Neural, Hormonal & Genetic Mechanisms (AQA A-Level Psychology): Revision Notes
Genetic Factors in Aggression
Understanding the role of genetic factors in aggression requires separating genetic influences from environmental ones. Psychologists use several research methods to investigate this, including twin studies, adoption studies, and specific gene research focusing on the MAOA gene.
Genetic factors are inherited DNA sequences that provide instructions for both general physical features (such as eye colour and height) and specific biological features (such as neurotransmitter levels and brain structure sizes). These genetic instructions may influence psychological characteristics including intelligence and mental health disorders, and are passed from parents to offspring through inheritance.
The MAOA gene is responsible for controlling the activity of the enzyme monoamine oxidase A in the brain. This gene has different variants, with the low-activity version being closely linked to aggressive behaviour patterns.
Twin studies
Research using twins provides valuable insights into the heritability of aggressive behaviour. Twin studies compare monozygotic (MZ) twins, who share 100% of their genes, with dizygotic (DZ) twins, who share only 50% of their genes on average. Since both types of twins typically share the same environment, any greater similarities in MZ twins compared to DZ twins suggests genetic influence.
Several twin studies indicate that heritability accounts for approximately 50% of the variance in aggressive behaviour, demonstrating a substantial genetic component to aggression.
Research Example: Coccaro et al. (1997)
Study Design: Adult male MZ and DZ twins were examined for both direct physical assault and verbal aggression.
Key Findings:
- Direct physical assault: 50% concordance for MZ twins vs 19% concordance for DZ twins
- Verbal aggression: 28% concordance for MZ twins vs 7% concordance for DZ twins
Conclusion: MZ twins showed much greater similarity in aggressive behaviour than DZ twins, supporting the role of genetic factors in aggression.
Adoption studies
Adoption studies examine similarities in aggressive behaviour between adopted children and their biological versus adoptive parents. If adopted children show more similarity to their biological parents (with whom they share genes but not environment), this suggests genetic influence. Conversely, greater similarity to adoptive parents suggests environmental influence.
Research Example: Rhee and Waldman (2002)
Method: Conducted a meta-analysis of adoption studies focusing on direct aggression and antisocial behaviour.
Finding: Genetic influences account for 41% of the variance in aggression, which aligns closely with findings from twin studies research.
The MAOA gene
Monoamine oxidase A (MAOA) is an enzyme that breaks down neurotransmitters in the brain after nerve impulses have been transmitted between neurons. This process involves breaking down neurotransmitters, particularly serotonin, so they can be recycled or eliminated through catabolism. The production of this enzyme is controlled by the MAOA gene.
A dysfunction in this gene's operation may result in abnormal MAOA enzyme activity, which affects serotonin levels in the brain. One variant of the MAOA gene, nicknamed the 'warrior gene', produces low MAOA activity in brain areas and has been linked to various forms of aggressive behaviour.
Key research findings
Research Example: Brunner et al. (1993)
Participants: 28 male members of a large Dutch family repeatedly involved in impulsively aggressive violent criminal behaviours including rape, attempted murder, and physical assault.
Discovery: These men had abnormally low levels of MAOA in their brains and possessed the low-activity version of the MAOA gene.
Research Example: Stuart et al. (2014)
Participants: 97 men involved in intimate partner violence (IPV) participating in a batterer treatment programme.
Finding: Men with the low-activity MAOA gene were found to be the most violent perpetrators of IPV, engaging in the highest levels of physical and psychological aggression and inflicting the most severe injuries on their partners.
Gene-environment interactions
Genetic influences on aggressive behaviour do not operate in isolation. The relationship between low MAOA gene activity and adult aggression appears to depend on environmental factors, particularly early life experiences.
Critical Concept: Gene-Environment Interaction
The relationship between genetics and aggression is not straightforward. Genetic predisposition only leads to aggressive behaviour when combined with environmental triggers - this is called diathesis-stress interaction.
