Genes do not directly cause aggression but influence elements of our biology that contribute to it. This can be demonstrated through the following methods; 1. Animal Breeding Studies In animal breeding studies, animals are selectively bred for certain traits. If it is possible to breed for aggression, this would suggest that genes contribute to aggressiveness. Lagerspetz (1979) bred 25 generations of mice. In each generation, she chose the least aggressive individuals to breed together and the most aggressive ones to breed together. The result was two very different strains.
One group of mice were super-aggressive, the other very docile. Even when cross-fostered to nonaggressive mothers, mice from the ‘aggressive’ strain still demonstrated more aggression. This showed that there is, at least in animals, a genetic component to aggressive behaviour However, a problem with research on animals is that it is difficult to generalise the results to humans, nevertheless, mice are often used in genetic studies because their genes are homologous to those of humans, this means they produce the same proteins, which are used in a similar way. 2.
Heritability Studies (Twin and adoption studies) Research on humans has also supported the notion that genes are an important factor in aggressiveness. Grove et al ( 1990) studied 32 sets of MZ twins who were separated and raised apart shortly after birth. A continuous score for anti-social behavior in both childhood and adulthood was derived by interviewing each subject with a standardised interview schedule; as such this assessment of antisocial behavior was a selfreport measure. Statistically significant heritabilities were obtained for anti-social behavior in both childhood (0. 1) and adulthood (0. 28). Rhee and Waldman (2002) conducted a meta-analysis of 51 twin and adoption studies. Aggression was measured (operationalised) through psychiatric diagnoses such as conduct disorder, criminal records and self report/reports by others. Overall they found that genetic variation appears to be responsible for 40% of cases. They also examined whether the results varied according to the different assessment methods. Interestingly they found heritability estimate was 39% for self reported aggression but 53% when reported by others.
However, researchers of such studies must rely on data from twins who have been separated at various ages and in a variety of circumstances, Some ‘separated’ identical twins may have spent considerable time in a similar environment before separation or they might have been ‘separated’ but have actually lived in very similar environments. They may also been reunited for a considerable time before their personality and intelligence were assessed, so that they had a chance to share an environment for a time and to become more alike.
With adoption studies it is important to remember that adoption agencies often practice selective placement, trying to place children with families who are similar in many ways to their genetic parents. Therefore the effects of genetic inheritance may be difficult to separate from the influences of the environment Furthermore reliance on self reported/reports by others measures means the data is susceptible to reporting bias. 3. XYY Chromosome Some men have an extra male Y chromosome, and since males are more aggressive than females anyway, this might suggest further aggression in XYY males.
Jacobs et al (1965) found the incidence of XYY syndrome was 3% in a prison population compared to 0. 1% of the normal population. These men were taller, had higher levels of testosterone and lower intelligence levels. However Witkins et al (1976) could find no link between XYY syndrome and increased aggression in prison inmates, but did find lower levels of intelligence. Inspection of crimes showed that they were not more violent in nature but they were poorly planned. The researchers suggest that lower IQs in XYYs make them more likely to be caught, hence more of them in the prison population.
In (1984) Theilgaard researched the personality traits of males with XYY, XY and XXY (thought to be more feminine). Theilgaard found that about 1 in 1000 males are XYY and that no other trait apart from height has been associated with XYY condition. Research shows no consistent link between XYY and aggression. Theilgaard showed that XYY males tend to give more aggressive interpretations of the images used in a thematic apperception test. ( see link – http://www. scienceclarified. om/dispute/Vol-1/Are-XYY-males-more-prone-to-aggressive-behaviorthan-XY-males. html) Molecular Genetics and linkage studies: MAOA gene (‘warrior gene’) New technology has enabled researchers to examine DNA at the molecular level. Researchers are now able to investigate the proteins a particular sequence of DNA codes for, which cells in the body express those proteins and the role of that protein in the body. Molecular geneticists are then able to collaborate with psychologists to examine any effects of genetic differences on behaviour.
