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This lesson examines the evidence for a genetic basis of aggression and the related evolutionary account that views aggression as an adaptive behaviour shaped by natural selection. We consider twin and adoption studies, the role of the MAOA gene (including Brunner et al.'s "warrior gene" research), the crucial concept of gene-environment interaction (Caspi et al., 2002), and evolutionary explanations of aggression connected to sexual jealousy and mate retention. The topic is treated scientifically throughout: we analyse heritability estimates, candidate genes and adaptive functions as research questions, and we are careful to distinguish what the evidence shows from the socially sensitive uses to which it could be put. The organising theme is that genes influence aggression only probabilistically and in interaction with the environment, so the most defensible reading is interactionist rather than genetically deterministic.
Key Definition: Genetic explanations propose that individual differences in the predisposition to aggression are partly inherited. Evolutionary explanations propose that aggression became species-typical because, on average, it conferred survival and reproductive advantages on ancestors, so genes supporting adaptive aggression were selected for and passed on.
This lesson covers two named strands of the AQA 7182 Paper 3 option Aggression: genetic factors in aggression, including the MAOA gene, and evolutionary explanations of human aggression. For the genetic strand you must be able to describe (AO1) the logic and findings of twin and adoption studies, the function of the MAOA gene and the MAO-A enzyme, Brunner et al.'s (1993) Dutch-family study, and the gene-environment interaction demonstrated by Caspi et al. (2002). For the evolutionary strand you must describe how aggression can be adaptive (resource and mate competition, dominance, mate retention) and explain partner-directed aggression via sexual jealousy and mate retention (Wilson & Daly). You then convert this into AO3: the equal-environments assumption in twin studies, the polygenic nature of aggression, social sensitivity and determinism, and — for the evolutionary account — falsifiability, the naturalistic fallacy and the problem of cultural variation. The recurring examiner theme is that heritability is not destiny and adaptiveness is not justification.
Twin studies estimate heritability by comparing concordance (or correlation) for aggression between monozygotic (MZ) twins, who share essentially 100% of their DNA, and dizygotic (DZ) twins, who share on average 50%. The logic is that, if MZ pairs are more alike for aggression than DZ pairs reared in comparably similar environments, the excess similarity reflects shared genes. Adoption studies complement this by separating genes from rearing environment: a correlation between an adopted child's aggression and that of their biological (rather than adoptive) parents implicates genetic transmission.
Miles and Carey (1997) conducted a meta-analysis of 24 twin and adoption studies of aggression. Aim: to estimate the relative contributions of genes and environment and to test whether they vary with age and method. Findings: roughly 50% of the variance in aggression was attributable to genetic factors, with the remainder due to environmental factors (shared and non-shared); genetic influence appeared stronger in adults while shared environment mattered more in children and adolescents; and self-report measures yielded larger genetic estimates than observational measures. Conclusion: there is a substantial but not overwhelming genetic component to aggression, the size of which depends on developmental stage and how aggression is measured.
Adoption evidence points the same way. Studies such as Hutchings and Mednick's Danish adoption research reported that boys with criminal (including violent) biological fathers were more likely to have criminal convictions themselves, even when raised by non-biological parents — consistent with heritable risk, while the residual role of the rearing environment keeps the picture interactionist. Adoption designs are valuable precisely because they break the usual confound between genes and environment: a biological parent contributes genes but (typically) not the rearing environment, so a child-biological-parent resemblance is hard to explain environmentally. Their limitations, however, must be noted for AO3 — adoptions are non-random (agencies often place children with families resembling the biological parents in social background, a phenomenon called selective placement), and adoptees and adoptive families may be unrepresentative, both of which can confound the genetic interpretation.
It is also worth distinguishing the two quantities these designs estimate. Heritability indexes the proportion of variance between individuals attributable to genetic differences in a particular population at a particular time; it is not a statement about how "genetic" any one person's aggression is, and it can change if the environment changes (for example, equalising environments tends to raise heritability because environmental variance shrinks). Keeping this definition precise is itself an examinable point, because it blocks the common misreading of "50% heritable" as "half of my behaviour is caused by genes."
Exam Tip: Miles and Carey's finding that the genetic estimate changes with age and measurement method is a strong AO3 point: it shows heritability is not a fixed property of "aggression" but a population statistic that depends on context and method. Use it to argue against treating "50% genetic" as a hard, universal fact.
The monoamine oxidase A (MAOA) gene, located on the X chromosome, codes for the enzyme MAO-A, which metabolises (breaks down) monoamine neurotransmitters — including serotonin, dopamine and noradrenaline — in the synapse after they have acted. A common low-activity variant (MAOA-L) produces less MAO-A enzyme, which is thought to disturb the regulation of these neurotransmitters (notably serotonin, linking this lesson directly to the serotonin mechanism in the previous one).
Brunner et al. (1993) investigated a large Dutch family in which several males across generations showed a pattern of impulsive aggressive and antisocial behaviour, together with mild cognitive impairment. Aim: to identify a genetic basis for this familial pattern. Findings: all affected males shared a rare point mutation in the MAOA gene that abolished MAO-A activity entirely, producing very high levels of unmetabolised neurotransmitters; unaffected male relatives did not carry it. Conclusion: a dysfunctional MAOA gene was associated with the aggressive phenotype in this family — the first identification of a specific gene linked to human aggression, and the origin of the popular (and misleading) "warrior gene" label.
Two cautions are essential for AO1 accuracy. First, Brunner studied a single family with an extremely rare complete-knockout mutation, so the findings cannot be generalised to ordinary aggression in the population. Second, the variant usually discussed in later research is the far commoner low-activity MAOA-L allele, which reduces rather than abolishes enzyme activity and which, on its own, does not reliably predict aggression — as the next study shows.
