Crime Research

The Warrior Gene: Genetics and Criminology

Anumber of research studies claim that a simple gene mutation affecting only men plays a crucial role in violent aggression. Furthermore, this is a gene variant that may be present in 30-40% of the male population. Labelled the warrior gene, a low activity version of this single MAOA gene suggests that nature and genetics may be much more important in why some individuals are violent than we previously thought.

MAOA – The Warrior Gene

The MAOA gene can be found on the X chromosome and is part of a family of genes which deal directly with chemical messengers such as dopamine, serotonin and norepinephrine in the brain. Monoamine oxidase A is an enzyme produced by these genes and research has discovered in rare cases this gene can be missing completely or, individuals can have different variants.

In men, who only have one X chromosome, they will either have a low activity variant known as MAOA-L or a high activity variant, MAOA-H. Women, who have two X chromosomes, are more likely to have at least one high functioning variant and are therefore less affected, although studies on women so far have been few.

A growing body of research suggests that the presence of MAOA-L translates to a vulnerability toward violent behaviour, particularly when provoked or challenged, hence the term the warrior gene. The presence of the warrior gene suggests that these crucial chemical neurotransmitters in the brain are not being mobilized correctly.  Serotonin is a neurotransmitter of particular importance to behaviour as it is most closely associated with stabilizing mood and calming us down in anger.

Research has shown that when serotonin levels are low there is less communication with the emotional centre of the brain, the amygdala. This suggests that people with low levels of serotonin may find it more difficult to control emotional responses to anger in the frontal area of the brain that have been generated within the amygdala.

In fact there have been many arguments within courtrooms over low levels of serotonin and a condition called ‘intermittent explosive disorder’. This is a disorder which is characterized by sudden explosive outbursts of rage often resulting in violence. These explosions are rarely appropriate to the circumstances and can show a great deal of violent aggression to unsuspecting recipients.

The case of Jon Hall for example, in 1994 in Tennessee. Hall was charged with the first degree murder of his wife, Billie Jo Hall. His defence attorneys presented two expert witnesses who testified Hall’s serotonin levels were very low and highlighted the links in scientific research between low serotonin levels and violent aggression.

The prosecution however presented their own expert witness who claimed a measure of serotonin from spinal fluid did not reflect the accurate levels present in synapses in the brain. Furthermore he testified, no ‘normal’ level of serotonin within the population had been confirmed. As a result, all arguments regarding low serotonin levels being part-way responsible for Jon Hall’s behaviour were thrown out as no full theory on intermittent explosive disorder was available. Jon Hall was convicted and sentenced to death.

The defence of an offender having the warrior gene follows a similar theory. If the gene is not as active as it should be, the associated chemical messengers such as serotonin are not functioning correctly. The impact of this on an individual is a lack of impulse control.  The warrior gene theory therefore, relates to more impulsive and sudden acts of violence rather than premeditated and planned acts of violence against another.

Related: Will Advances In Neurocriminology Give Rise To A Criminal Disorder Being Identified?

Genetic Research

It was Dutch Professor Han Brunner who made the discovery of a MAOA gene mutation in 1993, when he studied a Dutch family whose male generations displayed extreme violent aggression. It is this research that can be credited with creating the warrior gene debate. Nine of the males from the family were studied and though biological testing it was found these individuals all had a defect; they were missing the MAOA gene completely.

These males also displayed low IQ levels and were found to be introverted with little social inclusions and social activities. Their histories were littered with frequent violent outbursts with little justification, often occurring during periods of little sleep where they also experienced night terrors. The study found that the males in the family who did have the gene had a normal education, were holding down regular jobs and were not displaying the same kinds of anti-social behaviours as their affected male relatives.

Since this research was first published and picked up by various media sources, there has been an influx of men wanting to know if they have the warrior gene as an explanation for their behaviour.

Certainly in the United States, the opportunity to pay for DNA testing to establish the presence of the warrior gene is available. Quite why men would want to pay for such a test highlights the potential misconception that the presence of MAOA-L means any violent aggression is entirely genetic and out-with their control, essentially removing any blame or responsibility from the individual themselves. Mounting research however, suggests that having the warrior gene alone is not enough to cause any violent behaviour. It is more likely that this is a genetic predisposition that can be triggered by external environmental factors.

