Discuss the role of neural and/ or hormonal mechanisms in aggression (16 marks)
The limbic system is an area of the brain that helps to coordinate behaviours that satisfy motivational and
emotional urges, such as aggression and fear. There are two structures in the limbic system that are
associated with aggression. One is the amygdala, and this is responsible for evaluating the importance of
sensory information and prompting an appropriate response. An overreactive amygdala might produce faster
and heightened responses when the animal is mildly provoked. If the amygdala is surgically removed, the
animal no longer responds to stimuli that would have previously led to rage. For example, a study
discovered that the destruction of the amygdala in a monkey who was dominant in a social group caused it
to lose its dominance. The other structure associated with aggression is hippocampus. This is involved with
the formation of long-term memories, and so allows an animal to compare the conditions of a current threat
with similar past experiences. For example, if an animal had previously been attacked by another animal, the
next time they encounter that animal they may respond with aggression. Impaired hippocampal function
prevents the nervous system from putting things into a relevant and meaningful context, and so may cause
the amygdala to respond inappropriately to sensory stimuli, resulting in aggressive behaviour. For example,
a study found that habitually violent offenders exhibited abnormalities of hippocampal functioning. Lastly,
low levels of serotonin are associated with aggression. Serotonin typically inhibits the firing of the amygdala
(which controls fear, anger and other emotional responses). Low levels remove this inhibitory effect, with
the consequence that individuals are less able to control impulsive and aggressive behaviour. As a result,
when the amygdala is stimulated by external events, it becomes more active, making aggression more likely.
Hormones are also associated with aggression, in particular testosterone. Testosterone produces male
characteristics, one of which is thought to be aggressive behaviour. The male sex hormone is thought to
influence aggression from young adulthood onwards due to its action on brain areas involved in controlling
aggression. A researcher summarised evidence in this area by describing how removing the source of
testosterone in different species typically resulted in much lower levels aggression. Subsequently, reinstating
normal testosterone levels with injections of synthetic testosterone led to a return of aggressive behaviour.
A strength of neural influences on aggression is that there is evidence for the role of the amygdala in
aggression. A study found that reduced amygdala volume can predict the development of severe persistent
aggression. They carried out a longitudinal study of male participants from childhood to adulthood. This is a
strength in itself as longitudinal studies have high validity because it determines long-term changes. In this
study, some participants with varying histories of violence were subjected to a brain MRI. The results
showed that participants with lower amygdala volumes exhibited higher levels of aggression and the
relationship between amygdala volume and violence remained even when confounding variables were
controlled. This suggests that the amygdala plays a significant role in evaluating the emotional importance
of sensory information and that lower amygdala volume compromises this ability and makes a violent
response more likely.
Another strength is that there is evidence from studies of non-human animals on the neural influence
aggression. The study found that vervet monkeys fed on experimental diets high in tryptophan (which
increases serotonin levels in the brain) exhibited decreased levels of aggression. And individuals fed on diets
that were low in tryptophan exhibited increased levels of aggression. Additionally, a further study compared
a sample of 80 dogs that had been referred to vets for their aggressive behaviour, with a control sample of 19
dogs that did not show aggressive behaviour. The aggressive dogs averaged 278 units of serotonin, while the
non-aggressive dogs averaged 387 units. The findings of these studies suggest that the different levels of
aggression found could be attributed to the animals’ serotonin levels. However, it could be argued that
because these were non-human animals, the results cannot be generalised to humans therefore, the study
lacks external validity.
A weakness, however, is that there is inconsistent evidence to show a positive relationship between
testosterone and aggression as other studies find no such relationship. For example, positive correlations
have been reported between levels of testosterone levels and self-reported levels of aggression among prison
inmates and between testosterone levels and the likelihood of responding aggressively to provocation. On
the other hand, no correlation was found between testosterone levels and actual violent behaviour among
male inmates in prison. This suggests that the relationship between testosterone and aggression in humans
remains unclear, so we need to be cautious when using it as an explanation for aggressive behaviour.
