Genetics and aggression A2
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This document discusses biological explanations of aggression, focusing on genetic factors. It explains that some research has linked aggression to genes on the X and Y chromosomes as well as genes that influence the hormone testosterone. Specifically, it discusses studies finding a potential link between the XYY karyotype and aggression, though later research was less conclusive. It also summarizes research linking the MAO-A gene, sometimes called the "warrior gene", to increased aggression, especially when combined with childhood abuse. Animal breeding studies that selectively bred for aggressive traits in animals are also discussed as supporting a genetic role. However, issues with reductionism and the need to consider gene-environment interactions are raised.
Genetics and aggression A2
- 2. BIOLOGICAL EXPLANATIONS OF AGGRESSION Genetic factors in aggressive behaviour
- 3. aggression The Y-chromosome seems to be linked with aggression. Due to it not dividing properly when sperms are made, a very few men are XYY- and have a high chance of being more aggressive than XY (most men). XYY = (possibly)
- 4. CAN A GENE INFLUENCE THE LEVEL OF AGGRESSION? Genes can affect testosterone level Genes can affect how quickly testosterone circulate around the body Genes can affect the number of testosterone receptors and the sensitivity of these receptors.
- 5. IS AGGRESSION CAUSED BY GENETICS? Sandberg 1971 first identified what is known as 47 XYY Karyotype. Most individuals have 46 chromosomes, 23 from each parent, it is possible for a male to have an extra Y sex chromosome making them XYY
- 6. Normal male karyotype XYY karyotype
- 7. SUPER MALE SYNDROME: XYY KARIOTYPE Affected individuals are usually very tall and thin. Additional symptoms may include antisocial or behavioral problems and learning disabilities. Intelligence is usually normal, although IQ, on average, is 10 to 15 points lower than siblings.
- 8. A02: ARE XYY MORE AGGRESSIVE? Yes: Court-Brown 1965-67, found that of a sample of 314 patients in a high security hospital, 15 were found to have chromosomal abnormalities, including 9 who had an extra Y chromosome. Court-Brown concluded that those with XYY would be best kept hospitalised due to an increase likelihood of aggressive behaviour.
- 9. A02: ARE SUPER MALES MORE AGGRESSIVE? NO: Theilgaard (1984) examined the personality traits of a sample of XYY men and compared them to XY men, focusing on levels of aggression between the two groups. Her research concluded that in all occurrences of XYY (1 in every 1000) the only factor that seems to be linked is their height. Levels of aggressiveness fluctuated so no definite conclusion could be drawn.
- 10. MOLECULAR GENETICS New technology has enabled researchers to examine DNA at the molecular level. This type of research has produced a number of breakthroughs in the effect of genes on aggressive behaviour.
- 11. MAO-A MAO-A gene has been associated with increased aggressiveness. It is sometimes referred to as the ‘Warrior Gene’ The MAO-A gene produces Monoamine Oxidase, a chemical that is involved in the breakdown of the neurotransmitters serotonin, dopamine and norepinephrine in synapses.
- 12. WHAT IS SEROTONIN? Serotonin is a neurotransmitter. It can be found in the central nervous system (CNS) where it has various functions, including the regulation of mood” (wikipedia) Serotonin has effects all over the body. Low levels of serotonin in the brain can result in impulsive behaviour, aggression, overeating, depression, alcohol abuse and violent suicide
- 13. HOW DOES THIS WORK It is thought that having too little of the MAO-A gene leads to the brain being flooded with too much serotonin, norepinephrine and dopamine, which eventually leads to a lowered sensitivity to these neurotransmitters. low sensitivity is equivalent to low activity.
- 14. E.G. MAOA GENE Structural (left) and functional (right) MRI scan data shows that subjects with the violence-related version of the MAO-A gene (MAOA-L) had reduced volume and activity of the anterior cingulate cortex (blue area in front part of brain at left and corresponding yellow area in at right), which is thought to be the hub of a circuit responsible for regulating impulsive aggression. The color-coded areas show where subjects with the L gene type differed from subjects with the H gene type. Source: NIMH Clinical Brain Disorders Branch codes for the enzyme monoamine oxidase-A (MAO-A), which breaks down key mood-regulating chemical messengers, most notably serotonin. These, in turn, influence how the brain gets wired during development. The variations may have more impact on males because they have only one copy of this X-chromosomal gene, while females have two copies, one of which will be of the H variant in most cases.
