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.