Genetic Factors in Aggression (AQA A Level Psychology): Revision Note

A genetic explanation of aggression

• Genetic explanations of aggression assume that aggressive behaviour is the result of inherited traits/characteristics that have been adapted across generations because of their evolutionary usefulness

  • E.g., Aggression is a trait necessary for survival

• About 50% of the variance in aggressive behaviour is explained by genetics; 50% of the variance is explained by environmental factors (Tuvblad & Baker 2013)

• Genetic inheritance of traits is generally investigated using twin studies, as monozygotic (MZ) twins share 100% of their DNA and they not only provide a control sample for each other but can be compared to dizygotic (DZ) twins

  • Christiansen (1974) found 35% concordance for aggression between MZ twins but only 13% for DZ twins

  • Rushton et al. (1986) found a correlation coefficient of 0.40 for MZ twins but only 0.04 for DZ twins 

• Adoption studies are also used to investigate genetic factors in aggression, as they may highlight differences between biological and adoptive parents and their children

  • Rhee & Waldman’s (2002) meta-analysis of 52 twin and adoption studies showed that environmental factors can only be an explanation for aggression if the specific genetic factors are already present

    • Suggesting that the environment alone is not enough to trigger aggression 

The role of the MAOA gene in aggression

• The MAOA (monoamine oxidase) gene (also known as the ‘warrior gene’) is an enzyme which is implicated in the regulation of key neurotransmitters, particularly serotonin

  • Serotonin has been linked to mood, specifically in the balancing of mood

    • Low or disrupted levels of serotonin may be a factor in affective disorders such as depression, and also in aggressive behaviour

• A dysfunctional MAOA gene may lead to an imbalance of serotonin and, possibly, to an increase in aggression

• Some research suggests that a faulty MAOA gene alone is not enough to produce aggressive behaviour; rather, it is the combination of a traumatic childhood with the faulty gene that is responsible for extreme aggression (Caspi et al. 2002)

Research which investigates genetic factors in aggression

• Miles & Carey (1997) - Meta-analysis of 24 twin and adoption studies, which found strong support for a genetic explanation of aggression; additionally, they found that environment was more influential during a person’s youth, and genetics were more important (in determining aggressive behaviour) during adulthood

• Reif et al. (2007) - 45% of aggressive participants had a faulty MAOA gene (low-activity, short allele), which was also linked to childhood trauma, i.e., there are complex interactions between genes, serotonin and environmental factors 

• Brunner et al. (1993) found that aggressive males in one family all shared the same dysfunctional gene, which led to low levels of MAOA and, by extension, low levels of serotonin

Evaluation of genetic factors in aggression

Strengths 

• Biological evidence e.g. saliva samples and DNA analysis, is objective and can be measured by blind researchers, making it  reliable 

• Twin and adoption studies tend to use large samples (e.g., Christiansen’s study above used 7000 twin pairs), which generates robust quantitative data

Limitations

• If genetic factors alone were responsible for aggression, then concordance for MZ twins should always show a 100% concordance rate, which has never been the case

• It is unlikely that the MAOA gene is solely responsible for aggression, and research in trying to establish which other genes may be involved in aggression is ongoing

Issues & Debates

• Genetic explanations exemplify the nature side of the nature vs nurture debate, suggesting aggression is inherited through genes

  • However, research (e.g., Caspi et al., 2002) shows that environmental triggers like childhood trauma interact with genes, supporting an interactionist approach

  • This suggests aggression is shaped by both genetic vulnerability and environmental experience

• Genetic explanations imply a biological determinism, where individuals may be predisposed to aggressive behaviour due to their genes

  • This raises questions about moral responsibility, especially in legal contexts—can someone be held fully accountable if they carry a so-called “warrior gene”?