Genetic explanation of aggression
- 1. BIOLOGICAL EXPLANATION OF AGGRESSION DESCRIBE THE GENETIC EXPLANATION OF AGGRESSION EVALUATE GENETIC EVALUATION (A02)
- 3. THE CASE OF JEFFREY LANDRIGAN • 29 years old • On trial for murder of 42 year old male (Chester Dyer) • Previous convictions: Stabbing childhood friend to death- 1982 • Date of trial: 9th Nov 1990
- 4. • 09 – 11 - 1990 • Arizona, USA • Chester Dean Dryer was a 42 year old man who worked in a health club and was known to pick up men and take them home. Dyer called friends to tell them he was having sex with a man named Jeff. • He was later found strangled by an electrical cord and stabbed to death in his apartment. A deck of pornographic playing cards were strewn over the bed. • Landrigan's shoes matched a footprint left in sugar in Dyer's apartment. He told police that he had beaten Dyer after Dyer made sexual advances, but that another man had done the killing. • Landrigan was uncooperative and disruptive during his trial, and at sentencing told the Judge "If you want to give me the death penalty, bring it on, I'm ready."
- 5. THE CASE OF JEFFREY LANDRIGAN • Landrigan was abandoned by his parents as an infant. He was adopted at 9months of age into a loving and stable home. He never met his biological family. • Jeff stated that he felt loved but never really belonged. He had a drinking problem at age 10 and history of violence throughout his teens. • During imprisonment, he was told by a guard that he looked similar to a man on Death row in Arkansas, Darrell Hill. • Darrell Hill was Jeffrey’s biological father. They had never met but led parallel lives of drugs, alcoholism, violence and murder • He found out that his Grandfather had been shot dead by police whilst robbing a drug store. • Jeffrey appealed his sentence with the claim that his destiny was bound by his DNA and requested a re-trial
- 6. THE CASE OF JEFFREY LANDRIGAN • If the defense is plausible, why don’t more people use it? • 1. If we accept it, we’ll be forced to medicalize criminal behaviour and treat crime as a genetic disorder. • 2. If genetic, there can be no cure or reformation. Capital punishment may be used in more cases
- 7. GENES AND AGGRESSION… • Can affect levels of testosterone • Can affect speed of testosterone circulating the body • Can determine severity of the Stress Response through neurotransmitter and hormone levels. • Can affect sensitivity of receptors to hormones and NT
- 8. GENETIC EXPLANATION… • Is there a specific Gene for Aggression? • How could we test this theory to find out if there is an ‘Aggression’ gene?
- 10. • Twin studies are very useful in determining genetic influences of behaviour • Mz twins share 100% of their genes • Dz twins share 50% of their genes • Concordance rate = “The probability that a pair of individuals will both have a certain trait given that one of the pair has that trait”
- 11. TWIN STUDIES • If aggression is caused by a particular gene… • Mz’s should have 100% concordance rate of aggression… • i.e. if 1 Mz twin is aggressive, the other Mz twin will be too. • Dz’s should have a 50% concordance rate of aggression… • i.e. if 1 Dz twin is aggressive, there is a 50% chance the other will be too.
- 12. AGGRESSION IN TWIN STUDIES… • McGuffin & Gottesman (1985) • Studied both Mz and Dz twins who had been brought up in the same environment for levels of juvenile delinquency and crime. • Concordance rate for delinquency in Mz twins was 87% • Concordance rate for delinquency in Dz twins was 72%
- 13. • Is this convincing evidence for the role of genetic factors in aggressive behaviour? Why? • What do the two findings show? And what does the 72% concordance rate in Dz twins suggest? • What basic problems are there with investigating aggression amongst twins?
- 14. AGGRESSION IN TWIN STUDIES… • Mason & Frick (1994) • Meta-Analysis involving 3795 twin pairs from 12 twin studies and 3 adoption studies published since 1975. • Measured concordance rate of anti-social personality; aggression and criminality
- 15. MASON & FRICK (1994) • Results: • Concordance rate of aggressive behaviour calculated at 48%
- 16. MASON & FRICK (1994) • Results: • This effect was larger for more violent behaviour…
- 17. TWIN STUDIES… • Twin studies provide evidence for a genetic vulnerability to aggression (concordance of approx. 50%) • Mz twins have higher concordance rates than Dz twins suggesting heritability • However… • Mz twins are often treated more similarily than Dz twins which may influence similarity in aggressive behaviour • High concordance rate of aggression between twins may be due to their shared environment and upbringing
- 18. • Problem with twin studies is that they usually share the same environment too… so it’s difficult to accurately estimate the contribution of genetic factors on aggression.