Research Example: Frazzetto et al. (2007)
Finding: An association between higher levels of antisocial aggression and the low-activity MAOA gene variant in adult males was found, but only in those who had experienced significant trauma during the first 15 years of life, such as sexual or physical abuse.
Key Insight: Men who possessed the low-activity MAOA variant but had not experienced childhood trauma did not show particularly high levels of aggression as adults.
Evaluation
Isolating genetic factors
Establishing the influence of genes on aggressive behaviour presents substantial challenges. The main difficulty lies in separating genetic and environmental factors, as demonstrated in gene-environment interactions. An individual may possess a gene associated with aggression, but this behaviour only manifests when environmental conditions are conducive.
McDermott et al. (2009) showed that participants with the low-activity MAOA gene behaved aggressively in a laboratory-based money-allocation game, but only when they were provoked. Without provocation, they showed no more aggressive behaviour than other participants, highlighting the complexity of genetic influences.
Multiple genetic influences
While genetic effects on aggression are statistically reliable, they are also relatively small. This suggests that numerous other genes are likely involved in aggressive behaviour.
Complex Genetic Interactions
In Stuart et al.'s study, intimate partner violence in men was associated not only with the low-activity MAOA gene but also with the serotonin transporter gene (5-HTT), which influences serotonin activity in the brain. The combination of both genes showed the strongest link with IPV.
The search for additional genes remains an active research area. Vassos et al. (2014) conducted a meta-analysis and found no evidence of an association between any single gene and aggression. They suggest that even multiple genes are unlikely to fully explain a behaviour as complex as aggression, calculating that hundreds or thousands of genes may interact in intricate ways to determine aggressive behaviour.
Measuring aggression
Methods of measuring aggression vary considerably between studies, including self-reports, parent and teacher reports, and direct observations. In Rhee and Waldman's meta-analysis of 51 twin and adoption studies, genetic factors showed greater influence on aggression in studies using self-reports rather than parent or teacher reports.
If research findings vary depending on how aggression is measured, drawing valid conclusions about the role of genetic factors becomes very challenging.
Research support for MAOA gene
Additional research strengthens the evidence for the MAOA gene's role in aggression.
Research Example: Mertins et al. (2011)
Method: Studied participants with low-activity and high-activity variants of the MAOA gene in a money-distributing game where participants decided whether to contribute money for the group's benefit.
Finding: Males with the high-activity variant were more cooperative and made fewer aggressive moves than low-activity participants.
Significance: This finding confirms the importance of the MAOA gene by showing that the high-activity variant leads to behaviour that is opposite to that associated with the low-activity variant.
Non-human animal studies
Research with mice provides additional support for the MAOA gene hypothesis. Using genetic deletion techniques, researchers can 'knockout' single genes to observe effects on aggression.
Research Example: Godar et al. (2014)
Method: MAOA knockout mice were studied using genetic deletion techniques.
Findings:
- MAOA knockout mice have considerably increased brain serotonin levels
- These mice display hyperaggressive behaviour
- When serotonin was blocked using the drug fluoxetine, the mice returned to non-aggressive behaviour
Conclusion: This demonstrates that MAOA normally maintains appropriate serotonin levels by removing it from synapses. Without MAOA function, serotonin levels become excessive, leading to increased aggression.
Key Points to Remember:
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Twin studies suggest that genetic factors account for approximately 50% of the variance in aggressive behaviour, with MZ twins showing much higher concordance rates than DZ twins
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The MAOA gene controls an enzyme that breaks down neurotransmitters, particularly serotonin, with the low-activity variant being associated with increased aggression
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Gene-environment interactions are crucial - the low-activity MAOA gene only leads to aggression when combined with environmental factors such as childhood trauma
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Multiple genes likely contribute to aggression, making it difficult to identify single candidate genes, and the effects of individual genes are relatively small
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Research methodology affects findings, with different methods of measuring aggression producing varying results about genetic influence