BRENDGEN ET AL. (2005)

3.3.3 BRENDGEN ET AL. (2005) - EXAMINING GENETIC AND ENVIRONMENTAL EFFECTS ON SOCIAL AGGRESSION: A STUDY OF 6-YEAR-OLD TWINS

THE CONTEMPORARY STUDY: AGGRESSION IN 6-YEAR-OLD TWINS

Mara Brendgen and her team at the University of Montreal conducted this influential study on aggression in young children. Using a large sample of Canadian twins, the researchers distributed questionnaires to teachers and classmates to assess aggression. Brendgen’s focus was specifically on social aggression—such as taunting and malicious gossip—and its relationship with the physical aggression that had been the subject of extensive prior research.

This study holds significant value for psychology students for several reasons:

  • Advancing Research on Aggression: While previous studies concentrated on physical aggression and its biological and learned roots, Brendgen’s work breaks new ground by exploring social aggression as a distinct phenomenon. This innovative focus highlights the evolving nature of psychological research and the need to investigate underexplored behaviours.

  • Link to the Biological Approach: By employing a twin study methodology, the research demonstrates the Biological Approach’s emphasis on disentangling genetic (nature) and environmental (nurture) influences. This study isolates these variables by comparing monozygotic (MZ) twins, who share 100% of their genes, and dizygotic (DZ) twins, who share roughly 50%, to assess their influence on aggression.

  • Insights into Self-Report Methods: The study exemplifies both the strengths and limitations of self-report techniques. Teacher and peer questionnaires provided detailed data on aggression, but self-reported methods are inherently subjective and can be influenced by bias or inaccuracies, as the researchers did not observe the behaviours directly.

  • Relevance to Evolutionary Psychology: The research ties into the broader framework of evolutionary psychology, exploring how aggression may have a genetic basis due to its adaptive value for survival. For example, physical aggression might provide advantages in securing resources, while social aggression may play a role in maintaining status or social control within a group.

Brendgen et al.’s study serves as a cornerstone in aggression research, bridging the gap between physical and social aggression and shedding light on the complex interplay of biological and environmental factors. It highlights not only how aggression develops but also how research methods and theories in psychology continue to evolve

AIM

The aim of Brendgen et al. (2005) was to investigate the underlying influences of genetics and environmental factors on two distinct forms of aggression: social aggression, such as gossiping, spreading rumours, or excluding others, and physical aggression, such as hitting, kicking, or pushing. By focusing on these two types of aggression, the researchers sought to identify whether they arise from similar or distinct genetic and environmental mechanisms.

The study aimed to address three key questions:

  1. To what extent are social aggression and physical aggression influenced by genetic factors, shared environmental influences (e.g., family upbringing, cultural norms), and non-shared environmental influences (e.g., individual experiences, friendships)?

  2. Are the same genetic influences responsible for both social and physical aggression, or do they have separate genetic bases?

  3. How do environmental factors interact with genetic predispositions to influence the expression of these aggressive behaviours?

In particular, the study explored whether social aggression could develop from physical aggression over time through environmental shaping. For instance, children predisposed to physical aggression may shift toward more socially manipulative forms of aggression due to influences such as peer modelling, family dynamics, or social norms that discourage overt physical aggression. This aspect of the research was crucial for understanding the developmental pathways of aggressive behaviours and how interventions might differ depending on the underlying causes of each type of aggression.

By examining twins, including monozygotic (MZ) twins who share 100% of their genes and dizygotic (DZ) twins who share approximately 50%, the study aimed to disentangle the genetic contributions to aggression from environmental influences. This methodology allowed for a robust investigation of whether aggressive behaviours were primarily inherited or shaped by the children's surroundings and interactions.

The ultimate goal of the study was not only to understand the origins of these behaviours but also to inform strategies for intervention, particularly by identifying whether social aggression—due to its stronger environmental basis—might be more amenable to modification through targeted changes in upbringing, peer relationships, or educational approaches.

