Role of genetic factors in aggression

• Twin studies :

  • Twin studies have suggested that heritability (i.e. Genetic factors) accounts for 50% of the variance in aggressive behaviour
  • MZ twins share 100% of their genes but DZ only share 50%
  • Therefore, we would expect to find greater similarities in aggressive behaviour between MZ twins if aggression is mainly genetic
  • Coccaro et al. (1997) found concordance rates of 50% MZ twins and 19% for DZ twins for physical assault
• Adoption studies :

  • If adopted children show similarities in aggression with their biological parents, then this suggests a genetic influence
  • If adopted children show similarities with adopted parents then this suggests that environmental influences are operating
  • Rhee and Waldman (2002) carried out a meta-analysis of adoption studies for direct aggression and antisocial behaviour
  • They found that genetic influences accounted for 41% of the variance in aggression. This is in line with twin studies
• MAOA (Monoamine Oxidase A):

  
• MAOA:

  • Brunner et al. (1993) studied a Dutch family and found that many of its male members behaved violently and aggressively
  • These men had a defect in their MAOA gene meaning they had a low-activity version of the gene
  • They were also found to have abnormally low levels of MAOA in their brain 
  • Stuart et al. (2014) studied 97 men and found that men with the low-activity MAOA gene were found to be the most violent perpetrators of partner violence
• Gene Environment (GxE interaction ):

  • Genes do not work alone in influencing aggressive behaviour
  • Low MAOA gene activity is only related to adult aggression when combined with early traumatic life events
  • Frazzetto et al. (2007) found an association between higher levels of antisocial aggression and the low-activity MAOA gene variant in adult males 
• Gene environment:

  • However, this was only the case in those had experienced significant trauma  during the first 15 years of life
  • Those who did not experience such childhood trauma did not have particularly high levels of aggression as adults, even if they possessed the low-activity MAOA gene
  • This is strong evidence of a gene-environment interaction (diathesis-stress) 
• AO3:

  • limitation of genetic explanations of aggression is that it is difficult to separate genetic and environmental factors in their influence on aggression
  • For instance, a gene associated with aggression is only expressed when environmental factors trigger it.
  • McDermott et al showed this in their lab study. They found individuals with the low-activity MAOA gene behaved more aggressively than other participants but only when provoked
  • suggests that genetic explanations alone are insufficient for a complete explanation, as both environmental and genetic influences are needed.
• AO3:

  • limitation of genetic explanations of aggression is that research has failed to find a single gene responsible for aggression
  • In their meta-analysis, Vassos et al. (2014) could find no evidence of an association between any single gene and aggression
  • Instead, they calculate that hundreds of thousands of genes interact in complex ways to determine aggressive behaviour
  • Suggests Genetic explanations for aggression must consider the intricate nature of gene influence and acknowledge that a single gene causing aggression is unlikely to provide a valid explanation.
• AO3:

  • limitation of research into the role of genetic factors in aggression is that the method used to measure aggression varies across studies
  • For example, Rhee and Waldman found that studies using self-reports found that genetic factors had a greater influence of aggression than studies using parent or teacher reports
  • Therefore, it is very difficult to draw valid conclusions about the role of genetic factors in aggression because the method used to measure aggression is not standardised across all studies in this area
• AO3:

  • strength of genetic explanations of aggression is that there is research to support the role of MAOA gene in aggression
  • Mertins et al. (2011) found that individuals with the high-activity MAOA gene were more cooperative and less aggressive in a game than participants with low-activity MAOA genes 
  • This finding confirms the importance of the MAOA gene in aggressive behaviour and therefore increases the validity of the genetic explanation for aggression
• AO3:

  • Support for the role of the MAOA gene in human aggression comes from research on mice
  • Godar et al. (2014) deleted MAOA genes in mice and found that they had significantly higher serotonin levels and were more aggressive
  • This supports the idea that MAOA regulates serotonin levels in brain, preventing aggression. This is because when the MAOA was not produced, serotonin levels were not regulated and this led to aggression in the mice. This can explain why low-activity versions of the MAOA gene can lead to aggression in humans