BIOLOGICAL BIOLOGICAL EXPLANATION OF EXPLANATION OF
AGGRESSIONAGGRESSIONTHE ROLE OF GENETIC FACTORS
Learning objectivesLearning objectives
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.
The role of genes in aggressionThe role of genes in aggression
Genes do not directly cause aggressionGenes 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.
Is aggression passed on?Is aggression passed on?
Nelson 2006Found selective
breeding can lead to more aggressive behaviour in animals.
Selective breedingSelective breeding
Choosing animals with desirable characteristics for breeding.
Selective breeding for aggressionSelective breeding for aggression
Spanish fighting bullsChickens (cock
fighting)Fish (Siamese
fighting fish)Dogs (pit bull terriers)Compared to regular
animals they are more developed for fighting.
Lagerspetz 1979Lagerspetz 1979
Isolated mice after weaning
Tended to be aggressive when put with other mice (attacking, biting, speed).
interbredinterbred
These mice were interbred.
By the 19th generation rates of aggressive biting in aggressive mice was 52%
Only 5% in ordinary mice.
Aggressive miceAggressive mice
These mice also had:
Heavier testesHeavier forebrainsAltered levels of
neurochemical serotonin in the forebrain and noradrenaline in brain stem.
conclusionsconclusions
Supports the notions that genes influence both
Structural andFunctional aspects
of an animal’s biology.
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.
Lagerspetz 1981 Lagerspetz 1981
Genetic factors do not absolutely determine aggressive behaviour.
Environmental factors are also important...
The aggressive mice can be conditioned to be less aggressive
In the same way, wild animals have been tamed for years to become pets.
TWIN STUDIESTWIN STUDIES
FINDINGS EXTREMELY VARIABLE
Canter (1973)Correlation of 0.14
for MZs reared together.
O’Connor (1980) found 0.72
Twins reared apart showed slightly less variability.
Indicates a genetic contributionIndicates a genetic contribution
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
HeritabilityHeritability
Heritability varies according to assessment method
Self –report – 39%
Reported by others – 53%
Heritability
the extent to which genetic individual differences contribute to individual differences in observed behaviour (or phenotypic individual differences).
Is aggression caused by Is aggression caused by genetics?genetics?
Most individuals have 46 chromosomes (23 from each parent).
It is possible for a male to have an extra Y chromosome http://www.youtube.com/wa
tch?v=5CYdMspQzDw
Richard SpeckRichard Speck
July 13 1966Speck murdered 9
nursesHe was later found
to have the XYY chromosome
Aha!! This was later found to be a mistake!
Oh No!
Popular beliefs about 47 XYY Popular beliefs about 47 XYY malesmales
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.
Evidence – Theilgaard 1984Evidence – Theilgaard 1984
About one in a thousand males
Taller than usualBut not necessarily
more likely to show increased levels of aggression.
Evidence for XYY and aggression?Evidence for XYY and aggression?
Jacobs et al 1965
Found XYY syndrome at the rate of 3% in prisoners
Only 0.1% in normal population (One in a thousand)
3 in a hundred
Doh!Doh!
XYY males
less intelligentMore likely to commit
crimes but not generally violent ones.
More likely to get caught
Thematic apperception testingThematic apperception testing
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.
Rorschach testRorschach test
The effect of genes on The effect of genes on neurotransmittersneurotransmitters
MAOA gene
Studies have found a link with aggression.
MAOA gene regulates an enzyme in the brain (Monoamine Oxidase A)
This removes excess amounts of neurotransmitters.
Biochemistry and aggressionBiochemistry and aggression
plausible
Build up of neurotransmitters is related to mood
And can cause an aggressive reaction to a stressful situation.
Brunner et al (1993)
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.
Cases et al (1995)Disabled the MAOA
gene in X chromosome of mice and found males became highly aggressive.
Females didn’t.
Males Females
Lacked the monoamine oxidase A enzyme.
Levels of dopamine and serotonin increased.
Had the other functioning X chromosome to do the job.
Predisposes the Predisposes the male to anti-male to anti-social and social and aggressive aggressive behaviour.behaviour.
MAOA - L MAOA - H
Less of monamine oxidase
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.
More than twice as likely to have a conduct disorder as those with the MAOA-H gene.
Three times more likely to be convicted of violent crime by 26 years.
Furthermore, conclusion
12% of the sample had the MAOA-L gene and had been mistreated.
They committed 44% of the crimes.
The importance of interaction between genes and environmental influence.
One alone – not a predictor.
Two together – boom..
fMRI (functional magnetic fMRI (functional magnetic resonance imaging) resonance imaging)
Brain scans Structural changes
The MAOA-L variant may be linked to reductions of volume in areas of the brain.
Reduced volume of anterior Reduced volume of anterior cingulate cortex (Blue / yellow)cingulate cortex (Blue / yellow)
MAOA-L Normal brain
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 anti-social individuals.
MAOA-L gene displayed higher levels of aggression in response to provocation than MAOA-H.