Abstract had on the field of psychology,


In this essay I have analyzed and critically evaluated the findings of four researches conducted with the goal to find the link between genes and aggression. I have also tried to determine how likely it is that these genes make an individual more prone to violence, criminal acts, and risky behavior. The researches I chose to include in this essay were picked based not only on the influence they had on the field of psychology, but on how important their results were in relation to the warrior gene hypothesis as well. In other words, I have included the ones that, in my opinion, had/have??? results that provide us with strong evidence that there is at least a possibility the warrior gene hypothesis is true.

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In the first part of my individual essay, I introduce the reader to the topic and



Throughout human history, people have accused and blamed various factors to explain an individual’s aggressive and violent behavior. When someone murdered a person in Ancient Greece, it was believed that they had done so because they had “the sickness of anger”, which was also thought to spread through eye contact. In Medieval England, all a murderer had to do to get away with the crime they had committed was run to the nearest church, ask for forgiveness, and seek asylum. It was widely accepted back then that that was all it took since most people of the Middle Ages thought a demon was what induced a person to kill. In recent years though, the vast majority of such opinions and beliefs has gone extinct and that is because the science of psychology was born. Most people nowadays know that violence and anger are not the results of strange sicknesses or demonic possessions, but rather the outcomes of violent child-rearing or psychological and psychiatric issues.

In 2006, a new idea was presented during the 11th International Congress of Human Genetics in Brisbane, Australia, which proposed that a variant of a gene is responsible for violent behavior and high levels of aggression. The claim was made by a research team from the Institute for Environmental Science and Research (ESR) based on research results and it instantly triggered multiple protests from both scientific and political commentators. If this claim is true, it is capable of proving some of the most popular theories on anger that psychologists have provided us with wrong. The hypothesis is named the “warrior gene hypothesis” and it is one of the most controversial hypotheses in psychology’s history.

The Gene

Monoamine oxidase A gene (MAOA) was called the warrior gene for the first time at the 2004 Annual Meeting of the American Association of Physical Anthropologists in Tampa, Florida, by a scientific author named Ann Gibbons. By 2006, the warrior gene had made headlines in scientific articles and journals after research results by the Institute for Environmental Science and Research (ESR) linked it to violent and criminal behaviour, and MAOA became a conversation topic globally.

Monoamine oxidase genes (MAO) code for enzymes called monoamine oxidases. Monoamine oxidases (MAOs) are enzymes that are involved in the breakdown of neurotransmitters such as serotonin and dopamine and are, therefore, capable of influencing feelings, mood, and behaviour of individuals. The levels of these MAOs in the brain and other tissues are important because these levels determine just how quickly metabolism of these neurotransmitters occurs or whether metabolism occurs at all. It appears that the levels of these MAO enzymes in the brain tissue can have marked effects on behaviours ranging from anxiety and panic disorder to aggression and violence.

According to the warrior gene hypothesis, the warrior gene is an allele containing a polymorphism (allele 3-MAOA30bp repeated-rpt) that is linked to low levels of MAOA and high levels of dopamine. An allele is a variant form of a gene (one member of a pair) that can result in different observable phenotypic traits (i.e. pigmentation). This means that the warrior gene comprises particular variations in the X chromosome gene that produces monoamine oxidase A (MAOA). Since MAOA is located on the X chromosome, males inherit only a single maternal copy, which makes them more vulnerable to its effects. The variants, known collectively as MAOA-L, produce less of the protein that breaks down these signaling chemicals with a low level of the enzyme, which in turn causes them to build up. This means that, if existent, the warrior gene can influence human behaviour and make it violent and highly aggressive.

Ribbon diagram of a monomer of human monoamine oxidase A

1st Research (Brunner et al 1993)

The first research done on MAOA and its connection to abnormal behaviour was conducted in 1993 by a team of researchers from the University Hospital Nijmegen. It was the first gene to be linked to antisocial behaviour. The researchers studied a large Dutch family that was notorious for violence; two were arsonists, one tried to run over an employer with a car, another raped his sister and tried to stab the warden of a mental hospital with a pitchfork. After collecting 24-hour urine samples, the researchers analysed them only to find out they indicated significantly disturbed monoamine metabolism. It was noted that the results were associated with a deficiency of enzymatic activity of MAOA. All five affected males in the family had a point mutation identified in the eighth exon of the MAOA structural gene, which changes a glutamine to a termination codon. Thus, the research team came to the conclusion that isolated complete MAOA deficiency in this family was associated with a recognizable behavioral phenotype that includes disturbed regulation of impulsive aggression.

The MAOA gene is located on the short arm (p) of the X chromosome at position 11.3.

2nd Research (Caspi et al 2002)

In 2002, psychopathologist Caspi A. and colleagues announced they had studied a large sample of male children to determine why some children who are maltreated grow up to develop antisocial behavior, whereas others do not. Participants were 398 males from the Christchurch Health and Development Study who had been observed during the course of 30 years. Each participant had been exposed to sexual and physical abuse during childhood. They were studied at birth, 4 months, 1 year, and annually until the age of 16, and again at 18, 21, 25 and 30 years.