METHOD

The study involved a sample of 234 pairs of six-year-old twins from Canada, recruited to examine the genetic and environmental influences on aggression. The twin pairs included both monozygotic (MZ) twins, who are genetically identical and share 100% of their genes, and dizygotic (DZ) twins, who share approximately 50% of their genes, similar to ordinary siblings. By comparing the similarities in aggression between MZ and DZ twins, the researchers aimed to disentangle the relative contributions of genetic factors (heritability) and environmental factors (shared and non-shared) to both physical and social aggression.

The data collection involved multiple sources of assessment to ensure reliability and depth. Teachers rated the twins’ physical and social aggression based on observed behaviours in the classroom. Physical aggression was assessed through behaviours such as hitting, pushing, or physically intimidating peers, while social aggression was identified through actions such as spreading rumours, deliberately excluding others, or manipulating social relationships. These ratings were collected using a structured questionnaire designed to capture the frequency and intensity of aggressive behaviours in an educational setting.

To complement the teacher ratings, peer evaluations were gathered using a standardised peer nomination procedure. In this method, classmates were asked to identify peers who frequently exhibited aggressive behaviours, both physical and social. This approach allowed the researchers to triangulate their data and account for different perspectives on the children’s behaviours, reducing the potential for bias that might arise from relying on a single source.

The data were then analysed using advanced statistical models. These models estimated the proportion of variance in aggression attributable to genetic factors (heritability), shared environmental factors (influences common to both twins, such as parenting or home environment), and non-shared environmental factors (unique influences affecting one twin but not the other, such as different friendships or individual experiences). This comprehensive analysis allowed Brendgen et al. to explore not only the degree to which genetics and environment influenced physical and social aggression but also the potential overlap in the genetic and environmental determinants of the two forms of aggression.

This methodological rigour provided a nuanced understanding of the interplay between genetic predispositions and environmental contexts in shaping aggressive behaviours during early childhood

RESULTS

The findings of the study demonstrated a significant difference in the roles of genetic and environmental factors in physical and social aggression. For physical aggression, genetic factors were found to play a dominant role, with heritability accounting for 63% of the variance. This means that much of the variation in physical aggression among the children studied could be attributed to inherited genetic traits. The remaining variance was largely explained by non-shared environmental influences—factors unique to individual twins, such as differences in peer interactions, personal experiences, or specific situational exposures. Shared environmental factors, such as family upbringing or household dynamics, had a minimal influence on physical aggression.

In contrast, social aggression was found to be far less influenced by genetic factors. Heritability accounted for only 20% of the variance in social aggression, indicating a much smaller genetic contribution. Instead, shared environmental factors, such as parental modelling, sibling relationships, or cultural norms within the family or community, played a more significant role in shaping socially aggressive behaviours. These findings highlight that social aggression is more malleable and shaped by the collective environment shared by both twins, in contrast to the stronger genetic basis of physical aggression.

The study also uncovered a moderate genetic overlap between social and physical aggression. This suggests that some of the genetic predispositions associated with physical aggression may also influence social aggression. However, the expression of social aggression was found to be heavily moderated by environmental factors. For example, children who were genetically predisposed to physical aggression were more likely to develop social aggression if their environment encouraged behaviours like gossiping, exclusion, or manipulation. Such environmental influences could include observing peer modelling of social aggression, exposure to family dynamics that normalise such behaviour, or school environments where relational aggression is tolerated or rewarded.

These findings underline the importance of considering both genetic predispositions and environmental contexts when addressing aggressive behaviours in children. While physical aggression may be harder to modify due to its strong genetic basis, social aggression presents greater opportunities for intervention through changes in family dynamics, peer interactions, and broader environmental factors

CONCLUSION

Brendgen et al. concluded that social aggression is primarily shaped by environmental factors, while physical aggression is more strongly influenced by genetic factors. The findings suggest that physical aggression may act as a precursor to social aggression, with environmental influences playing a critical role in shaping the transition between these two forms of aggression.

