Genetic Factors in Aggression

• Twins

  geneticists have investigated aggression amongst twins, expecting there would be higher concordance amongst MZ than DZ twins

  since MZ twins are 100% genetically identical in contrast to DZ twins, higher concordance support the role in aggression

  Coccaro et al compared MZ vs DZ twin pairs.

  looked at concordance of criminal behaviour for both twins.

  MZ twins were found to have 50% concordance whereas DZ twins were only 19% for direct physical aggression

  verbal aggression it was 28% (MZ) and 7% (DZ)
• MAOA gene


  MAOA - protein that helps to breakdown excess neurotransmitters -> usually helps reuptake process in synaptic transmission ensuring there's regulated amounts of serotonin, dopamine + noradrenaline

  when MAOA gene is faulty/dysfunctional, prevents NTs being broken down -> leads to an imbalance of chemical causing aggression

  e.g. high levels of dopamine and low levels of serotonin are linked to aggression

  Brunner did a study on 28 males in a large Dutch family, who displayed high levels of aggression e.g. assault, rape, attempted murder

  they found that 5 of the males were found to have the dysfunctional, low activity version of the MAOA gene.

  females in the family weren't affected by genetic dysfunction
• Adoption studies


  similarities in behaviour between adopted child + biological parents = genetic influences

  similarities in behaviour between adopted child + adoptive parents = environmental influences

  Rhee & Waldman - conducted a meta-analysis of adoption studies of direct aggression + anti-social behaviour

  found genetic influences accounted for 41% of the variances of aggression, more or less in line with findings from twin studies
• Strength - Research support


  genetic explanation of aggression is there's a wealth of research evidence in support of theory.

  Godar et al used selective breeding to remove the MAOA gene in mice, to see what effect this would have on their behaviour.

  mice became more aggressive and they had disrupted levels of serotonin.

  also found that when the mice were given medication to help balance their serotonin levels, aggressive behaviour stopped.

  support role of genetics in aggression and influence that genetics can have on NTs

  HOWEVER, there are extrapolation issues with research on animals that affect how well results and conclusions can be generalised to humans

  e.g. aggression humans demonstrate is both psychological and physical & since this distinction can't be made in an animal sample, results lack validity needed to help us learn why humans become aggressive and what to do to reduce aggression
• Weakness - ignores the role of social + environmental factors


  no acknowledgement of the role of social and environmental factors

  e.g. someone way become aggressive due to financial pressures or in response to stressors, rather than because of a genetic dysfunction

  means whilst genetic explanation offers a scientific approach to understanding aggression and is compatible with aims of science, it doesn't include massive role that nurture/experiences can have on behaviour

  may be more suitable to take a diathesis-stress/interactionist approach to explain aggression. e.g. someone may have a genetic vulnerability to aggression that's only triggered when a stressor or trauma is experienced.

  this is a better alternative to genetic theories,, since it acknowledges the role of nature & nurture
• Weakness - biologically reductionist


  it's criticised for being biologically reductionist.

  means that it reduces a complex trait like aggression down to the lowest level of explanation, choosing to focus on genetics at the expense of other influences such as neuroanatomy, stress or relationships.

  limitation - there are several factors that influence our behaviour and different people are also likely to be aggressive for different reasons.

  means an explanations of aggression needs to acknowledge how complex the trait is and avoid breaking it down into just one, testable cause
• Strength - research support for role of MAOA gene


  research such as Brunner's Dutch family study, Stuart et al's domestic violence study and Frazzetto's gene-interaction study all support the idea the low-activity variant of the MAOA gene is associated with aggression due to its effect on serotonin. there's also evidence that the opposite is true.

  Mertins et al found that males with a high-activity MAOA gene were more co-operative when playing a money-distributing game compared with low-activity MAOA males

  these findings suggest the link between the low-activity variant MAOA gene and aggressive behaviour in males is reliable and this genetic explanation of aggression is valid