The monoamine oxidase A (MAOA) gene regulates the enzyme monoamine oxidase A, which breaks down important neurotransmitters in the brain, including dopamine, norepinephrine, and serotonin which are associated with mood. Humans have various forms of the gene, resulting in different levels of enzymatic activity. • MAOA – L is a low activity form that produces less of the monoamine oxidase A enzyme. • MAOA – H is a high activity form that produces more of monoamine oxidase A enzyme. Several studies have found a correlation between the low-activity form of MAOA and aggression in observational and survey-based studies.
It is thought that having too little MAO leads to the brain is flooded with too much serotonin, norepinephrine and dopamine, which eventually leads to a lowered sensitivity to these neurotransmitters (see documentary) Brunner et al ( 1993 ) conducted a study with a large Dutch family, who had a long history ( over five generations ) of anti-social behavior. Over the course of four years Brunner and his colleagues analyzed the X chromosomes of 28 members of the Dutch family . In their study they found a point mutation in the structural gene for MAOA, a neurochemical in the brain.
Recall that an MAOA deficiency is associated with impulsive, aggressive behavior. The same mutation was not found in any of the control group of ‘non-aggressive’ males. However, these results have not been confirmed in any additional family studies, which lead to a need for more studies to determine if other families share similar results. However, this one family study does seem to suggest that genetics play an important role in antisocial or criminal behavior ( see link – http://discovermagazine. om/1993/oct/aviolenceinthebl293) Gene – environment Interaction Finally, Caspi & Moffitt (2002) have argued that aggression is in fact an interaction of genes and the environment. Their study found a link between a genetic variant causing low levels of MAOA and increased levels of antisocial behavior in people who had been abused as children. The MAOA gene metabolises neurotransmitters such as serotonin, which regulates impulsive behavior. Variants of MAOA have been associated with aggression.
They studied a representative birth cohort of 442 male children in New Zealand to determine why some children who suffer childhood abuse grow up to develop antisocial behaviors whereas others do not. The children were studied for 26 years, from birth to adulthood. Their life histories had been recorded at various intervals since they were born. Moffitt’s team measured instances of maltreatment during the first ten years of life, including rejection by the child’s mother assessed when the child was three years old, frequent changes of the primary caregiver, and physical and sexual abuse.
They also evaluated the test subjects’ records of antisocial behavior, which is generally defined as “behavior that violates the rights and safety of others,” according to Moffitt. Specifically, they used the American Psychiatric Association’s criteria for adolescent conduct disorder and antisocial personality disorder, court records for violent crimes, and evidence of aggressive personality traits from a psychological assessment at age 26. The researchers determined which version of the MAOA gene each test subject had. 30% of the men carried MAOA variant (low levels).
Of this 30%, 85% showed some form of anti-social behavior overtime. The findings provide initial evidence that genotypes can moderate children’s sensitivity to environmental stress. These findings may partly explain why not all victims of maltreatment grow up to victimize others; some genotypes may promote resistance to stress and trauma. It may also show that a caring environment may control the negative effects of the MAOA gene. However replication studies of the interaction between MAOA genotype and maltreatment are few, showing both similar and non-similar results.
Conclusion There is a great deal of evidence that genes do play a significant role in aggressiveness. Animal breeding studies have shown that it is possible to select for aggressive behavioural traits and family studies in humans have shown that aggressiveness is highly heritable. Some of the actual genetic mechanisms responsible for aggression have been revealed by molecular genetics; however, the importance of environmental factors has also been highlighted by researchers such as Caspi and Moffat.
It appears that genes do increase the risk of aggression, but only when combined with enviromental risk factors, such as abuse and a neglectful family environment. Whilst our legal system is based on the notion of free will, genetic theories challenge the notion of free will. Accepting genetic explanations of aggression suggests we are not responsible for aggressive behaviours. This was demonstrated in the case of Bradley Waldrup who violently murdered his estranged wife’s