The fear is that this MAOA-L gene is being labelled as a ‘violence gene’ – if you have it you are more than likely heading into violent criminality. but the reality is much more complex involving both genetic and environmental factors.  Furthermore, if up to 40% of the population do have the warrior gene as suggested, and it was a cause of violent aggression, much more violence within the general population would be expected.

The Gene-Environment Interaction

Professor of Psychology and Neuroscience, Avshalom Caspi at Duke University, conducted an influential study published in 2002 where he followed research being carried out in New Zealand. From 1972 researchers had been closely monitoring a group of children since birth and through their development as they got older. Specifically, the study looked at maltreatment during childhood and the presence of the warrior gene.

They found that children who had suffered trauma or maltreatment were more likely to develop anti-social behaviour as adults. Those who suffered maltreatment and had the warrior gene showed even higher levels of anti-social behaviour and criminality. The risk of developing such behaviours it seems were already present with just the maltreatment history however, combined with the warrior gene, this risk is considerably higher.

Of the total number of boys studied (422), 12% were found to have both the warrior gene and mistreatment in their childhoods, but this 12% (55) were said to be 10 times more likely to be convicted of the violent crime by the age of 26 years old. Professor Caspi states that even though only a fairly small number of the boys studied had the warrior gene, this small number “accounted for 44% of the cohort’s violent convictions”.

This is not the only study to highlight the role of environment when studying violent aggressive behaviour, suggesting that nature and genetics are only one part of the puzzle.

Related: Sociopathic Children and Psychopathic Behaviour

A study carried out in Finland examined the genes of 900 convicted offenders. Offenders were categorized according to the level of violence in their offences. The research found that those who fell into the ‘extremely violent’ category were the same inmates who also had the warrior gene.

However, the life experiences of these offenders also needs to be taken into account. The fact those particular offenders all had the warrior gene is interesting, but life experiences may have also played a part in their resulting behaviour. There could be many other common factors amongst this group of offenders which has contributed to why they have displayed such violence. Maltreatment, abuse, mental health and, as the study itself highlights, substance misuse are important factors. In Finland, the majority of offenders have a substance misuse history and a combination of substance misuse with the warrior gene may be a better conclusion than attributing the extremely violent behaviour of these offenders entirely to the warrior gene alone.

So Are Criminals Born or Made?

Whether criminals are born or made has long been a topic of debate among criminologists and psychologists. In the 19th century prediction of criminal behaviour came from physical features. Strong jaw lines, heavy eyebrows and projecting ears were the feature of Italian criminologist Cesare Lombroso’s theory on criminals and criminality.

Things have moved on since then and the influence of environmental factors such as upbringing, child abuse, social interactions and substance misuse has been well-established; however it has not ended the curiosity about the biology of criminals and how much such biology, and particularly genetics, are involved in criminal behaviour.

Where such theory and science becomes especially important is in the courtroom when an individual is on trial and, in some cases, where their life is at stake. The case of Bradley Waldroup in 2006 in Tennessee is one where the gene-environment argument was crucial to his outcome. Bradley Waldroup was a man who murdered a friend of his wife’s by shooting her eight times before attacking her with a knife. He then attempted to murder his wife by attacking her with a machete.

There was no doubt he was guilty of murder and attempted murder however, his defence team focused on why he committed the offence and whether his explosion of violence was due to some factors out-with his control.

Analysis revealed Waldroup had the warrior gene putting him at higher risk of reacting violently. Armed with his history of child abuse, his defence presented a man to the jury who had been abused as a child affecting his psychological development as an adult.  Furthermore, he possessed genetics which predisposed him to impulsive violent aggression. The jury opted for a conviction for voluntary manslaughter instead of murder and a sentence of 32 years in prison over the death penalty.

When violent acts are committed, particularly violent crimes such as murder, if the issue of guilt is not in question, the focus often shifts to why a person may have committed such a crime rather than did they commit the crime.