- 15. AO2 SUPPORTING RESEARCH EVIDENCE Moffat et al. (2002) conducted a longitudinal study of 422 males in New Zealand. He studied their history of abuse and criminal convictions, their penchant for violence and any symptoms of antisocial personality disorder. His findings showed that there was a link between MAOA the risk of being convicted of a violent crime; however, this was only when participants also suffered abuse as children. This is a clear example of a gene-environment interaction, the gene only exerts its effect in particular environmental conditions. Video Clip: Jim Fallon on the "Warrior Gene"
- 16. EVALUATION Brunner et al. (1993) studied a large Dutch family where all the males had a mutant form of the MAOA gene. Moffitt et al. (1992) performed a longitudinal All had borderline retardation and reacted study on 442 New Zealand males from birth aggressively when angry, fearful or frustrated, to age 26, recording which participants as suggesting that abnormal MAOA activity is children suffered abuse and also what level associated with aggression. of activity of the MAOA gene participants had. It was found that those who had suffered abuse and had the low-activity version of the gene were nine times more likely to indulge in antisocial behaviour, including aggression. Participants who had been abused, but carried the high-activity version of the gene, were no more likely to be antisocial than those not suffering abuse, suggesting that the MAOA gene is involved in aggressive behaviour, but is sensitive to social experiences early in development.
- 17. AO2: Several previous studies had linked increased serotonin during development with violence and the L version of MAO-A. For example Caspi et al. (2002) discovered that the gene's effects depend on interactions with environmental factors: men with L were more prone to impulsive violence, but only if they were abused as children.
- 18. This is a clear example of a geneenvironment interaction, the gene only exerts its effect in particular environmental conditions. Video Clip: Jim Fallon on the "Warrior Gene"
- 19. ANIMAL BREEDING STUDIES In animal breeding studies, animals are selectively bred for certain traits. If it is possible to breed for aggression, this would suggest that genes contribute to aggressiveness
- 20. A02 SUPPORT: DMITRI BELYAEV 1950 Belyaev's aim was to reduce the aggressiveness of the Silver Fox to make them easier to breed for their much sought after silver fur. He found that after 18 generations the animals were tame: they approached humans, barked like dogs and even rolled over to have their tummies tickled. Unfortunately for Belyaev, the animals even looked like dogs, with floppy ears and a piebald coat – not the silver coat that the fur industry required. The silver fox breeding programme was also able to breed extremely vicious silver foxes, which supports the hypothesis that genes are involved in aggression.
- 21. AO2 SUPPORT FOR GENETICS: ANIMAL STUDIES Cases et al. (1995) studied mice genetically engineered to lack MAOA. The mice had a dramatically altered serotonin metabolism and severe behavioural alterations. When adult, they showed enhanced aggression and were aggressive during mating, giving support to human studies suggesting that Bock and Goode (1996) found that male aggression is a direct result of MAOA mice reared alone show a stronger tendency deficiency, rather than other genetic to attack other male mice then those reared influences or psychosocial factors with others. Other male mice, reared with their parents, had been shown when it was necessary to be aggressive and when it was not, giving support to the idea that aggression is a natural biological tendency shaped by learning experiences.
- 22. ISSUES AND DEBATES: EVALUATION The results of studies on animals cannot be extrapolated to humans because they are physiologically and cognitively different however for ethical reasons we could not carry out such studies on humans. Breeding aggressive animals can also raise ethical issues.
- 23. ISSUES AND DEBATES: THE EXPLANATION IS REDUCTIONIST This explanation does not take into account the role of psychological factors such as unconscious conflicts in early childhood. It does not take into account that aggressive behaviour can learnt through imitation (social learning theory). It does not take into account the role of anonymity and deindividuation which Zimbardo argues plays an important role in aggressive behaviour. It does not account for situational factors and tends to attribute aggressive behaviour to dispositional factors.
- 24. EVALUATION: DIATHESIS STRESS On its own, the MAOA gene variant has no effect, but if males who carry the MAOA gene were abused as children, there is a greatly increased chance of them committing violent crime. It seems that aggressiveness is influenced by a variation in the MAOA gene, which is sensitive to social experiences early in development; therefore it’s functional outcome depends on social context. This indicates an interaction of genetics and environment is at work in determining human aggression.
- 25. EVALUATION: REAL WORLD APPLICATION Waldroup shot his wife's friend Leslie Bradshaw, eight times, killing her before attempting to kill his wife by chopping her up with a machete Was he responsible or did his “warrior gene” make him do it? Practical application: In a 2009 criminal trial in the United States, in the case of Bradley Waldroup, an argument based on a combination of "warrior gene" and history of child abuse was successfully used to avoid a conviction of first-degree murder and the death penalty; however, the convicted murderer was sentenced to 32 years in jail.