genetic influence

• MAOA gene


  gene responsible for producing an enzyme called monoamine oxidase A ( MAOA) is associated with aggressive behaviour

  • regulates the metabolism (production & breakdown) of monoamines such as serotonin in the brain
• different forms of MAOA


  one form is a low-activity one (MAOA-L) which might lead to abnormal activity of the MAOA enzyme - causing abnormal levels of serotonin in the brain - which is associated with impulsive and aggressive behaviour = also known as the 'warrior gene'
• Dutch family study 


  in 1980s a study of a Dutch family found that lots of its male members behaved in violent and aggressive ways - and lots had been involves un serious crimes of violence like rape and arson

  • they had abnormal levels of MAOA enzyme & abnormal levels of serotonin due to having the MAOA-L gene
  • showed that the MAOA gene was passed onto men from the X chromosome of their mothers but only affected men due to only having one X chromosome
  • woman can also inherit gene but don't show the symptoms due to their second X chromosome carrying a good copy of the gene (Brunner et al)
• strength - supporting evidence

  twin studies , compare monozygotic and dizygotic twins. if the MZ twins more alike in aggressive behaviour then this should be because of genes rather than environment.

  O'Connor - concordance rates for aggressiveness were 72% for MZ twins vs 42% for DZ twins.

  • although MZ % is higher than DZ - concordance rate is never 100% = aggression cant be purely genetic
• criticism - reductionist 


  aggression only caused by genetic influence - argued that genetic influences aren't enough to cause aggression - env factors must interact with genetic factors to = aggression

  • longitudinal study . 1037 children followed over 25 years = Caspi et al, males who had been mistreated as boys were more likely to engage in anti-social behaviour - including violence if they had the MOAO-L gene. they were more than 3x more likely to have been convicted of violent crime by 26 =importance of both genes and env factors so diathesis-stress model might be better explanation
• criticism - biological determinism


  suggests we dont have freewill over our behaviour = leads to implications for crime and punishment

  • 2007 Bayout admitted to stabbing/killing a man - sentence id 9 years and 2 months. but an appeal court judge in Italy cut his sentence by a year after finding out he has gene variants linked to aggression.
  • socially sensitive & could lead to us questioning should unborn children be tested for aggression , should they not have child if they are etc
• criticism - methods of measuring aggression differ a lot between studies


  example - criminal convictions, self reports, teacher/parent reports, direct observations

  = all be used and its been found that results differ depending on type of measurement used - if research findings vary depending on how measured then its difficult to draw valid conclusions about the role of genetic factors
• MAOA-L gene codes for -> abnormal activity of MAOA enzyme -> leads to abnormal levels of serotonin -> causes aggression