Genetic factors in aggression

• The biological approach to aggression includes the belief that the propensity for aggressive behaviour lies in an individual's genetic make-up.
• Twin studies:

  Researchers compare the degree of similarity for aggression between sets of monozygotic twins and compare this to the similarity between sets of dizygotic twins. If the MZ twins are more alike in terms of their aggressive behaviour, then this should be due to genes rather than environment. Coccaro et al. (1997) studied aggressive behaviour using adult twin pairs and found that nearly 50% of the variance in direct aggressive behaviour could be attributed to genetic factors.
• Adoption studies:


  If a positive correlation is found between aggressive behaviour in adopted children and aggressive behaviour in their biological parents, a genetic effect is implied. In a study of over 14,000 adoptions in Denmark by Hutchings and Mednick (1975), it was found that a significant number of adopted boys with criminal convictions had biological parents with convictions for criminal violence, providing evidence for a genetic effect.
• Rhee and Waldman's (2002) meta-analysis of adoption studies found genetic influences accounted for 41% of the variance in aggression.
• The role of MAOA:

  Although no individual gene for aggression has been identified in humans, a gene responsible for producing an enzyme called MAOA has been associated with aggressive behaviour. MAOA regulates the metabolism of serotonin in the brain, and low levels of serotonin are associated with impulsive and aggressive behaviour.
• The role of MAOA

  Brunner et al. (1993) studied 28 male members of a Dutch family repeatedly involved in impulsively aggressive violent criminal behaviours (eg. rape, assault). These men had both abnormally low levels of MAOA in their brains and the low-activity version of the MAOA gene.
• MAOA-H and MAOA-L

  A study by Caspi et al. (2002) linking MAOA to aggressive behaviour, involved 500 male children. Researchers discovered a variant of the gene associated with high levels of MAOA (MAOA-H) and a variant associated with low levels (MAOA-L). Those with the MAOA-L variant were significantly more likely to grow up to exhibit anti-social behaviour.
• The 'warrior gene':


  MAOA-L is much more frequent in populations with a history of welfare, with about two-thirds of people in these populations having this version of the gene. This has led to it being referred to as the 'warrior gene'. McDermott et al. (2009) found that MAOA-L participants displayed higher levels of aggression, when provoked, than did MAOA-H subjects.
• AO3 - Problems of sampling:

  Many studies in this area have focused exclusively on individuals convicted of violent crime. Convictions for violent crime are relatively few compared to the vast number of violent attacks by individuals that never result in a conviction. They therefore represent a small minority of those regularly involved in aggressive behaviour.
• AO3 - Difficulties of determining the role of genetic factors:

  It is difficult to establish genetic contributions to aggressive behaviour for many reasons. More than one gene usually contributes to a given behaviour. As well as genetic factors, there are many non-genetic (i.e. environmental) influences on the manifestation of aggressive behaviour. These influences may interact with each other.
• AO3 - Findings depend on how aggression is measured:

  Methods of measuring aggression differ significantly between studies, and include self-report, parent and teacher reports and direct observations. In Rhee and Waldman's meta-analysis of 51 twin and adoption studies, genetic factors had a greater influence on aggression in studies using self-reports rather than parent or teacher reports. Findings vary depending on how aggression is measured so it is difficult to 'pin down' the real causes of this behaviour and draw valid conclusions about the role of genetic factors.
• AO3 - Evidence for the influence of the MAOA gene:

  Tiihonen et al. (2015) studied Finnish prisoners, revealing that the MAOA low-activity genotype (MAOA-L) in combination with another gene was associated with extremely violent behaviour. There was no substantial evidence for either of these genes among non-violent offenders, indicating that this combination of genes was specific for violent offending only.