Regression models were fitted to five antisocial behaviour outcomes (self-reported property offending; self-reported violent offending; convictions for property/violent offending; conduct problems; hostility), using measures of childhood exposure to sexual and physical abuse. After analysing the data gathered, it was revealed that there were
G × E interactions, with those having the low-activity MAOA variant and who were exposed to abuse in childhood being significantly more likely to report later offending, conduct problems and hostility. According to Caspi, these findings possibly indicate that there is a stable G × E interaction involving abuse exposure, MAOA and antisocial behaviour and that is why not all victims of maltreatment grow up to victimize others. He also noted that, for officially recorded convictions, the study found a significant interaction between MAOA and only one of three measures of maltreatment (sexual abuse). The weaker effects for convictions are likely to reflect both the limited time period over which officially recorded convictions were recorded, and the low base rate of convictions in the cohort.

Figure 3: Z–test values from Caspi’s research for tests of significance of MAOA activity level×abuse exposure (G × E) interaction from fitted models for varying antisocial behaviour outcomes and varying measures of abuse exposure.




3rd Research (Newman et al 2005)

In 2005, Newman TK. and his team of researchers from the Laboratory of Clinical Studies in Rockville, USA, decided to study the way in which MAOA influences aggressive behavior in male monkeys. With a sample of 45 rhesus macaques, raised with or without their mothers, the researchers observed the primate mammals as they engaged in competitive and social group aggression. After conducting a number of experiments on the monkeys, they determined functional activity of the MAOA gene promoter polymorphism and genotypes scored for assessing genetic and environmental influences on aggression.

Transcription of the MAOA gene in rhesus monkeys is modulated by an orthologous polymorphism (rhMAOA-LPR) in its upstream regulatory region. High- and low-activity alleles of this polymorphism show an interaction effect between genotype and environment on aggressive behavior, such that mother-reared male monkeys with the low-activity-associated allele had higher aggression scores. Newman concluded that such results suggest that the behavioral expression of allelic variation in MAOA activity is sensitive to social experiences early in development and that its functional outcome might partially depend on social context.

Figure 4: Transcriptional activity of the variants in human neuroblastoma cells  constitutively express MAOA (black bars) and MAOA-negative cos 7 (gray bars). Results are based on four independent experiments in triplicate. The MAOA transcription control region (rhMAP-1327) containing the 5-, 6-, and 7-repeat variants of the rhMAOA-LPR was ligated into the promoterless luciferase (luc ) expression vector pGL3 basic. rhMAOA-LPR, rhesus monkey monoamine oxidase A gene-linked polymorphic region. Three alleles of 5, 6, and 7 repeats, with allele frequencies of 35% for the 5-repeat, 25% for the 6-repeat, and 40% for the 7-repeat allele. The activity of both the 5-and 6-repeat variant was approximately 26% higher than that of the 7-repeat form.


 4th Research (Lea & Chambers 2007)

Although Lea R. and Chambers G. released a sample of their research report in 2006, the actual research was not entirely finished until 2007. Lea and Chambers chose to study the indigenous Polynesian people of New Zealand; the M?ori. The M?ori population of Aotearoa (New Zealand) represents the final link in a long chain of island-hopping voyages stretching across the South Pacific It is well recognised that historically they were fearless warriors.

Lea’s and Chamber’s research originally involved analysis of the MAOA gene as a genetic marker for alcohol and tobacco response traits with a view to improving the health of New Zealanders. They had no intension to study anything more than that, but their findings made them change the course/direction/aim of the research and have a more in-depth look into the warrior people. For their studies of alcohol response traits in males, they estimated the population prevalence of the MAOA alleles for M?ori by genotyping 46 unrelated male individuals. They found that the 3-repeat or low activity allele was present at a frequency of 56%; almost two-fold higher than the Caucasian frequencies that had been reported by Caspi et al in 2002.

(*CI=confidence interval)Individuals were recruited from the general Wellington population and were unrelated to each other. Therefore the sample was considered to be a fairly random, albeit small, sample of the M?ori population.



The couple of researchers characterised and published associations among polymorphisms spanning the entire MAOA gene and identified two additional polymorphisms that are suitable for scoring the most common haplotype (AGCCG). This haplotype was present in 70% of the M?ori they tested compared to 40% of the global (non-M?ori) sample previously tested. In a subsample of 17 M?ori males (selected because they had 8 M?ori great grandparents and thus reduced European admixture), the AGCCG haplotype frequency was increased in carriers of the “functional” 3-repeat allele compared to non-M?ori carriers.

The two researchers stated that, to them, these findings were evidence of positive natural selection acting at the MAOA and that it suggested that Polynesian males who embark on long, dangerous canoe voyages and engaged in war with other islander tribes carried the AGCCG haplotype, coupled with the 3-repeat allele of MAOA to New Zealand, where they both increased in frequency due to rapid population growth.





After carefully and thoroughly researching the warrior gene hypothesis by studying all the data research results have provided us with through the years, I came to the conclusion that there are enough to at least back up the warrior gene hypothesis. However,  I believe that most researches conducted


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