Genetic influences on aggression

Aggression refers to behaviour that aims to cause harm, which can be physical or psychological. This tendency appears across many animal species, including humans, and biological factors offer partial explanations. Genetic elements play a role, as shown by selective breeding in animals to enhance aggressive traits, such as in certain dog breeds trained for guarding. In humans, evidence emerges from twin and adoption studies, often using criminal behaviour as an indicator of aggression.

Christiansen (1977) twin study

• Method - Examined criminal records of 3586 twin pairs born in Denmark between 1881 and 1910, comparing concordance rates for criminality between monozygotic (MZ) twins, who share all genes, and dizygotic (DZ) twins, who share about half.
• Results - For males, MZ twins had a 35% concordance rate for criminality, compared to 12% for DZ twins. For females, MZ twins showed 21% concordance, versus 8% for DZ twins.
• Conclusion - Higher concordance in MZ twins suggests a genetic influence on criminal behaviour, as they are more likely to share aggressive tendencies if they have identical genes.
• Evaluation - The lack of 100% concordance in MZ twins indicates that environmental factors also contribute, as genetics alone cannot fully explain the patterns.

Strengths and limitations of genetic explanations

Strengths:

• Twin, adoption, and gene knockout studies consistently point to a hereditary component in aggression.
• Caspi et al (2002) examined men with a variant of the MAOA gene linked to low enzyme activity. Those with the gene alone showed no marked increase in antisocial behaviour, but when combined with childhood maltreatment, there was a significant rise, highlighting gene-environment interactions.

Limitations:

• These explanations overlook social and environmental contributions to aggression.
• Walters (1992) performed a meta-analysis of studies on genetics and crime, revealing only a small correlation overall, with even weaker links in more recent and rigorously designed research.

The role of the MAOA gene in aggressive behaviour

The MAOA gene regulates the production of monoamine oxidase A (MAOA), an enzyme that breaks down neurotransmitters like serotonin. Reduced MAOA levels, leading to lower serotonin processing, are associated with heightened aggression.

Brunner et al (1993) family study

• Method - Analysed genetic profiles of family members exhibiting violent crimes and impulsive acts.
• Results - Affected males had a mutation in the MAOA gene, resulting in deficient enzyme production.
• Conclusion - The faulty gene disrupts neurotransmitter regulation, contributing to aggressive outbursts.
• Evaluation - This provides direct evidence of a genetic basis, but the study's focus on one family limits generalisability.

Cases et al (1995) knockout study

• Method - Bred male mice lacking the gene responsible for MAOA production to observe behavioural changes.
• Results - These mice displayed increased aggression in adulthood compared to those with the gene intact.
• Conclusion - Removing the MAOA gene heightens aggressive tendencies, supporting its role in behaviour regulation.
• Evaluation - Knockout studies clarify gene functions but may not fully translate to complex human aggression.

Neural mechanisms involving the limbic system and amygdala

Certain brain regions are implicated in aggression, particularly the limbic system—a network of structures linked to emotions and memory. The amygdala, within this system, is strongly associated with aggressive responses.

Evidence from animal and human studies

Animal research shows that stimulating or damaging the amygdala can alter aggression. For example, electrical activation in specific amygdala areas may provoke attacks, while lesions can lead to submissiveness in some species, though responses vary.

In humans, a case study of a mass shooter revealed a tumour in the temporal lobe pressing on the amygdala, identified post-mortem, suggesting it contributed to his violent actions.

Mpakopoulou et al (2008) amygdalotomy study

• Method - Analysed pre- and post-surgery data from patients undergoing amygdalotomy for severe aggression.
• Results - Aggressive behaviours reduced by 33% to 100% across cases, with no effects on intelligence or learning.
• Conclusion - Disconnecting the amygdala can diminish aggression, indicating its central role in emotional regulation.
• Evaluation - The procedure's success supports neural explanations, but ethical concerns limit its use, and individual differences affect outcomes.

The influence of serotonin on aggression

Serotonin is a neurotransmitter believed to suppress aggressive impulses. Lower levels or reduced activity in the brain are connected to increased aggression, often measured by turnover rates (the speed of production and breakdown).

Animal studies linking serotonin to aggression

Higley et al (1996) rhesus monkey study:

• Method - Monitored 49 monkeys over four years, assessing serotonin turnover and aggressive interactions.
• Results - Monkeys with lower serotonin turnover exhibited more aggression, while those with higher turnover had better survival rates; fatalities from fights were concentrated in the low-serotonin group.
• Conclusion - Reduced serotonin activity correlates with heightened aggression and poorer outcomes.
• Evaluation - Longitudinal data strengthens the link, but animal findings may not directly apply to humans.

Valzelli and Bernasconi (1979) mouse study:

• Method - Selectively bred mice with low serotonin turnover and compared their behaviour when isolated to those with normal levels.
• Results - Low-turnover mice became more aggressive during isolation.
• Conclusion - Low serotonin predisposes individuals to aggression under stress.
• Evaluation - Isolation mimics environmental triggers, adding realism, but species differences reduce applicability.

Human studies through diet manipulation

Serotonin production relies on tryptophan, an amino acid. Diets depleting tryptophan can lower brain serotonin.

Moeller et al (1996) tryptophan depletion study:

• Method - Healthy male participants consumed a tryptophan-free amino acid mixture and completed aggression assessments.
• Results - Subjects reported increased aggressive tendencies post-consumption.
• Conclusion - Reduced serotonin via diet elevates aggression, supporting a causal link.
• Evaluation - Experimental control is strong, but effects may be short-term and not reflective of real-world scenarios.

Limitations of serotonin research

• Animal models demonstrate clear associations but lack generalisability to human contexts.
• Diet studies imply rather than prove direct causation.
• Many investigations rely on criminal records as aggression proxies, which may not capture nuances.
• Virkkunen et al (1987) found arsonists had notably low serotonin turnover compared to other violent offenders or controls, but this linked more to impulsivity than pure aggression severity, questioning specificity.
• Laboratory settings often have low ecological validity compared to natural observations.

Testosterone and its link to aggressive behaviour

Testosterone, an androgen hormone, drives male physical traits and is produced more in males, who statistically commit more violent crimes. This suggests a hormonal basis for gender differences in aggression.

Key studies on testosterone

Dabbs et al (1987) prison study:

• Method - Measured testosterone levels in saliva samples from male prisoners and correlated them with crime types.
• Results - Higher testosterone was associated with convictions for violent offences.
• Conclusion - Elevated testosterone may increase the propensity for aggression.
• Evaluation - The correlational design cannot confirm causation; other factors like environment could influence both.

Van Goozen et al (1994) hormone therapy study:

• Method - Assessed aggression proneness via questionnaires in 50 participants before and after three months of hormone therapy (testosterone for female-to-male, anti-androgens for male-to-female).
• Results - Testosterone recipients reported higher aggression, while anti-androgen recipients showed reductions.
• Conclusion - Manipulating testosterone levels directly affects aggressive tendencies.
• Evaluation - The experimental approach establishes cause and effect, but self-reports may be biased by expectations of gender norms.