Biological Psychology: Contemporary Study — Brendgen et al. (2005)

The classic study by Raine et al. (1997) put a striking claim on the table: that the brains of extremely violent offenders differ in measurable ways from those of other people. But Raine's PET study left two big questions unanswered. First, where do those differences come from — are they inherited, or are they written into the brain by experience? Second, is aggression a single thing, or are there different kinds of aggression with different causes? The prescribed contemporary study for Edexcel's Biological Psychology topic addresses both. Brendgen, Vitaro, Boivin, Dionne & Pérusse (2005), in their paper "Examining genetic and environmental effects on social aggression: a study of 6-year-old twins", used the powerful twin method to compare identical and non-identical twins on two forms of aggression — physical and social — and reached a surprising conclusion: physical aggression is substantially heritable, whereas social (indirect) aggression appears to be much more environmentally shaped. This lesson sets out the study's aim, method, results and conclusions accurately, evaluates it, and then draws the crucial comparison with Raine — showing how a modern, less invasive, more nuanced study both updates and complicates the biological explanation of aggression.

Key Definition: The twin method compares monozygotic (MZ, "identical") twins, who share ~100% of their genes, with dizygotic (DZ, "non-identical") twins, who share ~50%. If MZ twins are more alike on a trait than DZ twins, that greater similarity is taken as evidence of a genetic contribution, because both twin types (reared together) share their environment to a similar degree.

Spec Mapping

This lesson addresses the Edexcel 9PS0 — Paper 1, Topic 3: Biological Psychology requirement to study one contemporary study in depth, paired with the classic study: Brendgen et al. (2005), "Examining genetic and environmental effects on social aggression: a study of 6-year-old twins". You must know its aim, method/procedure (the twin design, sample of 6-year-old MZ and DZ twins, and the three measures — teacher, peer and self-report — of physical and social aggression), results/findings (a strong genetic contribution to physical aggression but a predominantly environmental contribution to social aggression), and conclusions, and you must be able to evaluate it and compare it with the classic study (Raine). It develops the topic's content on the genetics of aggression and the nature–nurture debate. In assessment-objective terms, you should be able to describe the study accurately (AO1), apply it to novel scenarios and to the biological explanation of aggression (AO2), and evaluate it and contrast it with Raine (AO3).

Connects to…

Classic study — Raine et al. (1997) (this topic): the paired classic study identifies neural correlates of extreme violence; Brendgen complements it by addressing where individual differences in aggression come from (genes versus environment) and by distinguishing types of aggression, together giving a fuller, multi-level biological picture.
Genetics and evolution of aggression (this topic): Brendgen is a direct application of the twin method and the concepts of heritability and gene–environment interaction covered in the genetics lesson, providing concrete evidence for how they are used.
The nature–nurture debate (Issues and debates): the study is a model illustration of nature and nurture operating on different behaviours, resisting a simplistic "aggression is genetic" reading and showing how behavioural genetics quantifies each contribution.

Background and Aim

Two developments in the early 2000s set the stage for Brendgen's study. First, behavioural genetics had repeatedly reported a genetic contribution to physical aggression — twin and adoption studies found that identical twins were more alike than non-identical twins in hitting, kicking and fighting. Second, developmental psychologists had increasingly recognised that aggression is not only physical. Social (or "indirect", "relational") aggression — spreading rumours, excluding others, damaging friendships, "telling secrets about a friend" — is a distinct, often more subtle form of harm, more common in girls and increasingly influential as children grow older. What was unknown was whether this social form of aggression had the same genetic roots as physical aggression, or whether it was learned — picked up from peers, models and the social environment.

Brendgen and colleagues set out to disentangle this. Their central aims were:

To assess the extent to which social aggression and physical aggression in young children are explained by genetic versus environmental factors, using the twin method.
To examine the relationship between physical and social aggression — in particular whether physically aggressive children also tend to be socially aggressive.
To compare teacher, peer and self ratings of the two forms of aggression, checking whether findings held across different informants.

A subtle but important point is that the study was designed to quantify the genetic and environmental contributions to each form of aggression separately, rather than to prove that "aggression is genetic". The prediction — informed by prior work — was that physical aggression would show a substantial genetic component, but the status of social aggression was genuinely open, which is what made the study worth doing.

Key Definition: Social (indirect/relational) aggression is behaviour intended to harm others through damage to their social relationships or standing — for example excluding someone, spreading rumours or manipulating friendships — as opposed to physical aggression, which involves direct bodily harm such as hitting or kicking.

Method

Design

The study was a natural/quasi-experiment using the twin method — a correlational, comparative design. It was quasi-experimental because the key comparison (MZ versus DZ) is a naturally occurring independent variable that cannot be manipulated. The analysis used model-fitting to partition the variance in each form of aggression into three components: genetic (heritability), shared environment (experiences that make twins reared together more alike, e.g. family), and non-shared environment (experiences unique to each twin, plus measurement error).

Participants

The sample was drawn from a large longitudinal study of twins in Quebec, Canada — the Quebec Newborn Twin Study. The analysed sample comprised 234 pairs of six-year-old twins (both MZ and DZ, of both sexes) attending kindergarten. Using six-year-olds was deliberate: this is an age at which social aggression is emerging and can be reliably observed among peers, while the children are young enough that gene–environment influences can be examined relatively early in development. Zygosity (whether a pair was MZ or DZ) was determined from physical-resemblance questionnaires and, in ambiguous cases, DNA analysis.

