Genetic Factors in Aggression, including the MAOA Gene
AO1- Description: How are Genetic Factors Studied?
(1) Twin Studies:
Several studies have suggested that heritability accounts for about 50% of the variance in aggressive behaviour. Monozygotic (identical) twins share 100% of their genes, while dizygotic (non-identical) twins share only about 50%, as a result we would expect to find greater similarities in aggressive behaviour between MZ twins if aggression is mostly influenced by genetic factors. Examples of concordance rates for different types of aggression are below.
Physical aggression MZ=50% and DZ=19%, Verbal aggression MZ=28% and DZ=7%.
(2) Adoption studies: Adoption studies can also help to separate out the effects of genes and environment. If a positive correlation is found between aggressive behaviour in adopted children and their biological parents, then this is suggestive of genes because the environmental factor has been removed.
Meta-analysis was carried out looking at adoption and its relationship to direct aggression and antisocial behaviour.
They found that genetic influences accounted for 41% of the variance in aggression.
Research Highlighting a Link Between Aggressions and Genetics (AO3):
Lagerspetz (1979) bred 25 generations of mice. In each generation she chose the most aggressive mice to breed together and the least aggressive ones to breed together. The result was two very different strains. One group of mice were super-aggressive, the other very docile. This research supports the role of genes in aggressive behaviour as it illustrates that when two mice holding the ‘aggressive gene’ reproduce, their offspring is significantly more likely to display high aggression (thus support a biological, genetic role in the presence of aggression).
Genetic Factors in Aggression (Evaluation – AO3)
Strengths (AO3 – Evaluation):
(1) Point: Twin studies provide further support for the genetic explanation of aggressive behaviour. Example: For example, McGue et al (1992) found a correlation of +0.43 for MZ twins and +0.30 for DZ twins on aggression scales. This research demonstrates s stronger link to aggressive behaviour for identical twins. Elaboration: This suggests that genes play some part in aggressive tendencies as MZ twins share more genes than DZ twins, therefore if there was no genetic element to aggression we would not see any difference in the correlation for MZ and DZ twins. This is positive as it suggests genes may at least play a part in aggression and therefore that the cause of aggression is, in some part, biological.
Weaknesses (AO3 – Evaluation):
(1) Point: A considerable amount of research into genetic factors in aggressive behaviour was conducted on animals, such as Lagerspetz’s research on mice.
Evidence: For example, this means that the research only investigates the genetic factors in the development of aggression for animals and fails to generate any information regarding human aggression and links to genetics.
Elaboration: This is a weakness because, the findings from this research cannot be extrapolated (generalised) to humans. Mice and humans are physiologically different and so, although there seems to be a genetic basis for aggression in mice, it doesn’t mean that such findings can be generalised to humans.
(2) Point: There are potential issues in how aggressive behaviour is measured
Evidence: For example, aggression is often measured through questionnaires or responses to a hypothetical scenario, and therefore predictive validity is poor.
Elaboration: This is a weakness because such measures of aggression are subjective (often influenced by individual judgement) and therefore it is difficult to draw conclusions about aggression from research that has used such measures.
(3) Point: Twin studies provide further support for the genetic explanation of aggressive behaviour. Example: For example, McGue et al (1992) found a correlation of +0.43 for MZ twins and +0.30 for DZ twins on aggression scales. This research demonstrates s stronger link to aggressive behaviour for identical twins. Elaboration: This suggests that genes play some part in aggressive tendencies as MZ twins share more genes than DZ twins, therefore if there was no genetic element to aggression we would not see any difference in the correlation for MZ and DZ twins. This is positive as it suggests genes may at least play a part in aggression and therefore that the cause of aggression is, in some part, biological.
MAOA Gene (AO1 – Description)
What is MAOA?
A gene (monoamine oxidase A), that has been implicated in aggression, mice that possessed the gene were extremely aggressive. The MAOA gene is thought to affect neurotransmitters.
How does the MAOA gene link to aggressive behaviour?
• The gene determines the production of the enzyme MAOA
• MAOA metabolises (‘mops up’) neurotransmitters (e.g. serotonin, dopamine and noradrenaline) in the brain after a nerve impulse, it breaks them down to be recycled or excreted.
• A dysfunction in the operation of this gene may lead to abnormal activity of the MAOA enzyme, which in turn affects levels of neurotransmitters in the brain.
• People with the low-activity form (MAOA-L) produce less of the enzyme and this is correlated with aggression. The high-activity form (MAOA-H) is not correlated with aggression.
Research showing a link between aggression and the MAOA gene (AO3 – Evaluation):
Han Brunner et al (1993) studied 28 males from a large Dutch family who were repeatedly involved in impulsively aggressive violent criminal behaviour. It was found that these men had abnormally low levels of MAOA in their brains and the low-activity version of the MAOA gene. This research supports the role of MAOA in aggression as when this gene is not functioning correctly, individuals display aggressive behaviour.
The MAOA gene and the environment (AO1 – Description):
Genes are crucial influences on aggressive behaviour, but they do not function in isolation. It appears to be the case that MAOA-L is only related to adult aggression when combined with early traumatic life events.
This indicates that an interaction of genetics and environment is at work in determining human aggression and there the phenotype differs from the genotype.
Possessing MAOA-L does not automatically make you aggressive.
MAOA Gene – AO3 – Evaluation:
(1) Point: Animal studies provide support for the role of the MAOA gene in the development of aggressive behaviour. Example: For example, Cases et al (1995) studied genetically engineered mice with low MAOA levels. They found the mice had abnormal serotonin levels and behaviour. As adult mice they demonstrated increased levels of aggression and were aggressive during mating. Elaboration: This is a strength because it highlights when there is a dysfunction with the MAOA gene, aggression occurs therefore highlighting a link between this gene and aggression.
(1) Point: The MAOA gene cannot account for all cases of aggression. Evidence: MAOA-L is only present in about 1/3 of men in Western population, who therefore have higher levels of neurotransmitters because they haven’t been metabolised. MAOA-L is rarely expressed in women. Elaboration: This is a weakness because it highlights that aggression is up to 2/3 of men and most women cannot be account for by the MAOA gene, this suggests therefore that there must be another explanation for aggression, making this explanation limited in its application.