Genetic Factors in Aggressive Behaviour

Genetic factors in aggressive behaviour, AO1 and AO2.

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AO1 - Twin and Adoption Studies

Twin studies

  • Most twin studies have focused on criminal behaviour generally
  • Coccaro: studying aggressive behaviour in adult twin pairs, found that nearly 50% of the variance could be attributed to genetic factors

Adoption studies

  • If a positive correlation is found between aggressive behaviour in adopted children and their biological parents, a genetic effect is implied
  • If a positive correlation is found between aggressive behaviour in adopted children and their adoptive parents, an environmental effect is implied
  • Hutchings and Mednick: studied over 14,000 adoptions in Denmark
  • Found a significant number of adopted boys with criminal convictions had biological parents (particularly fathers) with criminal convictions
  • This provides evidence for a genetic effect
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AO1 - The Role of MAOA

  • A gene responsible for producing MAOA has been associated with aggressive behaviour
  • MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are associated with impulsive and aggressive behaviour
  • Brunner et al: study of a Dutch family found that many of its male members behaved in a particularly violent and aggressive manner
  • A large proportion had been involved in serious violent crimes including **** and arson
  • These men were found to have abnormally low levels of MAOA in their bodies, and a defect in this gene was later identified
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AO1 - Gene-Environment Interaction

  • Caspi et al: studied 500 male children
  • Discovered one variant of the gene associated with high MAOA and one associated with low levels
  • Those with low levels of MAOA were significantly more likely to grow up to exhibit antisocial behaviour but only if they were maltreated as children
  • Maltreated children with high levels and non-maltreated children with low levels did not display antisocial behaviour
  • It is the interaction between genes and environment that determines aggression
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AO1 - Genetics and Violent Crime

  • Inherited temperamental/personality traits place some individuals more at risk of committing violent crime
  • Adoption studies have shown that the highest rates of criminal violence in adopted children occur when both their birth and adoptive parents have a history of this
  • However, other studies have found this link only to exist in cases of property crime, as opposed to violent crime
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AO2 - Difficulties of Determining the Role of Gene

  • It is difficult to establish genetic contributions to aggressive behaviour because:

1. More than one gene usually contributes to a given behaviour

2. There are many environmental influences on the manifestation of aggressive behaviour

3. These influences may interact with each other

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AO2 - Problems of Assessing Aggression

  • Many studies have relied on parental/self-reports or observational techniques
  • Miles and Carey: studies that used parental/self-report techniques were more likely to blame genetic factors, whilst observational studies leant more towards environment
  • Plomin et al: in a replication of the Bobo Doll study, twin pairs were encouraged to act aggressively towards it after watching an adult model
  • Researchers found no difference in correlations between MZ and DZ twins
  • Suggests that individual differences in aggression were more a product of environmental influences than genetic influences
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AO2 - The Inheritance of Criminal Violence

  • Studies often fail to distinguish between violent and non-violent crimes, making it difficult to gauge the genetic factors in specifically aggressive violence
  • They also fail to distinguish between criminals who are habitually violent and ones for whom their violent crime is a one-off
  • Walters: in a meta-analysis, found only a low-moderate correlation between heredity and crime
  • Better-designed and more recently published studies provided less support than earlier, more poorly-designed studies
  • In a report on youth violence, this statement was made: "the data do not suggest a strong role for heredity in violence"
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IDA - Problems of Sampling

  • Many of the studies in this area have focused on individuals convicted of violent crime
  • Convictions for violent crime are relatively few compared to the vast number of violent attacks that do not result in a conviction
  • They therefore represent just a small minority of those regularly involved in aggressive behaviour
  • Offenders designated as 'violent' on the basis of a court conviction may not necessarily be the most persistent, serious offenders
  • A convicted murderer might be labelled as violent even if he has a life otherwise free from violent crime
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IDA - Real-World Applications

  • There have been suggestions that public policy should be informed by the results
  • If people are predisposed towards aggressive or violent crime, then questions about the treatment of such behaviours inevitably arise
  • Some advocate genetic engineering; others go as far as 'chemical castration'
  • The conclusions that can be reached from this research are extremely tentative and have far-reaching ethical consequences (labelling someone a 'threat to society')
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