BIOLOGICAL EXPLANATIONS OF AGGRESSION
Genetic factors in aggressive
The Y-chromosome seems to be linked with
aggression. Due to it not dividing properly
when sperms are made, a very few men are
XYY- and have a high chance of being more
aggressive than XY (most men).
CAN A GENE INFLUENCE THE LEVEL OF
Genes can affect testosterone level
Genes can affect how quickly testosterone
circulate around the body
Genes can affect the number of testosterone
receptors and the sensitivity of these receptors.
IS AGGRESSION CAUSED BY GENETICS?
Sandberg 1971 first identified what is known
as 47 XYY Karyotype.
Most individuals have 46 chromosomes, 23
from each parent, it is possible for a male to
have an extra Y sex chromosome making them
SUPER MALE SYNDROME: XYY KARIOTYPE
Affected individuals are usually very tall and
Additional symptoms may include antisocial or
behavioral problems and learning disabilities.
Intelligence is usually normal, although IQ, on
average, is 10 to 15 points lower than siblings.
A02: ARE XYY MORE AGGRESSIVE?
Court-Brown 1965-67, found that of a sample of 314 patients
in a high security hospital, 15 were found to have chromosomal
abnormalities, including 9 who had an extra Y chromosome.
Court-Brown concluded that those with XYY would be best kept
hospitalised due to an increase likelihood of aggressive
A02: ARE SUPER MALES MORE AGGRESSIVE?
Theilgaard (1984) examined the personality traits
of a sample of XYY men and compared them to
XY men, focusing on levels of aggression between
the two groups. Her research concluded that in
all occurrences of XYY (1 in every 1000) the only
factor that seems to be linked is their height.
Levels of aggressiveness fluctuated so no definite
conclusion could be drawn.
New technology has enabled researchers to
examine DNA at the molecular level.
This type of research has produced a number of
breakthroughs in the effect of genes on aggressive
MAO-A gene has been associated with increased
It is sometimes referred to as the ‘Warrior Gene’
The MAO-A gene produces Monoamine Oxidase, a
chemical that is involved in the breakdown of the
neurotransmitters serotonin, dopamine and
norepinephrine in synapses.
WHAT IS SEROTONIN?
Serotonin is a neurotransmitter.
It can be found in the central nervous system (CNS)
where it has various functions, including the
regulation of mood” (wikipedia)
Serotonin has effects all over the body.
Low levels of serotonin in the brain can result in
impulsive behaviour, aggression, overeating,
depression, alcohol abuse and violent suicide
HOW DOES THIS WORK
It is thought that having too little of the MAO-A gene
leads to the brain being flooded with too much
serotonin, norepinephrine and dopamine, which
eventually leads to a lowered sensitivity to these
low sensitivity is equivalent to low activity.
E.G. MAOA GENE
Structural (left) and functional (right) MRI
scan data shows that subjects with the
violence-related version of the MAO-A
gene (MAOA-L) had reduced volume and
activity of the anterior cingulate cortex
(blue area in front part of brain at left
and corresponding yellow area in at
right), which is thought to be the hub of a
circuit responsible for regulating
impulsive aggression. The color-coded
areas show where subjects with the L
gene type differed from subjects with the
H gene type.
Source: NIMH Clinical Brain Disorders
codes for the enzyme monoamine
oxidase-A (MAO-A), which breaks
down key mood-regulating chemical
messengers, most notably serotonin.
These, in turn, influence how the
brain gets wired during development.
The variations may have more impact
on males because they have only
one copy of this X-chromosomal
gene, while females have two copies,
one of which will be of the H variant
in most cases.
AO2 SUPPORTING RESEARCH EVIDENCE
Moffat et al. (2002) conducted a longitudinal study of 422 males in
New Zealand. He studied their history of abuse and criminal
convictions, their penchant for violence and any symptoms of antisocial
personality disorder. His findings showed that there was a link between
MAOA the risk of being convicted of a violent crime;
however, this was only when participants also suffered abuse as
This is a clear example of a gene-environment interaction, the gene only
exerts its effect in particular environmental conditions.
Video Clip: Jim Fallon on the "Warrior Gene"
Brunner et al. (1993) studied a large Dutch
family where all the males had a mutant form of
the MAOA gene.
