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Genetic Factors
in Aggression
The Genetic Link in Aggression
What have twin and adoption studies shown?
Concordance
rate
McGuffin & Gottesman (1985) study of
concordance rates for aggressive and anti-social
behaviour in MZ and DZ twins
100%
80%
87%
60%
72%
40%
20%
MZs
DZs
The Genetic Link in Aggression
What have twin and adoption studies shown?
Hutchings & Mednick (1973) reviewed over 14,000
adoptions in Denmark
Number of
convictions
for violent
offences
amongst
biological
parents
(particularly
fathers)
They found a positive
correlation between the
two variables shown
Number of convictions for violent offences amongst adopted sons
The Genetic Link in Aggression
Genes have been linked to brain chemistry and increased
aggression
MAOA is an enzyme
that facilitates the
breakdown of excess
neurochemicals, such as
noradrenaline
What does this means
for those who have a
MAOA deficiency?
They are likely to have
excess levels of
noradrenaline
The Genetic Link in Aggression
Genes have been linked to brain chemistry and increased
aggression
noradrenaline
Too much
noradrenaline in
the synapses is
associated with
increased
aggression
There is evidence to suggest that MAOA deficiency is
genetically inherited:
The Genetic Link in Aggression
Genes have been linked to brain chemistry and increased
aggression
Four generations of males in a Dutch family
inherited a defect on their x-chromosome.
The MAOA gene lays in the vicinity of the
defect so it is thought that the men’s MAOA
is affected – for genetic reasons, not enough
of it is released.The men with the defective
gene (and not all men in the family
inherited it) showed aggressive, sometimes
violent behaviour including arson and
attempted rape.
Bruner tested the urine of the men in the
family: those with the defective gene had
higher levels of chemicals that result from
neurotransmitters, which indicates that the
men’s MOAO is deficient
The Genetic Link in Aggression
Genes have been linked to brain chemistry and increased
aggression
Further support for the
gene hypothesis comes
from 110 men who
showed an association
between abnormalities
in the MAOA gene and
aggressiveness and
impulse control
(Manuck et al, 2000)