Neural and hormonal mechanisms in aggression, including the roles of the limbic
system, serotonin and testosterone
● Serotonin- Under normal circumstances, the neurotransmitter serotonin works on the
frontal areas of the brain to inhibit the firing of the amygdala, the part of the limbic
system in the brain that controls fear, anger and other emotional responses.
Consequently, serotonin has a calming influence.
● Low levels of serotonin mean that people can’t control their impulsive and aggressive
behaviour. Serotonin also regulates the prefrontal cortex; therefore, lower levels of
serotonin affect our response to external stimuli, meaning the can’t control their
emotions as well and so becomes aggressive easily and can’t control their responses
in a ‘normal’ way. They can’t anticipate risk and therefore impulsively engage in
aggressive behaviour.
Evaluation
★ Godar et al (2014, see below)
★ Mann et al. (1990) administered the drug dexfenfluramine (which depletes serotonin in
the brain) to 35 healthy adults. The researchers then used a questionnaire to assess
hostility and aggression levels, which rose following administration of dexfenfluramine
amongst males, but interestingly not amongst females. The research by Mann et al.
(1990) demonstrates the issue of beta bias that is inherent in neural explanations of
aggression and shows that males and females may not be subject to the same
physiological factors when explaining aggression.
★ Ferrari et al. (2003) provide support for the role of serotonin in aggressive behaviour.
They allowed adult male rats to fight with another rat at a specific time for ten days. On
the eleventh day, the rat wasn’t allowed to fight. However, researchers found that the
rat’s dopamine levels had raised by 65%, and his serotonin levels were reduced by 35%.
Despite the fact that the rat was not fighting, the experience had changed the rat’s brain
chemistry. Ferrari et al.’s (2003) research raises the question of whether lower levels of
serotonin cause aggression or whether they are a response to aggression being carried
out. The issue of cause and effect is a key factor in the explanation of aggressive
behaviour. The aim of any science is to establish the cause by measuring the effect.
However, if the cause, i.e. lower levels of aggression, is the effect, this substantially
lowers the validity of the explanation as to the causes of aggression. Nevertheless, this
research does demonstrate the complexity of the role of serotonin in aggressive
behaviour
★ Lindberg et al. (1985) compared serotonin levels of 16 male violent criminals with 22
males who had attempted suicide and 39 non-violent males, finding the lowest levels of
serotonin by-product 5-HIAA among violent criminals and those who had attempted
suicide. This finding indicates that the level of serotonin in the brain negatively correlates
with aggression.
- Lindberg's study suffers from beta bias as the study only includes male participants, and
assumes that low serotonin levels will increase aggression in all humans, ignoring the
possibility that low serotonin could affect the female brain in a different, perhaps
non-aggressive way.
, ★ However most evidence linking low levels of serotonin and aggression is only
correlational and does not indicate causality.
● Testosterone is the sex hormone (androgen) that controls the development of male sex
characteristics. It has a role in regulating social behaviour by influencing parts of the
brain responsible for emotion and decision-making. Increased levels of aggression are
thought to be due to higher levels of testosterone.
● As testosterone levels are around 8 times higher in men than in women this suggests
that testosterone in males is the reason for observed sex differences in levels of
aggression between genders. High levels of testosterone lower activity in the
orbitofrontal cortex meaning that in an emotional situation there is more likely to be an
aggressive response. High levels can also reduce serotonin, leading to aggression.
Evaluation
★ Rissman (2006) (see below)
★ Wagner (1979) castrated mice and aggression levels went down, thus providing support
to the theory that testosterone is implicated in aggression. He then injected them with
testosterone aggressive behaviour increased, suggesting that high levels of testosterone
cause aggressive behaviour. (+ non correlational, so causal, - however is an animal
study)
★ This study is backed up by Van Goozen (1997) who conducted a natural experiment on
transgender sex-change patients. This is one of the few cases where research was
actually carried out on humans.
★ Findings revealed testosterone levels governed aggression. Males receiving
testosterone suppressants became less aggressive. Females receiving testosterone
became more aggressive.
★ Individuals with elevated testosterone levels exhibit signs of aggression, but rarely
commit aggressive acts, suggesting that social and cognitive factors play a mediating
role (Higley et al., 1996).
★ Dabbs and Morris (1990) 'Blocked pathways to success' study: When a rich boy with
high testosterone came home from the army he was less likely to get into trouble, but
when a poor boy with high testosterone came home he was more likely to get into
trouble.
★ The testosterone explanation has face validity as explains differences between genders,
and also uses scientifically valid measures, however is reductionist and biologically
deterministic, and overly reliant on animal studies and correlations.
● The limbic system is a collection of structures (including the hypothalamus, amygdala,
and hippocampus) in the brain that process emotional responses. It has been identified
as being responsible for aggressive behaviour, with the external layer of the brain (the
frontal cortex) working to control aggression produced by the limbic system.