Furthermore, the fundamental question is whether that individual was in full control of their actions at the time of the offence. For a person to be found guilty and convicted in a court of law it must be shown that they were fully aware of what they were doing. The Criminal Justice System is based around the idea that we can easily distinguish between a person who chooses how to behave and those who don’t. Under the insanity defence for example, for the defence to be successful it must be shown that their mental state at the time of the crime was so severe they were not consciously aware of their actions nor could they actively control them at that time.

Behavioural Genetics In The Courtroom

The introduction of behavioural genetics complicates this standard, bringing in extra factors with regards to an individual’s level of control under different circumstances and as a consequence, their level of criminal responsibility.

Related: Inside The Mind of a Psychopath – The Psychopathic Killer

Introducing the warrior gene as evidence in a criminal trial first took place in the United States in 1994 in the case of Stephen Mobley. He was man from an affluent background with no evidence of maltreatment during his childhood. In 1991 when he was 25 years old, he shot and killed the manager of a pizza shop after robbing the days takings. Put on trial for first degree murder, his defence team put family members on the stand testifying to generations of violence amongst the men of the family.

They applied to have genetic testing carried out looking specifically at the warrior gene. However their request was refused by the court on the basis that the research used to justify the request did not meet the requirements for permissible scientific evidence. Stephen Mobley was convicted of murder and executed in 2005.

The case that has caused the most controversy in using such genetic arguments in court is that of Abdelmalek Bayout in 2009 in Italy. Bayout murdered a man after an argument and initially received a 9 year prison sentence, already reduced after acceptance of mental health evidence.

In an appeal court, his sentence was reduced by a further year after his attorneys presented genetic evidence that Bayout had the warrior gene, predisposing him to impulsive aggression which he would find difficult to control. The judge in the case was swayed by the recent research of Caspi et al regarding the warrior gene and childhood maltreatment and made the decision to reduce Bayout’s sentence as a result.

A primary concern about this decision was no evidence about his childhood experiences was presented during his appeal. This suggests that while Bayout may have had the warrior gene, he did not have the maltreatment history in his childhood, that environmental factor, which was highlighted by Professor Caspi’s research as being significant to raise the risk of violent aggressive behaviour.

In 2011 Nature news blog reported on another case from Italy where in 2009 Stefania Albertani pled guilty to the murder of her sister and an attempt to murder her parents. Her sentence was reduced from life in prison to 20 years based on neuroscience and behavioural genetics evidence. Neuroscientific tests showed her brain scans were different in some areas from a standardised normal sample, and genetic testing highlighted she had the warrior gene. The result was an acceptance of partial mental illness influencing her behaviour and the reduction of her sentence.

Where genetics evidence has been presented as a defence, few cases have been successful however this may change in the future. Equally this is evidence which could be used for the prosecution who could just as easily suggest that such a genetic predisposition, that is out-with a defendants control, means they will always be a risk to society.

When discussing the warrior gene, so far it has to be said that while nature clearly plays a part, it is the nurture that is vital in impacting how an individual is going to turn out, and this includes violent behaviour and violent crime. A positive upbringing away from violence and abuse, even with genetics predisposing a person to potential violent behaviour, can mean a much more balanced well-adjusted adult.

Genetics, science, psychology and criminology are still trying to figure all this out and bringing such data into the courtroom at this stage may be premature. The research into behavioural genetics with regards to violent crime is still very early in its findings. Furthermore, the human mind and brain is a very complex set of interactions, mechanisms and operations. There is a long way to go before we could really understand the behaviours of individuals and what may have caused them. While certain traits can be identified, the exact behaviours and actions of any one individual can still not be fully predicted.

