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

• Twin studies suggest that heritability accounts for about 50% of the variance in aggressive behaviour, with the remainder explained by environmental factors

• 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

  • Coccaro et al. (1997) studied adult male MZ and DZ twins, and for aggressive behaviour defined as direct physical assault found concordance rates of 50% for MZ twins and 19% for DZs

• Because MZ twins share 100% of their genes but DZ twins share only 50% on average, we would expect to find greater similarities in aggressive behaviour between MZ twins if aggression is mostly influenced by genetic factors

  • This is because both MZ and DZ twins are raised together in the same environment, but MZ twins have a greater degree of genetic similarity than DZs

• 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 (2002) carried out a meta-analysis of adoption studies of direct aggression and antisocial behaviour, and found that genetic influences accounted for 41% of the variance in aggression, more or less in line with findings from twin studies

    • Similarities in aggressive behaviour between an adopted child and their biological parents suggest genetic influences are operating, while similarities with the adoptive parents suggest environmental influences

The role of the MAOA gene in aggression

• Monoamine oxidase A (MAOA) is an enzyme whose role is to 'mop up' neurotransmitters in the brain after a nerve impulse has been transmitted from one neuron to another

  • It breaks down the neurotransmitter, especially serotonin, into constituent chemicals to be recycled or excreted, a process called catabolism

• The production of this enzyme is determined by the MAOA gene, so a dysfunction in the operation of this gene may lead to abnormal activity of the MAOA enzyme, which in turn affects levels of serotonin in the brain

  • 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

• Frazzetto et al. (2007) found an association between higher levels of antisocial aggression and the low-activity MAOA gene variant in adult males, but this was only the case in those who had experienced significant trauma such as sexual or physical abuse during the first 15 years of life

  • Those who had not experienced such childhood trauma did not have particularly high levels of aggression as adults, even if they possessed the low-activity MAOA variant

    • This is strong evidence of a gene-environment interaction, sometimes described as diathesis-stress

Research which investigates genetic factors in aggression

• Stuart et al. (2014) studied 97 men who had been involved in inflicting intimate partner violence (IPV) and were part of a batterer treatment programme, and found that men with the low-activity MAOA gene were the most violent perpetrators of IPV

  • They engaged in the highest levels of physical and psychological aggression and inflicted the worst injuries on their partners

• Mertins et al. (2011) studied participants with low-activity and high-activity variants of the MAOA gene in a money-distributing game, in which they had to decide whether or not to contribute money for the good of the group

  • Men with the high-activity variant were more co-operative and made fewer aggressive moves than the low-activity participants, showing that the high-activity variant is associated with more prosocial and less aggressive behaviour

• Brunner et al. (1993) studied 28 male members of a large Dutch family who were repeatedly involved in impulsively aggressive violent criminal behaviours such as rape, attempted murder and physical assault, and found that these men had abnormally low levels of MAOA in their brains and the low-activity version of the MAOA gene

  • However, this is a single family with a rare mutation, so the findings may not generalise to aggression in the wider population

Evaluation of genetic factors in aggression

Strengths 

• Biological evidence such as saliva samples and DNA analysis is objective and can be measured by researchers blind to the participants' aggression scores, which increases the validity of the findings

• Twin and adoption studies tend to use large samples, such as Rhee & Waldman's meta-analysis of 51 twin and adoption studies, which generates robust quantitative data

Limitations

• Methods of measuring aggression differ significantly between studies, including self-reports, parent and teacher reports, and direct observations

  • E.g., in Rhee & Waldman's meta-analysis genetic factors had a greater influence on aggression in studies using self-reports rather than parent or teacher reports

    • If research findings vary depending on how aggression is measured, it becomes very difficult to draw valid conclusions about the role of genetic factors, or other factors for that matter

• The sizes of genetic effects are statistically significant but also small, as there are probably other genes involved in aggression

  • E.g., in Stuart et al.'s study IPV was associated not just with the low-activity form of MAOA but also with the serotonin transporter gene (5-HTT)

• Vassos et al. (2014) found no evidence of an association between any single gene and aggression in a meta-analysis, explaining this by pointing out that hundreds or thousands of genes interact in complex ways to determine aggressive behaviour, which casts doubt on the search for single candidate genes

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, as it is unclear whether someone can be held fully accountable if they carry a so-called 'warrior gene'