Neural and hormonal mechanisms
1. Hormonal influences.
Aggression occurs more often in males than females which is usually attributed to the
male sex hormone – testosterone.
a) Testosterone
Testosterone is thought to influence aggression from young adulthood onwards due
to its action on brain areas involved in controlling aggression.
Research. Salivary testosterone was measured in violent and non-violent criminals –
those with highest levels had history of violent crimes. Low levels – non violent
crimes. Similar results found in non prison population – Dabbs. Lindman – found
young males who behave aggressively when drunk had higher testosterone levels.
Wagner – if male mouse is castrated levels of aggression reduced, if mouse then gets
testosterone levels of aggression increase – suggesting an association – but cannot
prove cause and effect.
Nazca Booby bird practice obligate siblicide. 2 eggs are laid, several days apart. If both
hatch the elder chick will push out its sibling, leaving it to die of thirst or cold. The
parent booby will not intervene. This behaviour is linked to high levels of testosterone
in the hatchlings. The surviving chick will also behave as a bully when it grows up.
Higher levels could also cause social dominance, impulsiveness and competition. In
sport males and females in aggressive sports e.g. boxing had higher levels – also had
higher spatial ability so testosterone could have positive and negative effects.
Challenge hypothesis – Wingfield – in monogamous species (humans?) levels of
testosterone should only rise above base line breeding level in response to social
challenge. So if threat deemed relevant to reproductive capacity e.g. dispute over
female – surge is to be expected.
BUT testosterone may have a role to play but evidence is inconsistent. May make you
more competitive/dominant but not necessarily aggressive. Mazur suggests we should
distinguish between aggression and dominance aggressive when intention is to inflict
injury – act dominantly to maintain status. Higher levels may be effect of dominance
not the cause. Most studies showing a positive correlation have involved small
samples of men in prison using either self report or judgements based on severity of
crime
Link between testosterone and aggression in females may be even higher than for
men. Women with higher levels had higher occupational status (needed to be more
assertive?) but it might also make it harder for them to form alliances that usually
occur in groups of women.
In women PMT has been used successfully to reduce murder charge to manslaughter.
b) Cortisol appears to have mediating effect on aggression-related hormones like
testosterone. Studies have shown low levels of cortisol in habitual violent
offenders and violent school children.
Support – in a 4 year study of boys with behavioural problems- those with low
levels of cortisol began antisocial acts at a young age and showed 3x number of
aggressive symptoms compared to boys with higher or fluctuating cortisol – so
cortisol may also play role in likelihood of aggression (biological)
2. Neurotransmitters – these are chemicals that allow impulses within the brain to be
transmitted from one area of the brain to another.
a) Sorotonin
Thought to reduce aggression by inhibiting responses to emotional stimuli that might
lead to aggressive response. Low levels are associated with impulsive behaviour,
aggression and violent suicide. A meta analysis – Sverbo and Raine – examined levels
in antisocial children and adults – consistently found lower levels in individuals
described as being aggressive (no change in dopamine levels) lower serotonin levels in
all antisocial groups but especially in those who had attempted suicide. Suggesting
depletion leads to impulsive behaviour which may lead to aggression (all studies done
prior to 1992)
Some drugs thought to alter serotonin levels – Mann gave 35 healthy subjects
dexfenfluramine (depletes serotonin) using questionnaire to asses hostility and
aggression – found an increase in both in males but no chance in females.
SUPPORT from animal studies – Raleigh – vervet monkeys if fed on diet high in
tryptophan (which increases serotonin levels) showed decreased levels of aggression.
If low in diet showed increased levels of aggression. i.e. possibly due to serotonin
levels – animals bred for domestication and docile temperaments - concentration of
serotonin levels increases over the generations.
b) Dopamine
Some evidence of a link, increased use of amphetamines (increases dopamine
activity) also associated with increase in aggressive behaviour. Antipsychotic drugs
which reduce dopamine shown to reduce aggressive behaviour in violent delinquents.
BUT research fairly inconclusive about causal role of dopamine – may be a
consequence. In mice reward pathway in brain becomes engaged in response to
aggressive event and dopamine involved as the positive reinforce, i.e. seek out
aggressive encounter as it is rewarding.
Other AO2
Gun crime – perhaps the presence of a weapon increases testosterone levels which in turn
increases aggressive behaviour. Klinesmith – male college students –measured testosterone
in saliva then interacted for 15 mins with either a gun or child’s toy then re-measured. Those
with gun showed significant increase in testosterone and behaves more aggressively.
Clear links with non-human animals but not so clear in humans , humans have are more
complex social behaviour
Neural mechanisms - Brain Structure.
Amygdala
 Electrical stimulation produces aggression in humans and animals
 Wong found reduced amygdale size and activity in violent criminals
 Contradiction – Muller positive and negative pictures were presented to 6
male psychopaths and 6 normal control males. Brain activity was measured
using fMRI. Found increased activity in several brain areas including the
amygdale
 Exact role of amygdale is unclear, but seems to be part of a circuit of structures
responsible for aggression, which is controlled in a top-down fashion by the
prefrontal cortex.
Prefrontal cortex
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Thought to regulate amygdale- driven emotional responses, and damage
can result in loss of control, impulsivity, immaturity and emotionality
Anderson – damage to prefrontal cortex in infancy increases risk of
aggressive behaviour as adults – greater than damage when an adult. May
be due to the negative influence of brain damage on social development.
Raine found reduced brain activity in the prefrontal cortex of 41 murderers.
Supported by Volkow who found reduced prefrontal cortex activity in the
brains of 8 violent psychiatric prisoners.
Hypothalamus and amygdale associated with aggressive response. Individuals with
frontal lobe damage often show impulsive behaviour, irritability and shortness of
temper.
Phineas Gage 1848
Gage survived for 11 years following an accident but changed from a quiet hard –
working individual into a very negative and aggressive individual. But this is a case study so
generalisability is limited.
A lot of research is on animals so again problem of generalising to humans....................
To assume aggression is one cause – biological is too simple e.g. culture might influence
aggressive behaviour and situational factors e.g. alcohol disrupting information processing (
interpreting a comment as meaningful) noise can also explain aggressive behaviour.
Biological argument ignores argument showing aggressive behaviour may be a result of
frustration and becomes aggressive action through identification of aggressive cues in the
environment.
Positive – Reductionism
Phenomena can best be explained by analysing component parts, thus neglecting higher
levels of explanation e.g. psychological, social and cultural ones. Ethical issues of testing
animals – many techniques are painful and irreversible.
Reductionist – on its own biological explanation insufficient to explain the different aspects.
Lots of reasons other than hormone levels for aggression to occur e.g. social factors.
Disregards free will in the form of self control.
Most human research is correlational – cannot prove cause and effect
Measuring aggression is unreliable as behaviour is open to interpretation