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Outline and evaluate genetic factors in aggression
The biological approach to aggression includes the belief that genetic factors play a role in
aggressive behaviour and lies within an individual's genetic make-up. There is supporting evidence
of a genetic component of aggression. Psychologists have looked at twin studies to try and find
evidence for a genetic basis o compare the degree of similarity of aggression between sets of
monozygotic (MZ) twins and dizygotic (DZ) twins. In general it was found that aggressive behaviour is
more highly correlated in MZ twins than DZ twins. For example, Gottesman found a concordance rate
of 87% for aggressive behaviour for MZ twin pairs, compared with 72% for DZ twin pairs. However,
solely relying on evidence from twin studies can be problematic as it is hard to disentangle nature
and nurture. It has been suggested that MZ twins are treated more alike and share more similar
environments than DZ twins due to them acting more like 'one' person rather than two separate
people. This may affect how alike they are and how likely they both are to express aggression.
However, there are gender differences involved in twin studies. Button et al studies 258 twin pairs
and found that both aggressive and non-aggressive antisocial behaviour are subjected to significant
gender differences. The heritability of aggressive antisocial behaviour was significantly higher in girls
than boys, suggesting a stronger genetic effect on aggression in females than in males.
Adoption studies can help to disentangle the relative contributions of the environment and genetics.
If researchers find a greater similarity in levels of aggression between adopted children and
biological parents than between their adoptive parents, it suggests genetics are an important
influence. The Danish study by Mednick reviewed over 14000 adoptions in Denmark and found a
significant positive correlation between the number of criminal convictions for criminal violence
among biological parents and their adopted songs, thus supporting the genetic role in aggression.
However, a problem with this study is that its lacks ecological validity as the findings cannot be
applied to other countries. Also, measuring aggression levels using criminal convictions assumes that
the criminals were in fact guilty of the crimes convicted, and we know that occasionally people may
actually be innocent. Likewise, another problem with this sampling is that convictions for violent crime
are relatively few compared to the vast number of violent attacks by individuals that never result in
conviction. Also offenders designated as `violent' on the basis of a court conviction are not
necessarily the most serious, persistent aggressive offenders.
There are also various methodological problems involved in adoption studies. For example, one
problem is that in some countries such as the US, children given up for adoption showed a higher rate
of antisocial behaviour at the time adoption compared with the general population. Consequently,
correlations between adoptees and their biological parents may be due to either genetic factors or
environmental influences, such as antisocial behaviour as a result of feeling of abandonment.
Researchers have also identified a number of genes potentially for aggression. Although no individual
gene for aggression has been identified in humans, a gene responsible for the production of the
protein MAOA has been associated with aggressive behaviour. MAOA regulates the metabolism of
serotonin in the brain. Low levels of serotonin have been associated with aggression. In the 1980s, a
study of a Dutch family found that many of its male members behaved in a particularly violent manner
and most convicted of serious violent crimes. These men were found to have significantly low levels
of MAOA and a defect in this gene was later identified by Brunner. However, the role of
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Also Brunner himself
contended that it was unlikely that a single gene was the direct cause of a specific behaviour. Rather,
genetic deficiencies may exert some influence on an individual's behaviour, but they are not the sole
cause of behaviour.
Rather than either genetics or environment alone being responsible for the development of
aggression, some researchers believe that interactions between genes and environment influence
traits which are the diathesis-stress model. This theory is supported by research by Caspi et al.…read more