations, or simply a number of the known common variants in combination with the wrong environment.
References
1. Forzano F, Borry P, Cambon-Thomsen A, et al. Italian appeal court: a genetic predisposition to commit murder? Eur J Hum Genet 2010;18(5):519-21. doi: 10.1038/ejhg.2010.31 [published Online First: 2010/03/11]
2. Hunter P. The psycho gene. EMBO Rep 2010;11(9):667-9. doi: 10.1038/embor.2010.122 [published Online First: 2010/09/02]
3. Brunner HG, Nelen M, Breakefield XO, et al. Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A. Science 1993;262(5133):578-80. doi: 10.1126/science.8211186 [published Online First: 1993/10/22]
4. Kolla NJ, Bortolato M. The role of monoamine oxidase A in the neurobiology of aggressive, antisocial, and violent behavior: A tale of mice and men. Prog Neurobiol 2020;194:101875. doi: 10.1016/j.pneurobio.2020.101875 [published Online First: 2020/06/24]
5. Guo G, Ou XM, Roettger M, et al. The VNTR 2 repeat in MAOA and delinquent behavior in adolescence and young adulthood: associations and MAOA promoter activity. Eur J Hum Genet 2008;16(5):626-34. doi: 10.1038/sj.ejhg.5201999 [published Online First: 2008/01/24]
6. Caspi A, McClay J, Moffitt TE, et al. Role of Genotype in the Cycle of Violence in Maltreated Children. Science 2002;297(5582):851-54. doi: doi:10.1126/science.1072290
7. Tiihonen J, Rautiainen MR, Ollila HM, et al. Genetic background of extreme violent behavior. Mol Psychiatry 2015;20(6):786-92. doi: 10.1038/mp.2014.130 [published Online First: 2014/10/29]
8. Bevilacqua L, Doly S, Kaprio J, et al. A population-specific HTR2B stop codon predisposes to severe impulsivity. Nature 2010;468(7327):1061-6. doi: 10.1038/nature09629 [published Online First: 2010/12/24]
9. Blonigen DM, Hicks BM, Krueger RF, et al. Psychopathic personality traits: heritability and genetic overlap with internalizing and externalizing psychopathology. Psychol Med 2005;35(5):637-48. doi: 10.1017/s0033291704004180 [published Online First: 2005/05/28]
10. Frazier A, Ferreira PA, Gonzales JE. Born this way? A review of neurobiological and environmental evidence for the etiology of psychopathy. Personal Neurosci 2019;2:e8. doi: 10.1017/pen.2019.7 [published Online First: 2020/05/22]