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The biological model assumes that propensity of aggression is located within the biological makeup of
an individual rather than in the environment.
The study of aggression on nonhuman animals helps us understand aggressive behaviour in humans.
E.g selective breeding has been found to lead to more aggressive behaviour in animals and genetic
mutation, nicknamed "fierce", which causes violent behaviour in mice, has been identified.
Furthermore in twin studies, researchers compare the degree of similarity for a particular trait (such as
aggression) between sets of monozygotic (MZ) twins to the similarity between sets of dizygotic (DZ)
twins. Since both sets of twins share the environment, the difference between these two groups could
be attributed to genes. Adoption studies are also used to assess the possible contribution of genetics to
aggressive behaviour, looking at whether the aggressive behaviour correlates to that of the biological or
rearing families.
Supporting evidence to suggest that aggression is within our genes comes from Miles and Carey
(1997). They carried out a metaanalysis of 24 twin and adoption studies that had investigated the
genetic basis of aggression. The results suggested a strong genetic influence that could account for as
much as 50% of the variance in aggression. Age differences were notably important, with both genes
and family environment being influential in determining aggression in youth, but at later ages the
influence of rearing environment decreased and the influence of genes increased. This shows that
genes are not 100% responsible and that the environment does play a role within and this can be
shown within the transaction within age, whilst the influence of environment decreases as the age
increases which means whilst genes play an influential role environment does play a small part,
consequently these findings decrease the credibility of genetics playing a role with aggression, as there
is no evidence to suggest that genes are fully responsible for aggression.
However a drawback to this study is population validity because only 24 twin and adoption studies
were used which is a small sample, consequently the findings obtained based on the small sample
would be hard to generalise to the wider population, and as a result this decreases the credibility of the
findings of the study as well as he explanation that it supports towards the ides that genes play a role
towards ones aggression.
Although research has tried to identify an individual gene for aggression, evidence in this area is less
than conclusive. For example, research in the 1960's identified a link between aggression and
individuals with a genetic disorder in which males have an extra chromosome `Y' in the 23rd pair of
chromosomes, which results in a `XYY' set. Further research in this area however, has concluded that
no single characteristic, except height, has been associated with the `XYY' condition.
Current research does not suggest that there is a gene for aggression per se. Rather it is claimed that
there are some genetically inherited characteristics that when they interact with the environment, place
some individuals more at risk of behaving aggressively.
For example, researchers had found a gene, involved in the production of the protein MAOA
(monoamine oxidase A), which regulates the levels of serotonin in the brain, to be associated with
aggressive behaviour. Further research however, has found that only when interacting with the
environment, this is the case.

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Supporting the idea that there is a specific gene for aggression (Caspi et al., 2002), investigated 500
male children and found that those with low levels of MAOA and consequently low levels of serotonin
exhibited antisocial behaviour, only if they had been maltreated. Maltreated children with high levels of
MAOA and nonmaltreated children with low levels of MAOA did not exhibit antisocial behaviour.…read more


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