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Discuss genetic factors in aggression. (8+16 marks)
Human aggression depends in part on biological factors. Genetic influences on aggression
have been clearly demonstrated in non-human animals and twin studies suggest some sort
of biological component in aggressive behaviour.
Rhee and Waldman's (2002) meta-analysis of twin and adoption studies on anti-social
behaviours, which included aggressive behaviour, found that identical twins were more
similar in anti-social behaviour than fraternal twins. This supports genetics factors having
an influence on aggressive behaviour, even allowing for the more closely similar
environment of identical compared with fraternal twins. Furthermore, Rhee and Waldman
found that genetics influences are the cause of 41% of the variability in anti-social
Genes have also been linked to brain chemistry and increased aggression. MAOA is an
enzyme that facilitates the breakdown of excess neurochemicals, such as noradrenaline,
serotonin and other amines. Support from Brunner at al. (1993) study shows that the MAOA
deficiency is in fact genetic and means that those with MAOA deficiency are likely to have
raised levels of noradrenaline.
This was found in four generations of males in a Dutch family. The men inherited a
recessive, X-chromosome linked gene that appears to result in aggressive, sometimes
violent behaviour. Further support for the gene hypothesis comes from a study of 110 men
who showed an association between abnormalities in the MAOA gene and aggressiveness
and impulse control.
These findings are interesting because they seem to be showing a direct link between
genes, brain chemistry and aggression. However, both sets of participants in the research
constitute highly biased samples. This means that we cannot assume that findings can be
generalised to the population and so this sort of research has low ecological validity as
well as low mundane realism, for example, male-only samples cannot represent females
and so ignore half the population, resulting in gender bias.
Furthermore, the males in question were highly unusual. Their abnormal aggressiveness
does not relate to everyday aggressive behaviour and so cannot contribute a great deal to
our understanding of such aggression. There is also the possibility that the unusually
aggressive behaviours are now the result of expectation by the extended Dutch family and
so aggressive acts made my male children are not discouraged but accepted, which would
successfully reinforce such behaviours.
In addition, the link between raised noradrenaline levels and increased aggression are
correlational and therefore cannot be assumed to be directional or causal from
noradrenaline to behaviour. However, the fact that the raised levels are the result of
inheriting a maladaptive gene does strengthen the argument for a causal, directional link.
Differences in brain structure and functioning are also likely to be genetic, as a study by
Raine et al. (1997) suggests. They used PET scans to examine the brains of 39 males and 2
females, charged with committing murder and compared them with 41 controls. They
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found significant differences in the amygdala suggesting unusual emotional responses
such as a lack of fear which could lead to a lack of retribution and a lack of fear of breaking
social norms, all of which could increase actual aggressive behaviour.
However, an issue with this research is that the human brain is very complex and so it is not
at all likely that a single, simple brain mechanism is the explanation for all aggressive