1. BIOLOGICAL EXPLANATION OF AGGRESSION
THE ROLE OF GENETIC FACTORS

2. 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

3. The role of genes in aggression
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.
The role of genes in aggression

4. Is aggression passed on?
Nelson 2006
Found selective breeding can lead to more aggressive behaviour in animals.

5. Choosing animals with desirable characteristics for breeding.
Selective breeding

6. Selective breeding for aggression
Spanish fighting bulls
Chickens (cock fighting)
Fish (Siamese fighting fish)
Dogs (pit bull terriers)
Compared to regular animals they are more developed for fighting.
Selective breeding for aggression

7. Lagerspetz 1979 Isolated mice after weaning
Tended to be aggressive when put with other mice (attacking, biting, speed).
Lagerspetz 1979

8. interbred These mice were interbred.
By the 19th generation rates of aggressive biting in aggressive mice was 52%
Only 5% in ordinary mice.
interbred

9. Aggressive mice These mice also had: Heavier testes Heavier forebrains
Altered levels of neurochemical serotonin in the forebrain and noradrenaline in brain stem.
Aggressive mice

10. conclusions Supports the notions that genes influence both
Structural and
Functional aspects of an animal’s biology.
conclusions

11. 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.

12. 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

13. In the same way, wild animals have been tamed for years to become pets.

14. TWIN STUDIES FINDINGS EXTREMELY VARIABLE Canter (1973)
Correlation of for MZs reared together.
O’Connor (1980) found 0.72
Twins reared apart showed slightly less variability.
TWIN STUDIES

15. Indicates a genetic contribution
Greater association with MZ twins than DZ twins whether reared together or apart. MZ DZ
Reared together
Reared apart
0.72
0.64
0.42
0.34
0.39
0.46
0.06
0.14
0.53
0.30
Indicates a genetic contribution

16. Heritability 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).
Heritability

17. 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

18. Richard Speck July 13 1966 Speck murdered 9 nurses
He was later found to have the XYY chromosome
Aha!!
This was later found to be a mistake! Oh No!

19. 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.

20. Evidence – Theilgaard 1984 About one in a thousand males
Taller than usual
But not necessarily more likely to show increased levels of aggression.

21. Evidence for XYY and aggression?
Jacobs et al 1965
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?

22. Doh! XYY males less intelligent
More likely to commit crimes but not generally violent ones.
More likely to get caught
Doh!

23. Thematic 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.
Thematic apperception testing

27. Rorschach test

30. The effect of genes on neurotransmitters
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.
The effect of genes on neurotransmitters

31. Biochemistry and aggression
plausible
Build up of neurotransmitters is related to mood
And can cause an aggressive reaction to a stressful situation.
Biochemistry and aggression

32. 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.

33. 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.

34. Males
Females
Lacked the monoamine oxidase A enzyme.
Levels of dopamine and serotonin increased.
Had the other functioning X chromosome to do the job.

35. Predisposes the male to anti-social and aggressive behaviour.
MAOA - L
MAOA - H
Less of monamine oxidase
More of monoamine oxidase
Predisposes the male to anti-social and aggressive behaviour.

36. 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.

37. 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..

38. fMRI (functional magnetic resonance imaging)
Brain scans
Structural changes
The MAOA-L variant may be linked to reductions of volume in areas of the brain.
fMRI (functional magnetic resonance imaging)

39. Reduced volume of anterior cingulate cortex (Blue / yellow)
MAOA-L
Normal brain
Reduced volume of anterior cingulate cortex (Blue / yellow)

40. 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.