The limbic system is an area of the brain that helps to coordinate behaviours that satisfy motivational and
emotional urges, such as aggression and fear. There are two structures in the limbic system that are
associated with aggression. One is the amygdala, and this is responsible for evaluating the importance of
sensory information and prompting an appropriate response. An overreactive amygdala might produce faster
and heightened responses when the animal is mildly provoked. If the amygdala is surgically removed, the
animal no longer responds to stimuli that would have previously led to rage. For example, a study
discovered that the destruction of the amygdala in a monkey who was dominant in a social group caused it
to lose its dominance. The other structure associated with aggression is hippocampus. This is involved with
the formation of long-term memories, and so allows an animal to compare the conditions of a current threat
with similar past experiences. For example, if an animal had previously been attacked by another animal, the
next time they encounter that animal they may respond with aggression. Impaired hippocampal function
prevents the nervous system from putting things into a relevant and meaningful context, and so may cause
the amygdala to respond inappropriately to sensory stimuli, resulting in aggressive behaviour. For example,
a study found that habitually violent offenders exhibited abnormalities of hippocampal functioning. Lastly,
low levels of serotonin are associated with aggression. Serotonin typically inhibits the firing of the amygdala
(which controls fear, anger and other emotional responses). Low levels remove this inhibitory effect, with
the consequence that individuals are less able to control impulsive and aggressive behaviour. As a result,
when the amygdala is stimulated by external events, it becomes more active, making aggression more likely.
Hormones are also associated with aggression, in particular testosterone. Testosterone produces male
characteristics, one of which is thought to be aggressive behaviour. The male sex hormone is thought to
influence aggression from young adulthood onwards due to its action on brain areas involved in controlling
aggression. A researcher summarised evidence in this area by describing how removing the source of
testosterone in different species typically resulted in much lower levels aggression. Subsequently, reinstating
normal testosterone levels with injections of synthetic testosterone led to a return of aggressive behaviour.
A strength of neural influences on aggression is that there is evidence for the role of the amygdala in
aggression. A study found that reduced amygdala volume can predict the development of severe persistent
aggression. They carried out a longitudinal study of male participants from childhood to adulthood. This is a
strength in itself as longitudinal studies have high validity because it determines long-term changes. In this
study, some participants with varying histories of violence were subjected to a brain MRI. The results
showed that participants with lower amygdala volumes exhibited higher levels of aggression and the
relationship between amygdala volume and violence remained even when confounding variables were
controlled. This suggests that the amygdala plays a significant role in evaluating the emotional importance
of sensory information and that lower amygdala volume compromises this ability and makes a violent
response more likely.
Another strength is that there is evidence from studies of non-human animals on the neural influence
aggression. The study found that vervet monkeys fed on experimental diets high in tryptophan (which
increases serotonin levels in the brain) exhibited decreased levels of aggression. And individuals fed on diets
that were low in tryptophan exhibited increased levels of aggression. Additionally, a further study compared
a sample of 80 dogs that had been referred to vets for their aggressive behaviour, with a control sample of 19
dogs that did not show aggressive behaviour. The aggressive dogs averaged 278 units of serotonin, while the
non-aggressive dogs averaged 387 units. The findings of these studies suggest that the different levels of
aggression found could be attributed to the animals’ serotonin levels. However, it could be argued that
because these were non-human animals, the results cannot be generalised to humans therefore, the study
lacks external validity.
A weakness, however, is that there is inconsistent evidence to show a positive relationship between
testosterone and aggression as other studies find no such relationship. For example, positive correlations
have been reported between levels of testosterone levels and self-reported levels of aggression among prison
inmates and between testosterone levels and the likelihood of responding aggressively to provocation. On
the other hand, no correlation was found between testosterone levels and actual violent behaviour among
male inmates in prison. This suggests that the relationship between testosterone and aggression in humans
remains unclear, so we need to be cautious when using it as an explanation for aggressive behaviour.