Caspi et al. (2002), drawing on the longitudinal Dunedin cohort of over 1,000 New Zealanders followed from birth, tested whether the MAOA genotype interacts with early experience. Aim: to determine whether the low-activity MAOA allele predicts later antisocial behaviour only in combination with childhood maltreatment. Findings:
Conclusion: neither the gene nor the adverse environment alone was sufficient; aggression emerged from their interaction. This is the textbook demonstration of gene-environment interaction (G×E) and a clear application of the diathesis-stress logic to aggression — the genetic variant is a vulnerability that is only "switched on" by an environmental stressor.
| Group | MAOA Variant | Childhood Experience | Outcome |
|---|---|---|---|
| 1 | Low activity (MAOA-L) | Maltreated | Elevated antisocial/aggressive behaviour |
| 2 | Low activity (MAOA-L) | Not maltreated | Typical |
| 3 | High activity (MAOA-H) | Maltreated | Typical (genotype appears protective) |
| 4 | High activity (MAOA-H) | Not maltreated | Typical |
flowchart LR
A[MAOA-L allele\nlow-activity genotype] --> C{Childhood\nmaltreatment?}
B[Adverse early\nenvironment] --> C
C -- Both present --> D[Elevated risk of\nadult aggression]
C -- Gene only --> E[Typical outcome]
C -- Environment only,\nMAOA-H --> E
Key Definition: Gene-environment interaction (G×E) occurs when the effect of a gene on behaviour depends on the environment (and vice versa). For MAOA, the low-activity allele raises aggression risk only in the context of childhood maltreatment, so the same genotype yields different outcomes in different environments.
Evolutionary psychology treats aggression as a set of behaviours that became species-typical because they tended to raise survival and reproductive success in the environment of evolutionary adaptedness (EEA) — the ancestral conditions under which our psychology was shaped.
From this perspective, aggression is not a malfunction but a conditional strategy deployed when its expected fitness benefits outweighed its costs. Proposed adaptive functions include:
Because these benefits were, on average, greater for males competing for reproductive access, the account predicts a sex difference, with males more prone to direct physical aggression in status and mating contexts — a prediction broadly consistent with the cross-cultural over-representation of young males in violent confrontations. The same logic predicts that aggression should be context-sensitive rather than indiscriminate: it should be deployed when the likely payoff (status, resources, deterrence) exceeds the likely cost (injury, retaliation, loss of allies), which is why even an evolved disposition is expected to be expressed selectively.
Evolutionary psychologists have also applied this cost-benefit logic to bullying, reframing it not as a maladaptive pathology but as a strategy that could have raised an individual's status and access to resources at others' expense. On this view, bullying behaviour persists because, in ancestral conditions, dominating weaker rivals could deter competitors, advertise strength to potential mates and secure resources. A sex-differentiated prediction follows: male bullying is argued to function partly as a display of dominance attractive to potential mates, whereas female bullying is argued to lean towards relational tactics (exclusion, reputational harm) that protect the bully's own standing and, in some accounts, control rivals' access to mates. Volk et al. have argued along these lines that bullying shows the hallmarks of an evolved adaptation — it is common, early-emerging and associated with apparent reproductive and social advantages for the perpetrator. This application is useful for evaluation because it generates testable predictions (e.g., about the social rewards bullies obtain) rather than merely redescribing the behaviour, and because it has direct relevance to anti-bullying interventions, which may be more effective if they remove the status payoff of bullying rather than treating it solely as a skills deficit.
Wilson and Daly argued that male sexual jealousy is a key evolved driver of aggression directed at partners. The adaptive problem is paternity uncertainty: because fertilisation is internal, an ancestral male could never be certain a child was genetically his, and the fitness cost of unwittingly investing in a rival's offspring (cuckoldry) was severe. On this account, sexual jealousy evolved as an emotional mechanism motivating mate-guarding behaviours that function to deter a partner's infidelity.
Wilson and Daly's analyses of homicide data across cultures and historical periods reported recurring patterns: male-on-male killings were frequently rooted in status disputes and sexual rivalry, and partner killings were often preceded by sexual jealousy and estrangement (a partner leaving or threatening to leave). The cross-cultural recurrence of these patterns is used to argue for an evolved, rather than purely culturally specific, basis. It is important to treat such data clinically — as evidence about the triggers and demographics of lethal violence — rather than sensationally.
Mate-retention behaviours are argued to lie on a continuum from benign to coercive. The table below illustrates the range (after the mate-retention literature associated with Buss); the evolutionary claim concerns the underlying motivation, not an endorsement of any behaviour:
| Category | Example | Coerciveness |
|---|---|---|
| Vigilance | Monitoring a partner's whereabouts | Low |
| Monopolisation of time | Restricting a partner's social contact | Moderate |
| Jealousy induction | Provoking a partner to test commitment | Moderate |
| Emotional manipulation | Threats to self if the partner leaves | Moderate-High |
| Intersexual threats | Intimidating the partner directly | High |
| Intrasexual threats | Confronting or threatening rivals | High |
Shackelford et al. (2005) reported that men's reported use of mate-retention tactics involving direct guarding and intimidation was positively correlated with self-reported violence towards partners, and that women whose partners used more controlling tactics reported more partner-directed aggression. As a correlational finding this is consistent with the evolutionary account but cannot establish that an evolved jealousy mechanism caused the violence rather than, say, learned controlling norms.
Key Definition: Mate retention refers to behaviours that function to prevent a partner from forming a relationship with a rival. Evolutionary psychologists argue the underlying motivation (e.g., sexual jealousy) is an adaptation to the ancestral problem of infidelity and paternity uncertainty; the behaviours themselves range from benign to coercive and are not thereby justified.
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