Genetic Factors

• Twin studies

  • twin studies compare concordance rates between MZ and DZ rwins
  • if aggressive behaviour is due to genetic factors we would expect to see higher concordance rates in MZ twins
  • MZ and DZ twins are raised in the same environment but MZ twins have a greater degree of similarity than DZs

  Coccaro et al

  • a sample of adult male MZs and DZs were used
  • findings for direct physical aggression were :
  • MZ - 50%
  • DZ - 19%
• Adoption studies

  The similiaries in aggression between an adopted child and their biological parents suggest that their behaviour is due to genetic factors. Whereas similarities in aggression between an adopted child and their adopted parents suggest the behaviour is due to environmental influence.

  
  

  Hutchings & Mednick

  • a sample of over 14,000 adoptions in Denmark was used
  • the findings show that a significant number of adopted boys with criminal convictions had fathers that had convictions for criminal violence
• MAOA

  • MAOA is a gene that regulates the metabolism of serotonin
  • some people inherit the activity version of MAOA which leads to serotonin not being broken down in the brain
  • MAOA-L occurs in about 2/3rds of people in populations with a history of warfare - nicknamed the "warrior gene"
• MAOA is an enzyme that breaks down important neurotransmitters in the brain after a nerve impulse has been transmitted. The enzyme is controlled by the MAOA gene. There are variations of the gene in humans. People with the low activity form produce less of the enzyme. Excess amounts of neurotransmitters such as serotonin are not mopped up. The MAOA-L variant is extremely common and occurs in about 40% of the population.
• Studies have found a correlation between the low activity form of MAOA and aggression.

  Brunner et al

  • studied a sample of 28 males from a large Dutch family
  • a large proportion had been involved in serious crimes of violence (including physical assault, rape, attempted murder and arson)
  • found that these men had abnormally low levels of MAOA in their brains (monoamine oxidase A which mops up serotonin after release) and the MAOA-L version of the gene.
• Studies also suggest that low MAOA activity in combination with abuse experienced during childhood results in an increased risk of aggressive behaviour as an adult
• A03 -

  A problem with this research is that it suffers from sampling problems. Most studies focus on people convicted of violent crime. However, convicted violent criminals form only a small sample of those who actually commit violent crime. The research is not representative of all people who are regularly involved in aggressive behaviour.
• A03

  A problem with the research into genetics is that it is unlikely to ever identify a single gene responsible for aggression.

  Vassos et al carried out a meta analysis and could find no evidence of a single gene that was linked to aggression. They argued that even several genes are unlikely to fully explain behaviour as multi-faceted as aggression. In fact, they calculate that hundreds or thousands of genes interact in complex ways to determine aggressive behaviour.
• A strength of the genetic explanation is that it can be used to explain the gender differences in aggressive behaviour. The MAOA gene is linked to the X chromosome, of which men have one whereas women have two. In females, an unaffected second X chromosome with a normal MAOA gene will prevent the expression of an abnormal version of it. This could explain why males are typically more aggressive than females.
• It could be argued that genetic explanations of aggression are reductionist and oversimplify human behaviour. Genetic factors (diathesis) may interact with environmental factors (stress) to create an aggressive behaviour. A study involving 500 male children found that those with the low activity variant of the MAOA-L gene were more likely to grow and exhibit anti social behaviour, but only if they had experienced childhood mistreatment.
• A further problem is determinism. This allows no role for free will determining whether we behave aggressively, rather, dictates high levels of aggression for us if we are born with certain genes.