GENETIC FACTORS

• TWIN STUDIES


  suggest that heritability accounts for 50% of the variance in aggressive behaviour. Coccoro et al found concordance rates for physical aggression were 50% (MZ) and 19% (DZ). Verbal aggression  was 28% (MZ) and 7%(DZ). However as it is not 100% it suggests that environmental factors were involved i.e. adoption studies were carried out. 
• ADOPTION STUDIES


  • Similarities in aggressive behaviour with biological parents = genetic influences are operating
  • similarities with adoptive parents = environmental factors.

  RHEE + WALDMANS = meta-analysis of adoption studies found genetic influences accounted for 41% of the variance in aggression
• MAOA GENE 


  is a protein that is produced by a certain gene that has been attributed to aggressive behaviour. It regulates the metabolism of serotonin, noradrenaline and dopamine. Low levels of MAOA have been attributed to aggressive behaviour rather than the high levels. The synapse has become desensitised to the neurotransmitters. 
• (+) GENETIC FACTORS 


  ADOPTION STUDIES = HUTCHINGS + MEDNICK

  found that a number of adopted boys with criminal convictions had biological parents with convictions for criminal violence, when assessing 14,000 adoptions in Denmark.  This suggests that the family environment played no role in reducing aggression and therefore a genetic link is implied.
• (+) GENETICS 


  GODAR ET AL =

  showed that mice who were genetically modified to not produce MAOA were more aggressive. This suggests that because they naturally lacked the enzyme to metabolise serotonin, this led to them having more of this biochemical which increased their aggression levels.
• (-) GENETICS


  MCDERMOTT ET AL =

  found participants with low activity MAOA gene were only aggressive in a lab based money allocation game when they were provoked. This suggests that environmental factors play a role in activating any significant pre-disposition towards aggressive behaviour. 
• (-) GENTICS 


  MOFFITT ET AL =

  performed a study on 442 New Zealand males from birth to age 26, keeping a record of experiences of abuse and the presence of the shortened MAOA gene. 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.