Evidence
★ Bard and Mountcastle (1958) found that destruction of the hypothalamus has been
shown to produce permanent aggressive behaviour in cats.
, ★ Wong et al. (1997) undertook MRI scans of 19 violent male criminals in Broadmoor
hospital and compared the size of the amygdala with 20 ‘normal’ control subjects. He
found that the volume of the amygdala was significantly smaller in the 19 violent
criminals, thus supporting the role of the amygdala and limbic system in aggression. One
of the problems with this research is a lack of population validity: the sample was
relatively small, and thus the issue of whether these findings can be generalised to the
wider population can be raised. The research can be accused of beta gender bias, as
their research was confined to males; therefore, caution must be taken in using this
research to explain aggression in females. This is also an example of socially sensitive
research as it has negative implications for those studied (i.e: locking up all the people
with smaller amygdalas to save time and money), leads to questions about biological
determinism.
★ Summer’s case study of an aggressive 14 year old girl who had had epileptic fits since
the age of 3 found via MRI scan that she had a tumour in her limbic system. After this
was removed she returned to normal levels of aggression, suggesting that differences in
the limbic system may cause aggression. The biological approach to explaining
aggression threatens the idea that we act aggressively according to freewill.
Genetic factors in aggression, including the MAOA gene
● If aggression has a significant genetic component, then seeing aggression in one twin
should positively correlate with aggression in the other. Monozygotic twins should show
a higher concordance rate than dizygotic twins as they share 100% of their genes.
Evaluation
★ Coccaro et al. (1997) compared monozygotic versus dizygotic twin pairs. In this study,
twin pairs were examined for the concordance of criminal behaviour for both twins. The
MZ twin pairs were found to have a 50% concordance whereas DZ twins were only 19%.
This research supports the role of genetics in aggression as the twins that were
genetically identical, i.e. 100% of the same genes, were more likely to display criminal
behaviour than the twins that only shared 50% of the same genes.
★ This is backed up as using old Danish police records Christiansen (1977) demonstrated
that levels of criminality showed a stronger correlation between identical twins – with the
same genes – than between dizygotic twins. However criminality is not always the same
as aggression.
★ Other theories such as social learning theory lead us to question whether aggression is
purely genetic.
● The Supermale theory- The basic difference between men and women lies in their
genotype. Most people have 23 pairs of chromosomes and on these chromosomes are
our genes. One pair of chromosomes decides whether we are male (XY) or female (XX).
The Super-Male hypothesis (Sandberg, 1961) suggested the XYY Genotype leads to
aggression.
Evaluation
system, serotonin and testosterone
● Serotonin- Under normal circumstances, the neurotransmitter serotonin works on the
frontal areas of the brain to inhibit the firing of the amygdala, the part of the limbic
system in the brain that controls fear, anger and other emotional responses.
Consequently, serotonin has a calming influence.
● Low levels of serotonin mean that people can’t control their impulsive and aggressive
behaviour. Serotonin also regulates the prefrontal cortex; therefore, lower levels of
serotonin affect our response to external stimuli, meaning the can’t control their
emotions as well and so becomes aggressive easily and can’t control their responses
in a ‘normal’ way. They can’t anticipate risk and therefore impulsively engage in
aggressive behaviour.
Evaluation
★ Godar et al (2014, see below)
★ Mann et al. (1990) administered the drug dexfenfluramine (which depletes serotonin in
the brain) to 35 healthy adults. The researchers then used a questionnaire to assess
hostility and aggression levels, which rose following administration of dexfenfluramine
amongst males, but interestingly not amongst females. The research by Mann et al.
(1990) demonstrates the issue of beta bias that is inherent in neural explanations of
aggression and shows that males and females may not be subject to the same
physiological factors when explaining aggression.
★ Ferrari et al. (2003) provide support for the role of serotonin in aggressive behaviour.
They allowed adult male rats to fight with another rat at a specific time for ten days. On
the eleventh day, the rat wasn’t allowed to fight. However, researchers found that the
rat’s dopamine levels had raised by 65%, and his serotonin levels were reduced by 35%.
Despite the fact that the rat was not fighting, the experience had changed the rat’s brain
chemistry. Ferrari et al.’s (2003) research raises the question of whether lower levels of
serotonin cause aggression or whether they are a response to aggression being carried
out. The issue of cause and effect is a key factor in the explanation of aggressive
behaviour. The aim of any science is to establish the cause by measuring the effect.
However, if the cause, i.e. lower levels of aggression, is the effect, this substantially
lowers the validity of the explanation as to the causes of aggression. Nevertheless, this
research does demonstrate the complexity of the role of serotonin in aggressive
behaviour
★ Lindberg et al. (1985) compared serotonin levels of 16 male violent criminals with 22
males who had attempted suicide and 39 non-violent males, finding the lowest levels of
serotonin by-product 5-HIAA among violent criminals and those who had attempted
suicide. This finding indicates that the level of serotonin in the brain negatively correlates
with aggression.
