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Describe and evaluate how neural and hormonal mechanisms influence aggressive
behaviour (24 marks)
Neurotransmitters are chemicals that allow impulses within the brain to be transmitted from one area
of the brain to another. The main neurotransmitter believed to play an important role in aggression is
low levels of serotonin.
Serotonin is believed to reduce aggression by inhibiting responses to emotional stimuli that might
otherwise have led to an aggressive response.
Low levels of serotonin in the brain, particularly the pre-frontal cortex has been linked with higher
chances of impulse behaviour, aggression and even suicide as the inhibiting effect is lessened.
Mann et al found supporting evidence for this theory; in one study 35 healthy participants were
given a drug (dexfexfluramine) to deplete serotonin levels.
Judging their subsequent aggression based on hostility scores on a questionnaire, it was found that
lower levels of serotonin were found to be associated with higher levels of aggression in males.
However, Mann's study demonstrates a gender bias because an increase in aggression when
coupled with lowered serotonin only took place among male participants, suggesting that the role of
serotonin in aggression may possibly be different for males thus providing an incomplete explanation
of neurotransmitters in aggression.
Although, the gender bias can be explained by methodological issues with Mann's study instead.
Mann used questionnaires to measure aggression levels and participants may have exhibited
demand characteristics after expecting themselves to behave more aggressively after taking drugs,
therefore there might not be a gender bias as males would generally rate themselves more
aggressively than females in any self-report.
Other research support for the serotonin hypothesis comes from a study by Raleigh et al.
Researchers fed vervet monkeys diets that either decreased or increased serotonin levels.
For monkeys that ate the raised serotonin diets, they exhibited less aggressive behaviour.
Alternatively those who ate the lowered serotonin diets exhibited more aggressive behaviour,
supporting the theory that the serotonin neurotransmitter influences aggressive behaviour.
Though Raleigh's study establishes a clear link between serotonin and aggression in animals, the case
is not so clear (as evidenced in Mann's study) in humans. This is not to deny that the link exists in
humans, but rather that the complexity of the human social behaviour means a biological explanation
would be reductionist in explaining all types of aggression behaviour. For example, impulse
behaviour from the serotonin hypothesis would not be able to account for aggressive behaviour
over the internet; therefore there could be other explanations, such as the psychodynamic
explanation whereby Froid would argue that aggression is a result of repressed childhood conflicts
(encompassing the nurture side in the nature/nurture debate).
If low levels of serotonin are associated with low impulse control and aggressive behaviour, drugs
that clinically raise serotonin levels should produce a concurrent lowering in aggression. Bond (2005)
established that antidepressants which elevate serotonin levels reduce irritability and impulsive
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Alternatively, hormonal mechanisms also have an important role in influencing aggression.
The hormone testosterone is thought to increase aggression in adults due to its action on brain areas
involved controlling aggression.
For example, research by Dabbs found that criminals with the highest levels of testosterone tended
to have a history of violent crimes whereas those with the lowest levels had committed non-violent