- 19. • Adoption studies look for correlations in aggressive behaviour between children raised by non-biological parents, and aggression in the biological parents. • Nature vs. Nurture…
- 21. • Hutchings & Mednick (1973) • 14,000 adoptions in Denmark • Measured number of convictions for criminal violence
- 22. • Hutchings & Mednick (1973) • Results: • Significant positive correlation found between adoptees and biological parents for criminal convictions • No significant correlation between adoptee and adoptive parents.
- 23. • Cadoret (1995) • Best predictor for Aggressive behaviour in children is when the biological family were aggressive AND the adoptive experience marital dispute or drug abuse.
- 24. XYY SYNDROME • Sandberg (1971) • First identified the XYY genotype
- 25. XYY SYNDROME • Prevalance of approx. 1 in 1000 • Affected men are usually very tall and thin • Symptoms include, learning disabilities (IQ 10-15 points lower than siblings) and anti social behavioural problems
- 26. XYY SYNDROME • Out of a sample of 314 patients in a high security hospital, 9 were found to have XYY • Sandberg concluded that those with XYY should be hospitalised due to an increased likelihood of aggressive behaviour
- 27. HOWEVER… • Theilgaard (1984) • Compared a sample of XYY men with XY and found fluctuating levels of aggression within the sample • Conflicting evidence for aggression – XYY link
- 28. • Media may have influenced peoples perception of Xyy causing aggressive behaviour
- 29. MAOA • Gene – Monoamine Oxidase A (MAOA) • The gene controls an enzyme which breaks down the neurotransmitter Serotonin.
- 30. MAOA • Serotonin is a neurotransmitter • It modulates Vasoconstriction; Body temperature; Mood; Appetite; Anger and Aggression
- 31. MAOA • Brunner et al (1993) • Examined a Dutch family which had males displaying high levels of violence • There were high levels of domestic violence • One brother had tried to rape his sister • The father had been convicted for violent crimes many times
- 32. BRUNNER ET AL (1993) • All the aggressive males in the family had defected MAOA genes with very low activity from the gene • This results in increased serotonin levels as a result of a reduction of the enzyme.
- 33. MAOA – WARRIOR GENE • Lower-level activation of MAOA is associated with increased Aggression… • Low Level MAOA (LL-MAOA) affects 1/3 of Western societies • LL-MAOA effects 2/3 in countries with a history of warfare
- 34. LL-MAOA causes a genetic prediposition to aggression Increased aggressive behaviour occurs as a result of a combination between genes and environment
- 36. • The most aggressive strains of a certain species can be interbred and the offspring can be measured for aggressive behaviour.
- 37. • Lagerspetz (1979) • Mice classified as either ‘aggressive’ or ‘docile’ • The 2 strains were then interbred and ‘fostered’ • By the 19th generation, 52% of the ‘aggressive’ mice’s offspring showed biting behaviour compared to just 5% of the non- aggressive
- 38. • Lagerspetz (1979) • Demonstrates a genetic basis for aggressive behaviour in mice • Concordance rate was not 100%... Must be environmental factors too…
- 40. EXAM PREP • Discuss Genetic factors involved in Aggressive Behaviour • 4+16 marks
- 42. A01 – “DISCUSS GENETIC FACTORS INVOLVED IN AGGRESSION” (4 MARKS) • Aggression is the intent to verbally or physically harm others. Genetic explanations of aggression suggest that aggressive behaviour is as a result of a biological, genetic predisposition to aggression and ignores environmental explanations. The explanation assumes heritability of aggression amongst family members who share genetic make-up. Previous research into an inherited gene for aggression have found specific genetic mutations that may lead to increased aggressive behaviour in the males who carry the genotype XYY. The additional Y chromosome is associated with many symptoms including heightened levels of aggression. Other genes associated with aggression is Monoamine Oxidase A gene (MAOA) which is responsible for the breakdown of the neurotransmitter Serotonin. Lower activation of this gene and subsequent higher levels of serotonin (due to reduced enzyme activity from MAOA) have also been correlated with increased aggression in those who carry it. One third of people in Western countries carry the lowered activation variation of MAOA, compared with two thirds in countries with a history of warfare, leading the gene to be named the ‘Warrior gene’.
Editor's Notes
- #3: Can anyone explain what these are? (class discussion – hands up activity) Then go through and explain each. Then get them to link these to aggression, very basically e.g. Might be a gene that causes aggression, does anyone know any hormones that might explain it?
- #20: If more similar to adopted parents …. Environment; if more similar to biological parents … genes