genetic factors

• Diathesis - Stress:

  • genetic predisposition
  • environmental trigger
• Twin Studies

  • suggests heritability accounts for about 50% of the variance in aggressive behaviour
  • monozygotic (MZ) share 100% of their genes
  • dizygotic (DZ) share approx 50% of genes
  • MZ have greater similarity than DZ
• Coccaro et al. (1997)

  • studied all male monozygotic (MZ) and dizygotic (DZ) twins
  • physical aggression (assault), 50% concordance rate for MZ and 19% for DZ
  • for verbal aggression 28% for MZ, 7% for DZ
  • suggests a strong genetic link = concordance rates higher for MZs
  • suggests a role for genetics but not whole picture otherwise concordance rate will be 100%
• Adoption Studies

  • Similarities with their biological parents vs adoptive parents
  • Genetic vs environmental influence
• Rhee and Waldman (2002)

  • meta-analysis on adoption studies of direct aggression and antisocial behaviour
  • genetic influences account for 41% of the variance in aggression
• Mednick, Gabrielli and Hutchings (1987)

  • studied criminal records of all Danish children adopted outside their biological family between 1924 and 1947
  • having a criminal biological father increased the risk of criminality
  • the highest risk was for those with a criminal biological father AND criminal adoptive father
• The MAOA Gene

  • Monoamine oxidase A (MAOA) is an enzyme
  • It’s role is to ‘mop up’ neurotransmitters in the brain after a nerve impulse has been transmitted from one neurone to another
  • We need to control excesses of these neurotransmitters to control our mood
• Warrior gene

  • Variant of the MAOA gene – nicknamed the ‘warrior gene’
  • Leads to low MAOA enzyme activity in areas of the brain
  • Neurotransmitter imbalance
  • Associated with various forms of aggressive behaviour
• Brunner et al. (1993)

  • Studied 28 male members of a large Dutch family who were repeatedly involved in impulsively aggressive violent criminal behaviours such as rape, attempted murder and physical assault
  • Found that these men had abnormally low levels of MAOA in their brains and the low-activity version of the MAOA gene
• Stuart et al. (2014)

  • Studied 97 men who, because they had been involved in inflicting intimate partner violence (IPV) - were part of a treatment programme
  • Men with low-activity MAOA gene were found to be the most violent perpetrator of IPV
• Gene-environment (GE) Interactions

  It appears to be the case that the low MAOA gene activity is only related to adult aggression when combined with early traumatic life events
• Moffat et al (2002) found in a longitudinal study of 422 New Zealand males that there is a link between the MAOA gene and the risk of being convicted of a violent crime. However this was only when participants had suffered abuse as children.
• Frazzetto et al. (2007) found an association between higher levels of antisocial aggression and the low-activity MAOA gene variant in adult males, as expected.But this was only the case in those who had experienced significant trauma (i.e. sexual or physical abuse) during the first 15 years of life. Those who had not experienced such childhood trauma did not have particularly high levels of aggression as adults, even if they possessed the low-activity MAOA variant.
• Gene-environment interaction (sometimes described as diathesis-stress)
• Genetic factors

  Genes consist of DNA strands. DNA produces 'instructions' for general physical features of an organism (such as eye colour, height) and also specific physical features (such as neurotransmitter levels and size of brain structures). These may impact on psychological features (such as intelligence and mental disorder). Genes are transmitted from parents to offspring, i.e. inherited.
• MAOA gene

  The gene responsible for the activity of the enzyme monoamine oxidase in the brain. The low-activity variant of the gene is closely associated with aggressive behaviour.
• Genes come in different variants (high and low in the case of the MAOA gene)
• The low-activity variant (MAOA-L) results in low activity of the MAO-A enzyme (MAO-A deficiency)
• It is this MAOA-L variant that has been linked to high levels of aggressive behaviour
• The MAOA gene has been nicknamed 'warrior gene' because of research by Lea and Chambers (2007) which showed that the MAOA-L variant was possessed by 56% of New Zealand Maori men (compared with 34% of Caucasians)
• Link between MAOA-L and aggression by Brunner et al. (1993)


  Studied 28 men from a large Dutch family who were repeatedly involved in impulsively aggressive violent criminal behaviours such as rape, attempted murder and physical assault. These men had abnormally low levels of the enzyme MAO-A as well as the MAOA-L gene variant (sometimes referred to as the 'Brunner syndrome').
• Genes are crucial influences on aggressive behaviour but they do not function in isolation. It appears to be the case that MAOA-L gene activity is only related to adult aggression when combined with early traumatic life experiences.