STRENGTHS

STRENGTHS OF BRENDGEN ET AL. (2005)

The study by Brendgen et al. (2005) has several notable strengths that contribute to its robustness and validity as a piece of psychological research.

ROBUST RESEARCH DESIGN

The use of a twin design is a major strength, as it provides a robust framework for disentangling the relative contributions of genetic and environmental factors. By comparing monozygotic (MZ) twins, who share 100% of their genes, and dizygotic (DZ) twins, who share approximately 50% of their genes, the researchers were able to attribute differences in aggression to genetic influences versus environmental factors. This methodological approach is a gold standard in behavioural genetics research and allows for a nuanced understanding of how nature and nurture interact.

RELIABILITY OF MEASUREMENTS

The study utilised multiple informants—both teachers and peers—to assess aggressive behaviours.

  • Teacher Ratings: Teachers are well-positioned to observe children in structured social settings, such as classrooms, where aggression is likely to manifest consistently. Their professional training in behaviour management also lends credibility to their assessments.

  • Peer Ratings: Peer evaluations provided additional validity, as children often witness aggressive behaviours in less formal contexts, such as playtime or informal interactions.
    The use of two independent sources of data increases the reliability and validity of the findings, reducing the potential for bias that might arise from relying solely on one perspective.

DEVELOPMENTAL INSIGHTS

The study provided valuable insights into the developmental pathways of aggression, highlighting how physical aggression can act as a precursor to social aggression. This is particularly important for understanding how environmental factors shape behaviours over time. For example, children genetically predisposed to physical aggression may adopt socially manipulative forms of aggression in environments that discourage overt physical conflict. This developmental perspective makes the findings applicable to interventions designed to address aggression at different stages of childhood.

STATISTICAL RIGOUR

The researchers employed advanced statistical models to estimate the proportion of variance in aggression attributable to genetic, shared environmental, and non-shared environmental factors. This rigorous analytical approach ensured that the contributions of each factor were accurately quantified, enhancing the precision of the conclusions.

REPLICABILITY AND SIMILAR STUDIES

Brendgen et al.’s findings align with those of similar twin studies, strengthening the reliability of the results. For example:

  • Caspi et al. (2002) found that genetic factors significantly influence antisocial behaviour, while environmental factors moderate how these genetic predispositions are expressed.

  • Eley et al. (1999) demonstrated genetic influences on aggression in childhood, corroborating Brendgen et al.’s findings regarding the heritability of physical aggression.

The consistency of these findings across studies suggests that the genetic and environmental influences on aggression observed by Brendgen et al. are robust and replicable in different contexts.

APPLICABILITY TO INTERVENTIONS

The study’s distinction between physical and social aggression has practical applications in developing tailored interventions.

  • Physical Aggression: Given its stronger genetic basis, interventions may focus on managing impulsivity and providing structured environments to mitigate its expression.

  • Social Aggression: Since social aggression is more influenced by shared environmental factors, interventions targeting family dynamics, peer relationships, and school policies are likely to be more effective.

ETHICAL DESIGN

The study adhered to ethical guidelines by using observational methods and questionnaire-based assessments rather than direct experimental manipulation. This ensures that no harm was caused to the children involved. Additionally, informed consent was obtained from the parents or guardians of the twins.

CONTRIBUTION TO NATURE VS NURTURE DEBATE

By distinguishing between the genetic and environmental contributions to different types of aggression, the study adds depth to the long-standing nature vs nurture debate. It demonstrates how genetic predispositions interact with environmental factors to produce complex behaviours, reinforcing the idea that aggression results from both inherited and learned influences.

SUMMARY OF STRENGTHS

The study’s twin design, use of multiple informants, statistical rigour, and alignment with similar research make it a methodologically robust investigation into the origins of aggression. Its developmental insights and applicability to interventions further enhance its value, offering practical strategies for addressing aggressive behaviours in children. Additionally, its ethical and replicable nature strengthens its contribution to the field of developmental psychology and behavioural genetics.

