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Having examined the proximate neural and hormonal mechanisms of aggression in the previous lesson — the amygdala, serotonin, testosterone and cortisol that govern how aggression is produced in the moment — this lesson turns to the deeper questions of why those mechanisms exist and where individual differences in aggressiveness come from. Two related biological explanations are considered. The genetic explanation asks whether the predisposition to aggression is partly inherited, drawing on twin and adoption studies and, most famously, on the MAOA gene (the so-called "warrior gene"), together with the pivotal demonstration that this gene interacts with early experience. The evolutionary and ethological explanation asks why a capacity for aggression became species-typical at all, treating it as an adaptive behaviour shaped by natural selection — expressed through innate releasing mechanisms and fixed action patterns, regulated by ritualistic display, and deployed as a conditional, cost-sensitive strategy. Throughout, heritability estimates, candidate genes and adaptive functions are analysed as research questions, distinguishing what the evidence shows from the socially sensitive uses to which it could be put. The organising theme, which the Edexcel specification rewards, is that genes and evolved dispositions influence aggression only probabilistically and in interaction with the environment — so the most defensible reading is interactionist rather than deterministic.
Key Definition: Genetic explanations propose that individual differences in the predisposition to aggression are partly inherited. Evolutionary (and ethological) 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 addresses the Edexcel 9PS0 — Paper 1, Topic 3: Biological Psychology content on the role of evolution and genetics in aggression, including the evolutionary explanation of aggression and genetic explanations (the MAOA gene and twin/adoption evidence). You should 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, the gene–environment interaction demonstrated by Caspi et al. (2002), and the ethological/evolutionary account (aggression as innate and adaptive; innate releasing mechanisms, fixed action patterns, ritualistic aggression; aggression as a conditional strategy for resources, status and mating). You should be able to apply (AO2) this to described scenarios — interpreting a described family pattern, a genotype result, or a piece of observed animal behaviour. You should be able to evaluate (AO3) the account, including the equal-environments assumption in twin studies, the polygenic nature of aggression, the strength of gene–environment-interaction evidence, generalising from animals to humans, the falsifiability of evolutionary accounts, the naturalistic fallacy, and social sensitivity and determinism. The recurring examiner theme is that heritability is not destiny and adaptiveness is not justification.
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Twin studies estimate heritability by comparing concordance for aggression between monozygotic (MZ) twins, who share ~100% of their DNA, and dizygotic (DZ) twins, who share on average 50%. The logic is that, if MZ pairs are more alike 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. Danish adoption research (of the kind associated with Hutchings and Mednick) 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 because they break the usual confound between genes and environment: a biological parent contributes genes but (typically) not the rearing environment. Their limitations must be noted for AO3, however — adoptions are non-random (agencies often place children with families resembling the biological parents in social background, a phenomenon called selective placement), 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. Keeping this definition precise 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 explanation directly to the serotonin mechanism in the previous lesson).
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<br/>low-activity genotype] --> C{Childhood<br/>maltreatment?}
B[Adverse early<br/>environment] --> C
C -- Both present --> D[Elevated risk of<br/>adult aggression]
C -- Gene only --> E[Typical outcome]
C -- Environment only<br/>MAOA-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 raised survival and reproductive success in ancestral conditions. Ethology — the study of animal behaviour in natural settings — supplies much of the supporting evidence and the key mechanisms, on the assumption that processes conserved across species may illuminate aggression in humans too.
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 — broadly consistent with the cross-cultural over-representation of young males in violent confrontations. The same logic predicts aggression should be context-sensitive rather than indiscriminate: deployed when the likely payoff exceeds the likely cost (injury, retaliation, loss of allies), which is why even an evolved disposition is expressed selectively.
Konrad Lorenz, in On Aggression (1966), argued that aggression is an innate tendency, present in all members of a species, that is adaptive because it serves survival functions, and that within a species is typically expressed in ritualised rather than lethal form. Three ideas are central.
Innate releasing mechanisms (IRMs) and sign stimuli. Lorenz proposed that aggression is triggered not by general arousal but by specific environmental cues called sign stimuli (or releasers). A sign stimulus activates an innate releasing mechanism (IRM) — an inbuilt neural network that, once triggered, "unlocks" a particular pattern of aggressive behaviour. In the three-spined stickleback, the red underbelly of a rival male in breeding condition acts as the sign stimulus, releasing territorial aggression.