X-linked borderline mental retardation with prominent behavioral disturbance: phenotype, genetic localization, and evidence for disturbed monoamine metabolism

Journal: American Journal of Human Genetics
Authors: H G Brunnet et al, Department of Human Genetics, University Hospital, The Netherlands

We have identified a large Dutch kindred with a new form of X-linked nondysmorphic mild mental retardation. All affected males in this family show very characteristic abnormal behavior, in particular aggressive and sometimes violent behavior. Other types of impulsive behavior include arson, attempted rape, and exhibitionism. Attempted suicide has been reported in a single case. The locus for this disorder could be assigned to the Xp11-21 interval between DXS7 and DXS77 by linkage analysis using markers spanning the X chromosome. A maximal multipoint lod score of 3.69 was obtained at the monoamine oxidase type A (MAOA) locus in Xp11.23-11.4. Results of 24-h urine analysis in three affected males indicated a marked disturbance of monoamine metabolism. These data are compatible with a primary defect in the structural gene for MAOA and/or monoamine oxidase type B (MAOB). Normal platelet MAOB activity suggests that the unusual behavior pattern in this family may be caused by isolated MAOA deficiency.

Read the full paper


  1. Baum., ML. (2011) The Monoamine Oxidase A (MAOA) Genetic Predisposition To Impulsive Violence: Is It Relevant To Criminal Trials? Neuroethics, DOI 10.1007/s12152-011-9108-6.
  2. Beaver, KM., DeLisi, M.. Vaughn, M.G., and Barnes, J.C., (2009) Monoamine Oxidase A genotype is Associated With Gang Membership And Weapon Use, Comprehensive Psychiatry, 51(2), pp130-134.
  3. Bernet, W., et al (2007) Bad Nature, Bad Nurture and Testimony regarding MAOA and SLC6A4 Genotyping at Murder Trials, Journal of Forensic Science, (52)6.
  4.,. (2011). Italian court reduces murder sentence based on neuroimaging data: News blog.
  5. Caspi, A., et al (2002) Role of Genotype in the Cycle of Violence in Maltreated Children, Science, vol 297, pp851 – 854.
  6. Feresin, E. (2009). Lighter sentence for murderer with ‘bad genes’. Nature. doi:10.1038/news.2009.1050.
  7. Hebebrand J, Klug B (September 1995). Specification of the phenotype required for men with monoamine oxidase type A deficiency. Hum. Genet. 96 (3): 372–6.
  8. Hogenboom, M. (2014) Two Genes Linked With Violent Crime, BBC News, Science & Environment. Accessed 9 November 2015.
  9. Hunter, P. (2010). The psycho gene. EMBO Reports, 11(9), 667–669.
  10. (2015). Violent Effects of Abuse Tied to Gene.
  11. (2010). Can Your Genes Make You Murder?
  12. McDermott, R., Tingley, D., Cowden, J., Frazzetto, G., & Johnson, D. D. P. (2009). Monoamine oxidase A gene (MAOA) predicts behavioral aggression following provocation. Proceedings of the National Academy of Sciences of the United States of America, 106(7), 2118–2123.
  13. Mairi Levitt,. (2015). Crime Genes. Retrieved 13 November 2015.
  14. Seo, D., and Patrick, C.J., (2008) Role of Serotonin and Dopamine System Interactions in the Neurobiology of Implusive Aggression and its Comorbidity with other Clinical Disorders, Aggress. Violent Behav. 13(5), pp383-395.
  15. Society For Neuroscience. Brain Chemicals Involved In Aggression Identified: May Lead To New Treatments. ScienceDaily. ScienceDaily, 7 November 2007.
  16. University of Cambridge,. (2015) Serotonin levels affect the brain’s response to anger. Retrieved 14 November 2015.

To cite this article: Guy, F. (2016, Apr 10) The Warrior Gene: Genetics and Criminology. Crime Traveller. Retrieved from

Recommended Book: The Psychopath Inside by Prof. James Fallon – While researching serial killers, neuroscientist and professor James Fallon uncovered a pattern in their brain scans that helped explain their cold and violent behaviour. Astonishingly, his own scan matched that pattern. Fallon set out to reconcile the truth about his own brain with everything he knew as a scientist about the mind, behaviour and personality.

1 Comment

Click here to post a comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

  • I ɑm in fact delighted to rеad this blog post which carrieѕ
    lots of ѵaluable informatiօn, thanks for providing these statistics.

Site Categories