Materials and Procedure — three measures of aggression

The two forms of aggression were each measured using three different informants, a design feature that guards against the bias of any single source:

Measure	Who provided it	What it captured
Teacher rating	Kindergarten teachers	Rated each twin on items for physical aggression (e.g. "gets into fights", "hits, bites, kicks") and social aggression (e.g. "tries to make others dislike a child", "says bad things behind others' backs").
Peer nomination	Classmates	Children were shown photographs of their classmates and asked to circle the peers who best fitted descriptions such as "hits, bites or kicks" (physical) and "tells others not to play with a child" or "spreads nasty rumours" (social).
Self-report	The twins themselves	Younger children's self-reports were collected but treated cautiously, as six-year-olds are less reliable reporters of their own behaviour.

The peer-nomination procedure is especially valued because it draws on the observations of many classmates who see the child's behaviour daily, making it more ecologically valid than a single adult's judgement and harder to fake. Ratings for physical and social aggression were kept separate so that the genetic and environmental contributions to each could be estimated independently. The researchers then applied statistical model-fitting to the pattern of MZ and DZ correlations to estimate how much of the variation in each form of aggression was due to genes, shared environment and non-shared environment.

Exam Tip: The multiple-informant design (teacher + peer + self) is a genuine methodological strength you can use in AO3: it reduces reliance on one biased source and lets the researchers check whether findings converge across raters.

Results

The central finding was a striking dissociation between the two forms of aggression.

Physical aggression — substantially genetic

Variation in physical aggression was explained to a substantial degree by genetic factors (a moderate-to-strong heritability), with the remainder due largely to non-shared environment. In other words, identical twins were considerably more alike in physical aggression than non-identical twins, pointing to a meaningful inherited component.

Social aggression — predominantly environmental

Variation in social aggression, by contrast, was explained mainly by environmental factors — and importantly by the non-shared environment (the unique experiences of each twin) rather than by genes. MZ twins were not markedly more alike than DZ twins on social aggression, indicating little genetic contribution; instead, social aggression appeared to be learned from the social environment.

Form of aggression	Main genetic contribution?	Main environmental contribution?	Interpretation
Physical	Yes — substantial heritability	Non-shared environment (remainder)	Partly inherited; individual differences owe much to genes
Social (indirect)	No — little/none	Yes — mainly non-shared environment	Largely learned from unique social experiences

The relationship between the two forms

The study also found that children who were physically aggressive were often also socially aggressive — the two correlated positively. However, the model-fitting suggested that this overlap was explained partly by the genetic influence on physical aggression, whereas the distinctively social component of aggression (over and above physical aggression) was driven by the environment. Put differently: a child's physically aggressive tendencies may be partly inherited and may "spill over" into social aggression, but the specifically social, relational style of harming others is chiefly picked up from experience.

Conclusions

Brendgen and colleagues drew several conclusions:

Physical and social aggression have different origins. Physical aggression has a substantial genetic basis, whereas social aggression is predominantly environmentally determined (chiefly by non-shared experiences). Aggression is therefore not a single, uniformly heritable trait — its form matters.
Social aggression is largely learned. Because MZ twins were not much more alike than DZ twins on social aggression, it appears to develop through social learning — observation and imitation of models, peers and the social world — rather than being inherited. This is consistent with social-learning accounts of relational aggression.
The two forms overlap but are distinct. Physically aggressive children are often also socially aggressive, and some of that link is genetic (via physical aggression), but the specifically social form has its own, largely environmental, aetiology. This suggests physical aggression may be developmentally "primary", with social aggression emerging as a partly learned elaboration.
Aggression is best explained by gene–environment interplay. The findings support neither pure nature nor pure nurture, but a behavioural-genetic model in which genes and environment each contribute — and contribute differently to different behaviours.

The developmental implication is significant. If social aggression is largely learned, then it should be modifiable through environmental intervention — changing peer norms, modelling and social skills — offering a more optimistic message than a purely genetic account of aggression would allow.

Evaluation (AO3)

A key strength of the study is its use of multiple informants (teacher, peer and self-report), which increases the validity of the aggression measures. Rather than relying on a single, potentially biased source, Brendgen collected teacher ratings, peer nominations and self-reports, and gave particular weight to peer nominations because many classmates observe the child daily. Because the measures could be checked for convergence across raters, the classification of children as physically or socially aggressive is more trustworthy than a single adult's judgement. The implication is that the study's central finding — the dissociation between physical and social aggression — rests on a robust, triangulated measurement base rather than on one informant's perspective.

A further strength is the large, well-characterised twin sample, which gives the study strong statistical power and internal validity. With 234 twin pairs, zygosity carefully established (by resemblance questionnaire and, where needed, DNA), and formal model-fitting to partition variance, the study could estimate genetic and environmental contributions with reasonable precision. Because both MZ and DZ twins were reared together, the design holds shared environment broadly constant, so greater MZ similarity can be attributed more confidently to genes. The implication is that the different heritabilities of physical and social aggression are unlikely to be statistical flukes and reflect a genuine difference in the aetiology of the two behaviours.

However, the twin method rests on the "equal environments assumption", which may be violated and can inflate heritability estimates. The method assumes MZ and DZ twins share their environments to the same degree, but MZ twins are often treated more alike, spend more time together and are more frequently mistaken for each other than DZ twins. If MZ twins experience more similar environments, then their greater similarity in physical aggression may partly reflect environment, not genes. The implication is that the heritability of physical aggression may be overestimated, so the "genetic" conclusion should be treated as an upper bound rather than a precise figure.

A limitation of generalisability arises from the specific sample. The participants were six-year-old twins from Quebec, so the findings may not generalise to older children, to adults, or to other cultures, and twins themselves may differ from singletons (for example in language development or peer dynamics). Aggression also changes with age — physical aggression typically declines and social aggression rises through childhood — so the balance of genetic and environmental influences found at age six may not hold at, say, fifteen. The implication is that the conclusions are best treated as specific to early childhood rather than as a universal account of aggression across the lifespan.

Biological Psychology: Contemporary Study