Moffitt et al. (1992) performed a longitudinal
All had borderline retardation and reacted
study on 442 New Zealand males from birth
aggressively when angry, fearful or frustrated,
to age 26, recording which participants as
suggesting that abnormal MAOA activity is
children suffered abuse and also what level
associated with aggression.
of activity of the MAOA gene participants had.
It was found that those who had suffered
abuse and had the low-activity version of the
gene were nine times more likely to indulge
in antisocial behaviour, including aggression.
Participants who had been abused, but
carried the high-activity version of the gene,
were no more likely to be antisocial than
those not suffering abuse, suggesting that
the MAOA gene is involved in aggressive
behaviour, but is sensitive to social
experiences early in development.
Several previous studies had linked increased
serotonin during development with violence and
the L version of MAO-A. For example Caspi et al.
(2002) discovered that the gene's effects depend
on interactions with environmental factors: men
with L were more prone to impulsive violence, but
only if they were abused as children.
This is a clear example of a geneenvironment interaction, the
gene only exerts its effect in
Video Clip: Jim Fallon on the
ANIMAL BREEDING STUDIES
In animal breeding studies, animals are
selectively bred for certain traits. If it is possible
to breed for aggression, this would suggest that
genes contribute to aggressiveness
A02 SUPPORT: DMITRI BELYAEV 1950
Belyaev's aim was to reduce the aggressiveness of the Silver Fox to make
them easier to breed for their much sought after silver fur.
He found that after 18 generations the animals were tame: they approached
humans, barked like dogs and even rolled over to have their tummies
Unfortunately for Belyaev, the animals even looked like dogs, with floppy
ears and a piebald coat – not the silver coat that the fur industry required.
The silver fox breeding programme was also able to breed extremely vicious
silver foxes, which supports the hypothesis that genes are involved in
AO2 SUPPORT FOR GENETICS:
Cases et al. (1995) studied mice genetically
engineered to lack MAOA. The mice had a
dramatically altered serotonin metabolism
and severe behavioural alterations. When
adult, they showed enhanced aggression and
were aggressive during mating, giving
support to human studies suggesting that
Bock and Goode (1996) found that male
aggression is a direct result of MAOA
mice reared alone show a stronger tendency
deficiency, rather than other genetic
to attack other male mice then those reared
influences or psychosocial factors
with others. Other male mice, reared with
their parents, had been shown when it was
necessary to be aggressive and when it was
not, giving support to the idea that
aggression is a natural biological tendency
shaped by learning experiences.
ISSUES AND DEBATES: EVALUATION
The results of studies on animals cannot be
extrapolated to humans because they are
physiologically and cognitively different
however for ethical reasons we could not carry
out such studies on humans. Breeding
aggressive animals can also raise ethical
ISSUES AND DEBATES:
THE EXPLANATION IS REDUCTIONIST
This explanation does not take into account the role of
psychological factors such as unconscious conflicts in
It does not take into account that aggressive behaviour
can learnt through imitation (social learning theory).
It does not take into account the role of anonymity and
deindividuation which Zimbardo argues plays an important
role in aggressive behaviour.
It does not account for situational factors and tends to
attribute aggressive behaviour to dispositional factors.
EVALUATION: DIATHESIS STRESS
On its own, the MAOA gene variant has no effect,
but if males who carry the MAOA gene were
abused as children, there is a greatly increased
chance of them committing violent crime.
It seems that aggressiveness is influenced by a
variation in the MAOA gene, which is sensitive to
social experiences early in development; therefore
it’s functional outcome depends on social context.
This indicates an interaction of genetics and
environment is at work in determining human
EVALUATION: REAL WORLD APPLICATION
Waldroup shot his wife's friend Leslie Bradshaw,
eight times, killing her before attempting to kill his
wife by chopping her up with a machete
Was he responsible or did
his “warrior gene” make
him do it?
Practical application: In a 2009 criminal trial in the United
States, in the case of Bradley Waldroup, an argument based on
a combination of "warrior gene" and history of child abuse was
successfully used to avoid a conviction of first-degree
murder and the death penalty; however, the convicted murderer
was sentenced to 32 years in jail.