Genetic and Evolutionary Explanations of Aggression

This lesson examines the evidence for a genetic basis of aggression and evaluates evolutionary explanations that view aggression as an adaptive behaviour shaped by natural selection. We consider twin studies, the role of the MAOA gene, gene-environment interactions, and evolutionary theories of aggression related to sexual jealousy and mate retention.

Key Definition: Evolutionary explanations propose that aggression evolved because it conferred survival and reproductive advantages on our ancestors. Genes associated with aggression were passed on because aggressive individuals were more successful at competing for resources and mates.

---

Genetic Influences on Aggression

Twin Studies

Twin studies compare concordance rates for aggression between monozygotic (MZ) twins (who share 100% of their DNA) and dizygotic (DZ) twins (who share approximately 50%). If aggression has a genetic component, MZ twins should show higher concordance rates than DZ twins.

Miles and Carey (1997) conducted a meta-analysis of 24 twin and adoption studies examining genetic and environmental contributions to aggression. Their key findings were:

• Approximately 50% of the variance in aggression could be attributed to genetic factors
• The remaining 50% was attributable to environmental factors (both shared and non-shared)
• Age moderated the relative importance of genes and environment — genetic factors were more influential in adults, while shared environment was more important in children and adolescents
• The method of measurement mattered: studies using self-report measures found stronger genetic effects than those using observational measures

Exam Tip: Miles and Carey's (1997) finding that genetic influence varies by age and measurement method is an important evaluation point. It demonstrates that genetic explanations are not absolute and that methodological choices affect conclusions.

The MAOA Gene — "The Warrior Gene"

The monoamine oxidase A (MAOA) gene codes for the enzyme MAO-A, which breaks down neurotransmitters such as serotonin, dopamine, and noradrenaline in the synapse. A variant of this gene produces low levels of MAO-A, resulting in higher concentrations of these neurotransmitters.

Brunner et al. (1993) studied a Dutch family in which several male members displayed abnormally aggressive behaviour, including arson, attempted rape, and impulsive aggression. Genetic analysis revealed that all affected males had a point mutation in the MAOA gene, resulting in a complete lack of MAO-A enzyme activity. This was the first study to identify a specific gene linked to aggression.

However, Brunner et al.'s study was based on a single family, raising questions about generalisability. The complete absence of MAO-A is extremely rare — the more common variant associated with aggression is the low-activity MAOA-L allele, which produces reduced (but not absent) enzyme activity.

Gene-Environment Interaction — Caspi et al. (2002)

Caspi et al. (2002) conducted a landmark study that examined the interaction between the MAOA gene and childhood maltreatment. Using a longitudinal sample of over 1,000 New Zealand males followed from birth, they found:

• Males with the low-activity MAOA-L allele who had been severely maltreated as children were significantly more likely to develop antisocial behaviour and aggression in adulthood
• Males with the high-activity MAOA-H allele who had been maltreated were not significantly more likely to become aggressive
• Males with the low-activity allele who had not been maltreated also did not show elevated aggression

This study powerfully demonstrates that genes and environment interact — neither the gene nor the environment alone was sufficient to produce aggression. It is a classic example of the diathesis-stress model applied to aggression.

Key Definition: Gene-environment interaction occurs when the effect of a gene on behaviour depends on the environment, and vice versa. In the case of MAOA, the genetic variant only increases aggression risk in the context of childhood maltreatment.

Group	MAOA Variant	Childhood Experience	Outcome
1	Low activity (MAOA-L)	Maltreated	High aggression
2	Low activity (MAOA-L)	Not maltreated	Normal aggression
3	High activity (MAOA-H)	Maltreated	Normal aggression
4	High activity (MAOA-H)	Not maltreated	Normal aggression

Evaluation of Genetic Explanations

Strengths:

• Twin studies consistently show a genetic component to aggression (~50% heritability)
• The identification of specific genes (e.g., MAOA) provides a biological mechanism
• Gene-environment interaction models (Caspi et al., 2002) are more sophisticated than pure genetic determinism

Limitations:

• Aggression is polygenic — it is influenced by many genes, each with a small effect. Focusing on a single gene oversimplifies the picture
• Socially sensitive — labelling certain genetic variants as "warrior genes" could stigmatise carriers and be misused by the criminal justice system to deny free will
• Twin studies assume that MZ twins share environments to the same extent as DZ twins (equal environments assumption) — this may not be valid
• Genetic explanations risk determinism — the implication that aggression is "in the genes" and therefore inevitable

---

Evolutionary Explanations of Aggression

Evolutionary psychology views aggression as a behaviour that evolved because it provided survival and reproductive advantages in the environment of evolutionary adaptedness (EEA).

Aggression as Adaptive Behaviour