Genetic Factors in Aggressive Behaviour

A01 and A02 included. MAOA, twin studies and adoption studies

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Biological explanations of aggression
Assumptions made:
1. INFLUENCE aggressive behaviour- Genes cause aggressive behaviour in people e.g. MAOA is
linked to high aggressive behaviour
2. TRANSMITTED from one generation to another- The use of twin studies has shown us that
genes for aggressive behaviour are transmitted and affect offspring when passed down.
3. SOCIAL FACTORS ignored or played down- When speaking in terms of the biological
explanation, social explanations are ignored and the biology is focused on.
One candidate gene named MAOA is thought to contribute to a higher risk of anti-social behaviour.
Found on the X chromosome, this gene's role is to regulate the enzyme also known as monoamine
oxidase A (MAOA) which breaks down neurotransmitters, such as dopamine and serotonin, after
they have transmitted an impulse from one nerve cell to another. MAOA regulates the levels of
serotonin in the brain and so if there are low levels of MAOA there are low levels of serotonin. Low
levels of serotonin are associated with aggressive behaviour therefore low levels of MAOA are
associated with aggressive behaviour.
Brunner et al. A family in the Netherlands had many males with a history of extreme violence.
Brunner analysed the x chromosome of 28 people in the family and found that the violent men had
abnormally low levels of MAOA. This suggests that there is a link between the MAOA gene and
Correlational not causal- There is a link found between MAOA and aggression but we
cannot assume that the relationship is causal as there are other factors to consider such as
environmental factors. This type of behaviour is widespread and may be found in the family due to
shared environmental factors e.g. bad parenting
Supporting study: Cases et al. ­ They disabled the MAOA gene in mice and measured the
levels of serotonin and dopamine. They found that the levels of serotonin and dopamine increased
and aggressive behaviour in the mice increased. This suggests MAOA reduces aggression by
regulating levels of serotonin and dopamine which supports the theory.
However, the use of mice in the study is a weakness. Mice have a different physiology so
may react differently to changes in neurotransmitters therefore the study is questionable support.
Unable to predict aggression- This information has little use because we cannot identify
whether the levels of MAOA or if it is a defect gene cause aggression therefore we cannot predict if
someone is more aggressive due to the MAOA gene.

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Twin studies
Twin studies are used to determine whether genetic influence is greater than environmental
influence. In humans, aggressive behaviour is highly correlated in identical twins, who share 100%
genes, compared to fraternal twins, who share 50% genes.
McGuffin and Gottesman found a concordance rate of 87% in MZ twins (identical) and 72% for DZ
twins. The concordance rates are very similar suggesting the genetic influence is low. This indicates
that family environments shared across siblings exerts an important influence on juvenile
delinquency.…read more


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