AGGRESSION: Year 13
 Genes
are the ‘hand behind the scenes’...
directing testosterone’s actions...
 Genes
determine how much testosterone
or oestrogen is produced and how
quickly it circulates around the body.
 Genes determine the synthesis of
testosterone receptors, and how many
and how sensitive such receptors are.
 Testosterone may affect brain function
and contribute to aggression but genes
regulate how much testosterone is made
and how effectively it works.
 Early
research focussed on chromosomal
abnormalities – specifically the XYY
genotype, as being correlated with
aggression.
 Males with an extra Y (male)
chromosome
 Over represented in prisons
 XYY also linked with low intelligence
 BUT a definitive link has not been proved
 Selectively
bred animals
 Turner (2007) – aggressiveness in pigs
 Suggests that pig farmers may try to
selectively breed non-aggressive pigs and
aggressiveness should fall by 5% per year
during the early years of selection
Evaluation?
 1995
– mice that lack this suffer serious
anger management problems
 The
enzyme made by the gene mops up
the excess neurotransmitters, so mice
who didn’t have the gene had unusually
high levels of neurotransmitters such as
serotonin, noradrenaline, dopamine
 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.
 Sensitive to social experiences in early
development.
 Functional
context!
outcome depends on social
 Scientists
have managed to clone genetic
DNA
 Mutant mice have been produced that
lack a gene for the serotonin receptor
 These mice have normal behaviour
generally, but are twice as aggressive as
‘normal’ mice
 Bock & Goode, 1996
 MAOA-deficient
male mice were quick to
attack an intruder in a resident intruder
test and failed to establish the usual
dominant-submissive relationships,
which meant that these mice were injured
more when confined with other male
mice
Evaluation?
 This
gene has been implicated in human
aggression (Cases et al., 1995)
 Participants: 1037
children (442 boys, 595
girls) all born in 1972 in Dunedin, New
Zealand
 Length of study: 26 years from birth
 What did the researchers look at?
Whether the participants had high or low
levels of MAOA and the upbringing of the
children (were they abused?) and anti
social behaviour.
Diagnosis of
conduct
disorder
during
adolescence
Conviction
for a violent
crime
Tendency
towards
violent
behaviour
Signs of
antisocial
personality

What did they find out about men who had the
genotype for high MAOA activity?
• In all four of the areas above, men who had been maltreated or
abused as children were far less likely to show antisocial
behaviour as adults

What did the find out about men who didn’t have the
genotype for high MAOA activity?
• Men who had been maltreated were more likely to show
antisocial behaviour as adults.

Maltreated men without the genotype for high MAOA
activity only made up about 12% of the group studies –
however, they accounted for 44% of the group’s
convictions for violent crime.



It is important to note that the MAOA gene variant had
no effect on its own, but if men who carried the gene
that generated low levels of MAOA activity were
abused as children then they were over three times
more likely to commit violent crime.
A gene that generated high levels of MAOA seemed to
give ‘trauma resistance’ – people who had been abused
in childhood were protected against the potential
negative effects.
The gene is found on the X chromosome, and it’s
thought that it doesn’t have an effect on girls because
the other X chromosome cancels out the effect, as it
were. Boys do not have the luxury of two X chromosome
so the effects are more noticeable.
 Dutch
family – all males have mutant
MAOA gene.
 All borderline retardation
 Reacted aggressively when angry, fearful
or frustrated.
 Impulsive
aggression (domestic / work
based) have genetic component related
to seretonergic system.
 Aggressive patients with personality
disorders had a G-allele variant of a
serotonin gene HTR1B
Mednick
et al (1984)
• 14,000 male adoptees
Percentage of criminal convictions
30
25
20
15
10
5
0
Percentage of criminal
convictions
Reductionist
Ethics
Extrapolation
– animal studies
Research support
Methodological issues
• Sample
• Operationalisation etc
 Critically
consider the role of genetic
factors in aggressive behaviour.
• Hand behind the scenes
• XXY
• Inheritability
• MAOA
• Twin studies
• Adoption studies
AO2