Fixed action patterns (FAPs). Once an IRM is triggered, aggression is expressed as a fixed action pattern (FAP) — a stereotyped, species-typical sequence that, once begun, tends to run to completion.
| Feature | Meaning |
|---|---|
| Innate | Present without learning, in all typical members of the species |
| Stereotyped | The same form each time it occurs |
| Ballistic | Once initiated, it tends to run to completion regardless of changes in the stimulus |
| Universal | Found across members of the species |
| Specific trigger | Released by a particular sign stimulus via an IRM |
flowchart LR
A[Sign stimulus<br/>e.g. red belly of<br/>rival stickleback] --> B[Innate Releasing<br/>Mechanism IRM]
B --> C[Fixed Action Pattern<br/>stereotyped aggressive<br/>sequence]
C --> D[Runs to completion<br/>ballistic]
Tinbergen's stickleback experiments. Niko Tinbergen tested the IRM/sign-stimulus idea experimentally. He presented territorial male sticklebacks with a range of model "dummies": some were realistic in shape but lacked a red underside; others were crude, unrealistic shapes that nevertheless had a red underside. Males attacked any model bearing a red belly, even highly unrealistic ones, but largely ignored accurate models lacking red. This isolated the sign stimulus releasing aggression as specifically the red underbelly, not the overall shape of a fish — strong experimental support for the idea that innate aggression is released by specific cues via an IRM, and a model demonstration of the FAP being elicited in stereotyped form.
Exam Tip: Tinbergen's stickleback work is the best concrete evidence to pair with Lorenz's IRM/FAP concepts. Use it to show that the sign stimulus (red belly), not realism, drives the response — this earns AO1 credit and sets up AO3 about whether such tightly stimulus-bound responses generalise to flexible human aggression.
A key part of Lorenz's theory is that intra-species aggression is usually ritualised — expressed through displays of strength (posturing, vocalising, mock combat) that allow a weaker individual to submit before serious injury occurs. He argued this was adaptive: killing conspecifics would reduce the species' reproductive success, so selection favoured appeasement displays (signals of submission) that terminate aggression. Red deer clash antlers, but the weaker stag typically withdraws before fatal injury; wolves present a submission posture (exposing the throat) that inhibits further attack by the dominant animal. Ritualisation converts potentially lethal conflict into a signalling contest in which contestants assess each other's fighting ability, so the likely loser concedes before injury — adaptive for both parties, since the winner avoids a costly fight and the loser lives to compete another day.
Key Definition: Ritualistic aggression is aggression expressed through stereotyped displays (posturing, mock combat, appeasement gestures) that settle disputes over status or territory without serious injury, terminating when the weaker individual submits.
A central application of the evolutionary account to human aggression concerns behaviour 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. Wilson and Daly argued that male sexual jealousy evolved as an emotional mechanism motivating mate-guarding behaviours that deter a partner's infidelity, and their cross-cultural analyses of homicide reported recurring patterns: male-on-male killings frequently rooted in status disputes and sexual rivalry, and partner killings often preceded by sexual jealousy and estrangement (a partner leaving or threatening to leave). Such data should be treated clinically — as evidence about the triggers and demographics of lethal violence — not sensationally. Shackelford et al. (2005) found that men's use of guarding and intimidation tactics was positively correlated with self-reported violence towards partners; as a correlation, this is consistent with the 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.
Twin studies support a genetic contribution but rest on the contestable equal-environments assumption. Miles and Carey (1997) found roughly 50% heritability, but the twin method assumes MZ and DZ pairs experience equally similar environments, so any excess MZ similarity must be genetic. This matters because MZ twins are often treated more alike, share more peers and are more likely to be confused for one another, so part of their greater behavioural similarity may be environmental, inflating heritability estimates. The implication is that "50% genetic" should be read as an upper-bound population estimate under specific assumptions, not as a fixed biological fact — a point reinforced by Miles and Carey's own finding that the estimate varies with age and measurement method.
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