Download BIOLOGICAL EXPLANATION OF AGGRESSION

BIOLOGICAL
EXPLANATION OF
AGGRESSION
THE ROLE OF GENETIC FACTORS
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Understand how biological and physiological
processes affect aggressive behaviour.
Understand how biological explanations of
behaviour are split into genetic, biochemical
and neuro-anatomical explanations.
Describe the various biological and
physiological processes that lie behind
aggressive behaviour.
Evaluate the effectiveness of the various
biological explanations of the cause of
aggression.
Learning objectives
Genes do not directly cause aggression
 Genes affect muscle and bone
development necessary for aggression.
 Genes influence neurochemical, hormonal
and sensory systems.
 It is a combination of these that
contribute to an animal’s aggressive
behaviour.
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The role of genes in aggression
Is aggression passed on?
Nelson 2006
 Found selective
breeding can lead
to more aggressive
behaviour in
animals.
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Choosing animals with desirable
characteristics for breeding.
Selective breeding
Spanish fighting bulls
 Chickens (cock
fighting)
 Fish (Siamese
fighting fish)
 Dogs (pit bull
terriers)
 Compared to regular
animals they are
more developed for
fighting.
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Selective breeding for aggression
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Lagerspetz 1979
Isolated mice after
weaning
Tended to be
aggressive when
put with other mice
(attacking, biting,
speed).
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interbred
These mice were
interbred.
By the 19th
generation rates of
aggressive biting in
aggressive mice
was 52%
Only 5% in ordinary
mice.
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These mice also
had:
Heavier testes
Heavier forebrains
Altered levels of
neurochemical
serotonin in the
forebrain and
noradrenaline in
brain stem.
Aggressive mice
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Supports the
notions that genes
influence both
Structural and
Functional aspects
of an animal’s
biology.
conclusions
Aggression can be passed from one
generation to another.
 Animal studies such as Cairns and Nelson
have shown this.
 But there are environmental influences as
well such as upbringing and social
influences (SLT, deindividuation, cue
arousal, relative deprivation etc.)
 These are played down by the genetic
explanation.
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Genetic factors do
not absolutely
determine
aggressive
behaviour.
Environmental
factors are also
important...
The aggressive
mice can be
conditioned to be
less aggressive
Lagerspetz 1981
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In the same way,
wild animals have
been tamed for
years to become
pets.
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FINDINGS
EXTREMELY
VARIABLE
Canter (1973)
Correlation of 0.14
for MZs reared
together.
O’Connor (1980)
found 0.72
TWIN STUDIES
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Twins reared apart
showed slightly less
variability.
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Greater
association
with MZ
twins than
DZ twins
whether
reared
together or
apart.
MZ
DZ
Reared
together
Reared
apart
Reared
together
Reared
apart
0.72
0.64
0.42
0.34
0.39
0.46
0.42
0.06
0.14
0.53
0.30
0.39
Indicates a genetic contribution
 Heritability
varies
according to
assessment
method
 Self –report –
39%
 Reported by
others – 53%
Heritability
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Heritability
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the extent to which
genetic individual
differences contribute
to individual
differences in
observed behaviour
(or phenotypic
individual
differences).
Is aggression caused by
genetics?
Most individuals
have 46
chromosomes (23
from each parent).
 It is possible for a
male to have an
extra Y
chromosome
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http://www.youtube.com/wa
tch?v=5CYdMspQzDw
Richard Speck
July 13 1966
 Speck murdered 9
nurses
 He was later found
to have the XYY
chromosome
 Aha!!
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This was later found
to be a mistake!
Oh No!
Popular beliefs about 47 XYY
males
The media have
encouraged belief
that the XYY male
is more aggressive
than normal males.
 Popularised in TV
series such as
Doomwatch, XYY
Man, Alien 3.
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Evidence – Theilgaard 1984
About one in a
thousand males
 Taller than usual
 But not necessarily
more likely to show
increased levels of
aggression.
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Jacobs et al 1965
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3 in a hundred
Found XYY syndrome
at the rate of 3% in
prisoners
Only 0.1% in normal
population (One in a
thousand)
Evidence for XYY and aggression?
XYY males
less intelligent
 More likely to commit
crimes but not
generally violent
ones.
 More likely to get
caught
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Doh!
Theilgaard found that XYY men tend to
give more aggressive interpretations to
the images used in these tests.
 But consider the validity and reliability of
the test.
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Thematic apperception testing
Rorschach test
MAOA gene
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Studies have found a
link with aggression.
MAOA gene regulates
an enzyme in the
brain (Monoamine
Oxidase A)
This removes excess
amounts of
neurotransmitters.
The effect of genes on
neurotransmitters
plausible
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Build up of
neurotransmitters is
related to mood
And can cause an
aggressive reaction
to a stressful
situation.
Biochemistry and aggression
Brunner et al (1993)
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Discovered a
mutation in the MAOA
gene in a Dutch
family.
Family records
showed history of
violence in the males
(often associated
with aggressive
events.
Only affects men
Evidence
Men have only one X
chromosome.
Might inherit the gene
but have a second X
chromosome carrying
a good copy of the
gene.
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Cases et al (1995)
 Disabled the MAOA
gene in X
chromosome of mice
and found males
became highly
aggressive.
 Females didn’t.
Males
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Lacked the
monoamine oxidase A
enzyme.
Levels of dopamine
and serotonin
increased.
Females
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Had the other
functioning X
chromosome to do
the job.
MAOA - L
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Less of monamine
oxidase
Predisposes the
male to antisocial and
aggressive
behaviour.
MAOA - H
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More of monoamine
oxidase
Evidence
With MAOA-L
Caspi et al (2002)
 Longitudinal study of
1,037 children over
25 years
 Males severely
maltreated as boys
more likely to engage
in violent behaviour
as adults.
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More than twice as
likely to have a
conduct disorder as
those with the MAOAH gene.
 Three times more
likely to be convicted
of violent crime by 26
years.
Furthermore,
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12% of the sample
had the MAOA-L gene
and had been
mistreated.
They committed 44%
of the crimes.
conclusion
The importance of
interaction between
genes and
environmental
influence.
 One alone – not a
predictor.
 Two together –
boom..
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Brain scans
Structural changes
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The MAOA-L variant
may be linked to
reductions of volume
in areas of the brain.
fMRI (functional magnetic
resonance imaging)
MAOA-L
Normal brain
Reduced volume of anterior
cingulate cortex (Blue / yellow)
Meyer-Lindberg et al
(2006)
McDermort et al
(2008)
Significant reductions
in volume in
prefrontal cortex and
amygdala in MAOA-L
participants.
 Involved in emotion
and are often
impaired in antisocial individuals.
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MAOA-L gene
displayed higher
levels of aggression
in response to
provocation than
MAOA-H.