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Outline and evaluate the role of genes in
Genes do not directly cause aggression but influence elements of our biology that contribute to it. A
combination of structural and functional genetic effects contributes to aggressive behaviour.
Evidence for the effect of genes comes from 4 different sources of research. The first is selective
breeding which is the choosing of animals with aggressive characteristics and mating them together
with other to enhance this trait. Lagerspetz (1979) selectively bred mice to be 50% more aggressive
than normal mice within 19 generations. These mice had heavier testes and forebrains and altered
levels of the neurochemicals serotonin and noradrenaline. This supports the genes argument as the
altered structures and hormones demonstrate that you can pick the characteristics and behaviours
that show aggression from the genes you breed together.
The second contributing piece of evidence for genes comes from twin studies. Selective breeding is
impossible in humans so the next best thing is to study people with known genetic factors, for
example, using twin studies. This is especially useful if the twins are reared apart as it eliminates the
role nurture may play in aggression as the twins would have been brought up in different ways. Rhee
and Waldman (2002) found a heritability estimate for identical twins to be 39% for self reported
aggression but 53% when aggression levels were reported by others. As identical twins share 100%
if their genes this supports the genes argument because there is a strong similarity between their
levels of aggression.
XYY chromosomes also provide evidence for the genes argument. Some men have an extra male Y
chromosome and since males are naturally more aggressive than female, this might suggest further
aggression in XYY males. Jacobs et al (1965) found the incidence of XYY chromosome was 3% in a
prison population compared to 0.1% of the normal population. These men were taller, had higher
levels of testosterone and lower intelligence levels. Because more of a prison is made up of XYY
males than a normal population then you can suggest it is the extra Y chromosome that codes for
higher aggression levels as you assume it is aggression and aggressive behaviours that leads to
people being put in prison.
The final piece of evidence for the role of genes in aggression is the idea of the MAOA gene. A
number of studies have linked aggression to the mono amine oxide A gene which regulates the
enzyme mono amine oxidase A. This enzyme breaks down several neurotransmitters for example
serotonin and dopamine which are associated with mood. A build up of these neurotransmitters can
cause people to behave aggressively. Cases et al (1995) disabled the MAOA genes in the X
chromosome of mice and found that without the mono amine oxidase A enzyme, levels of dopamine
and serotonin increases and males became highly aggressive. Restoring the function of the gene
returned male mice to a normal state. This supports the argument for genes as when the gene was
deactivated the male mice were more aggressive as they had excess levels of serotonin and
dopamine. One the gene was restored the aggression was lowered.
A strength of the genes argument is that there is evidence in favour in the form of concordance
rates. Evidence from twin studies show a consistency in the genetic influence over aggression. For
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O'Connor (1980) found that MZ twins reared together and apart were similar in their
aggressive characteristics and was higher than the rates in DZ twins. This suggests that there is a
strong genetic component in aggression as MZ twins share 100% of their genes compared to 50% in
DZ twins suggesting that aggression is caused by nature.
There is further supporting evidence for the role of genetics from Brunner who used case studies.…read more