LIMITATIONS OF BRENDGEN ET AL. (2005)

While Brendgen et al.’s study provides valuable insights into aggression, it is not without limitations. These issues pertain to the methodology, generalisability, and interpretation of the findings.

AGE-SPECIFIC FOCUS

The study exclusively examined six-year-old children, which limits its developmental generalisability. Aggressive behaviours evolve significantly over time, with physical aggression typically declining as children age and social aggression becoming more prominent. As such, the findings cannot reliably explain aggression in adolescents or adults, where different social dynamics, cognitive development, and hormonal changes influence behaviour.

POTENTIAL BIAS IN RATINGS

The reliance on teacher and peer ratings introduces the possibility of subjective bias.

  • Teacher Bias: Teachers may have unconsciously overestimated or underestimated certain children’s aggression due to familiarity, implicit bias, or reliance on classroom behaviour rather than a comprehensive view of the child’s social interactions.

  • Peer Bias: Peer nominations could have been influenced by popularity dynamics, peer pressure, or interpersonal conflicts, potentially skewing the data.
    Although using multiple informants strengthens reliability, these biases cannot be fully eliminated.

CULTURAL LIMITATIONS

The study was conducted in Canada, and its findings may not apply to other cultural contexts. Aggression is influenced by cultural norms, values, and child-rearing practices, which vary globally. For instance, cultures that emphasise collectivism may discourage overt aggression more strongly than individualistic societies. This cultural variability limits the external validity of the study.

LIMITATIONS OF TWIN STUDIES

While twin studies are a powerful tool in behavioural genetics, they are not without methodological issues:

  • Shared Environmental Influences: Twins, even dizygotic (DZ) ones, often grow up in the same household and share similar environments, making it difficult to separate genetic influences from shared environmental ones. For example, parenting styles, socio-economic conditions, and cultural zeitgeists affect both twins similarly.

  • Ordinary Sibling Comparison: Including ordinary siblings (non-twin siblings) in the study would have provided an additional layer of comparison. Ordinary siblings share 50% of their genes, like DZ twins, but often grow up in slightly different family environments due to age gaps or different social contexts. This could have highlighted environmental influences more clearly.

  • Cause and Effect: Twin studies show correlation rather than causation. For example, a high heritability estimate for aggression does not necessarily mean genes cause aggression; environmental interactions might still play a significant role.

LIMITED SCOPE OF MEASURES

The study focused on specific forms of aggression (physical and social) but did not account for contextual aggression (e.g., reactive vs proactive aggression) or other influential behaviours like verbal aggression. This narrow focus might oversimplify the complexity of aggressive behaviours.

LACK OF LONGITUDINAL DATA

The study did not follow the participants over time, making it difficult to determine how genetic and environmental influences on aggression change with age. For instance, physical aggression may decrease in later childhood, while social aggression may increase. A longitudinal design would have provided deeper insights into the developmental trajectories of aggression.

GENERALISABILITY TO CLINICAL POPULATIONS

The study involved a general population sample and did not include children with clinical diagnoses, such as conduct disorder or oppositional defiant disorder, where aggression might manifest differently. The findings may therefore not apply to clinical populations where aggression is more severe or pathological.

ENVIRONMENTAL FACTORS UNDER-EXPLORED

While the study highlighted the role of shared and non-shared environmental factors, it did not delve deeply into specific environmental variables, such as parenting styles, peer relationships, or socio-economic conditions. Understanding these factors could have provided more actionable insights for interventions.