- Lindberg's study suffers from beta bias as the study only includes male participants, and
assumes that low serotonin levels will increase aggression in all humans, ignoring the
possibility that low serotonin could affect the female brain in a different, perhaps
non-aggressive way.
, ★ However most evidence linking low levels of serotonin and aggression is only
correlational and does not indicate causality.
● Testosterone is the sex hormone (androgen) that controls the development of male sex
characteristics. It has a role in regulating social behaviour by influencing parts of the
brain responsible for emotion and decision-making. Increased levels of aggression are
thought to be due to higher levels of testosterone.
● As testosterone levels are around 8 times higher in men than in women this suggests
that testosterone in males is the reason for observed sex differences in levels of
aggression between genders. High levels of testosterone lower activity in the
orbitofrontal cortex meaning that in an emotional situation there is more likely to be an
aggressive response. High levels can also reduce serotonin, leading to aggression.
Evaluation
★ Rissman (2006) (see below)
★ Wagner (1979) castrated mice and aggression levels went down, thus providing support
to the theory that testosterone is implicated in aggression. He then injected them with
testosterone aggressive behaviour increased, suggesting that high levels of testosterone
cause aggressive behaviour. (+ non correlational, so causal, - however is an animal
study)
★ This study is backed up by Van Goozen (1997) who conducted a natural experiment on
transgender sex-change patients. This is one of the few cases where research was
actually carried out on humans.
★ Findings revealed testosterone levels governed aggression. Males receiving
testosterone suppressants became less aggressive. Females receiving testosterone
became more aggressive.
★ Individuals with elevated testosterone levels exhibit signs of aggression, but rarely
commit aggressive acts, suggesting that social and cognitive factors play a mediating
role (Higley et al., 1996).
★ Dabbs and Morris (1990) 'Blocked pathways to success' study: When a rich boy with
high testosterone came home from the army he was less likely to get into trouble, but
when a poor boy with high testosterone came home he was more likely to get into
trouble.
★ The testosterone explanation has face validity as explains differences between genders,
and also uses scientifically valid measures, however is reductionist and biologically
deterministic, and overly reliant on animal studies and correlations.
● The limbic system is a collection of structures (including the hypothalamus, amygdala,
and hippocampus) in the brain that process emotional responses. It has been identified
as being responsible for aggressive behaviour, with the external layer of the brain (the
frontal cortex) working to control aggression produced by the limbic system.
Evidence
★ Bard and Mountcastle (1958) found that destruction of the hypothalamus has been
shown to produce permanent aggressive behaviour in cats.
, ★ Wong et al. (1997) undertook MRI scans of 19 violent male criminals in Broadmoor
hospital and compared the size of the amygdala with 20 ‘normal’ control subjects. He
found that the volume of the amygdala was significantly smaller in the 19 violent
criminals, thus supporting the role of the amygdala and limbic system in aggression. One
of the problems with this research is a lack of population validity: the sample was
relatively small, and thus the issue of whether these findings can be generalised to the
wider population can be raised. The research can be accused of beta gender bias, as
their research was confined to males; therefore, caution must be taken in using this
research to explain aggression in females. This is also an example of socially sensitive
research as it has negative implications for those studied (i.e: locking up all the people
with smaller amygdalas to save time and money), leads to questions about biological
determinism.
★ Summer’s case study of an aggressive 14 year old girl who had had epileptic fits since
the age of 3 found via MRI scan that she had a tumour in her limbic system. After this
was removed she returned to normal levels of aggression, suggesting that differences in
the limbic system may cause aggression. The biological approach to explaining
aggression threatens the idea that we act aggressively according to freewill.
Genetic factors in aggression, including the MAOA gene
● If aggression has a significant genetic component, then seeing aggression in one twin
should positively correlate with aggression in the other. Monozygotic twins should show
a higher concordance rate than dizygotic twins as they share 100% of their genes.
Evaluation
★ Coccaro et al. (1997) compared monozygotic versus dizygotic twin pairs. In this study,
twin pairs were examined for the concordance of criminal behaviour for both twins. The
MZ twin pairs were found to have a 50% concordance whereas DZ twins were only 19%.
This research supports the role of genetics in aggression as the twins that were
genetically identical, i.e. 100% of the same genes, were more likely to display criminal
behaviour than the twins that only shared 50% of the same genes.
★ This is backed up as using old Danish police records Christiansen (1977) demonstrated
that levels of criminality showed a stronger correlation between identical twins – with the
same genes – than between dizygotic twins. However criminality is not always the same
as aggression.
★ Other theories such as social learning theory lead us to question whether aggression is
purely genetic.
● The Supermale theory- The basic difference between men and women lies in their
genotype. Most people have 23 pairs of chromosomes and on these chromosomes are
our genes. One pair of chromosomes decides whether we are male (XY) or female (XX).
The Super-Male hypothesis (Sandberg, 1961) suggested the XYY Genotype leads to
aggression.
Evaluation
