Genetic factors in aggression

• • It is possible that there is a specific gene or group of genes that, if inherited, can lead to an increase in aggressive tendencies.
  • Genes can have an impact on brain chemistry and/or brain structure & this can lead to changes in behaviour.
  • Genes can be passed from parent to offspring, so if aggression runs in families, this can support the idea that genes play a role.
• Twin & adoption studies:

  • Coccaro studied aggressive behaviour in adult twins & found that genes accounted for 50% of the variance in direct aggression towards others.
• Evidence (twin & adoption studies):

  • Hutchings & Medrick studied 14,000 adoption records in Denmark & found that a significant number of boys with criminal convictions had biological fathers with criminal convictions for violence- indicating a genetic effect.
  • However, Brennan compared the criminal history of adopted males with their biological fathers & adoptive fathers, and found genetic influences were only significant for property crime, not violent crime.
• Evaluation for evidence (twin & adoption studies):

  • Not too valid; is property crime an accurate measure of aggression?
  • Large sample size- beneficial
  • Gender bias- only boys in sample.
• MAOA- the 'warrior' gene:

  • Brunner found a Dutch family where aggression seemed to run in the family with many of its male members involved in serious & violent crimes.
  • They all had low levels of Monoamine Oxidase A and a defect in the related gene (MAOA) was identified.
  • MAOA regulates the metabolism of serotonin, which at low levels, can lead to impulsivity & aggressiveness.
• There are 2 versions of the MAOA gene:

  • MAOA-H, gene for high levels (high= healthy).
  • MAOA-L, gene for low levels (low= aggressive).
  • Low levels said to be important in aggressive behaviour & those with MAOA-L are more likely to be aggressive.
• Evidence for MAOA gene:

  • Caspi used 500 boys & looked for the gene associated with high levels of MAO and a gene related to low levels of MAO (MAOA-H & MAOA-L).
  • Found that those with MAOA-L were more likely to behave antisocially but only if mistreated as a child.
  • All other combinations did not display this behaviour.
  • Tithonen studies Finnish prisoners and found that the MAOA-L gene in combination with another gene called CDH-13 was associated with extreme violence & there was no evidence for these genes in non-violent criminals.
• Evaluation of genetic factors in aggression- weakness:

  • It's really difficult to establish genetic contributions to aggression & separate this from the environment.
  • This is because more than one gene is likely to contribute to the behaviour and there are many environmental factors that have an impact.
  • It's likely to be an interaction between genes & environment as illustrated by the research of Caspi.
• Evaluation of genetic factors in aggression- weakness:

  • It's difficult to measure aggression.
  • Some studies have used parental or self-report techniques & others have used observational methods and genetic contributions are assessed to have a larger contribution if the study used parental/ self-report techniques.
  • This makes it difficult to assess the actual genetic & environmental contributions.