SUMMARY OF LIMITATIONS

Brendgen et al.’s study, while methodologically sound, is limited by its age-specific focus, potential biases in data collection, cultural constraints, and the inherent challenges of twin studies. These include difficulties in separating shared environmental influences from genetic ones and the absence of comparative data from ordinary siblings. The study’s lack of longitudinal data and its limited scope of aggression measures further restrict the applicability of its findings to broader populations or contexts. Addressing these limitations in future research would enhance our understanding of the interplay between genetics, environment, and aggression

APPLICATIONS

The findings have practical applications in educational and parenting strategies. Interventions targeting social aggression should prioritise modifying environmental factors, such as peer influence and family dynamics, rather than focusing on genetic predispositions. Early prevention strategies that identify children predisposed to physical aggression may help reduce the likelihood of developing social aggression through targeted environmental changes.

EXEMPLAR ESSAY

Evaluate the contemporary study from biological psychology. (8 marks)

An 8-mark "evaluate" question awards 4 marks for AO1 (Describe) and 4 marks for AO3 (Evaluate). A conclusion is necessary to achieve the top band (7–8 marks).

DESCRIPTION

Brendgen et al. conducted a study involving 234 pairs of twins, drawn from the Quebec Newborn Twin Study, when the children were 6 years old. Parental consent was obtained to allow the children to participate in school-based assessments.

The study used two types of data collection methods. Teacher ratings were gathered through a questionnaire, which included questions assessing behaviours such as how frequently the child engaged in fights. These ratings provided scores for both physical and social aggression. Peer ratings were collected by showing classmates photographs of their peers. The children were then asked to circle the photographs of those they thought were most likely to engage in behaviours like spreading malicious rumours or physically hitting others. This method classified children as being either socially or physically aggressive.

The findings indicated that physical aggression was strongly influenced by genetic factors, with heritability accounting for approximately 60% of the variance. In contrast, social aggression was influenced by genetic factors in only 20% of cases, with environmental factors playing a more significant role.

EVALUATION

Brendgen’s study demonstrated high reliability due to its use of standardised survey methods that are straightforward to replicate. For example, the photo-based peer nomination procedure is a well-established technique, particularly effective for younger children who may struggle with more abstract tasks.

The study is also valid because it employs a twin study methodology. By comparing monozygotic (MZ) twins, who share 100% of their genetic material, with dizygotic (DZ) twins, who share roughly 50%, the researchers could attribute behavioural differences to either genetic or environmental factors. This design strengthens the study’s conclusions about the genetic basis of aggression.

However, correlations do not establish causation. For example, the results may be influenced by biases in the environment. Classmates and teachers might stereotype one twin based on the behaviour of their sibling, especially in the case of identical twins who are often mistaken for each other.

Ethical concerns arise from the peer nomination process, which required children to identify classmates they associated with aggressive behaviours, such as spreading secrets. This could potentially harm relationships, create stigma, or contribute to bullying.

CONCLUSION

While Brendgen et al. highlighted important differences in the genetic and environmental contributions to social and physical aggression, they acknowledged that the sample size was too small to generalise the findings widely. As this was one of the first studies to examine social aggression using this approach, further research with larger, more diverse samples is necessary to confirm these findings.

Rebecca Sylvia

I am a Londoner with over 30 years of experience teaching psychology at A-Level, IB, and undergraduate levels. Throughout my career, I’ve taught in more than 40 establishments across the UK and internationally, including Spain, Lithuania, and Cyprus. My teaching has been consistently recognised for its high success rates, and I’ve also worked as a consultant in education, supporting institutions in delivering exceptional psychology programmes.

I’ve written various psychology materials and articles, focusing on making complex concepts accessible to students and educators. In addition to teaching, I’ve published peer-reviewed research in the field of eating disorders.

My career began after earning a degree in Psychology and a master’s in Cognitive Neuroscience. Over the years, I’ve combined my academic foundation with hands-on teaching and leadership roles, including serving as Head of Social Sciences.

Outside of my professional life, I have two children and enjoy a variety of interests, including skiing, hiking, playing backgammon, and podcasting. These pursuits keep me curious, active, and grounded—qualities I bring into my teaching and consultancy work. My personal and professional goals include inspiring curiosity about human behaviour, supporting educators, and helping students achieve their full potential.

https://psychstory.co.uk
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