Brendgen et al. (2005) Social Aggression (Edexcel A-Level Psychology): Revision Notes
Brendgen et al. (2005) Social Aggression
Study overview
This landmark study examined the genetic and environmental origins of social aggression in young children using a twin study methodology.
Researchers: Brendgen et al.
Title: Examining genetic and environmental effects on social aggression: a study of 6-year-old twins
Year: 2005
Aim
Previous research into aggression has primarily focused on physical forms of aggression, with less attention paid to verbal and social forms. Brendgen and colleagues investigated the origins of social aggression, which involves socially manipulative behaviours such as ignoring others, spreading rumours or threatening to withdraw friendship. Social aggression can be expressed both overtly and covertly.
The study had three key aims:
- To determine whether social aggression is caused by genetic factors or environmental factors
- To establish if social aggression shares the same underlying causes as physical aggression
- To examine whether one type of aggression leads to another type
Participants
The sample was recruited from the Quebec Newborn Twin Study (QNTS), comprising twins born between November 1995 and July 1998. Initially, 322 pairs of twins were tested, but complete data at all stages was only available for 234 twin pairs.
Sample Composition
The final sample consisted of:
- 44 pairs of monozygotic (MZ) males
- 50 pairs of MZ females
- 41 pairs of dizygotic (DZ) males
- 32 pairs of DZ females
- 67 pairs of mixed-sex DZ twins
Data was collected longitudinally at 5, 18, 30, 48 and 60 months, with the final data gathered at age 6 years, which formed the focus of this study.
Procedure
Two ratings of each twin's behaviour were collected - one from their teacher and one from their classmates. Ratings were gathered in the spring term of the school year to ensure the twins were well known by those providing the ratings.
Teacher ratings were based on agreement with statements from two standardised scales:
- Preschool Social Behaviour Scale (PSBS-T; Crick et al., 1997)
- Direct and Indirect Aggression Scales (Bjorkvist, Lagerspetz et al., 1992)
These scales included items such as "To what extent does the child try to make others dislike a child" (social aggression) and "To what extent does the child get into fights" (physical aggression). Teachers scored each statement on a 3-point scale (0=never/sometimes; 2=often).
Peer ratings were conducted by giving each child in the twins' classes a booklet containing photos of every child in the class. Each child was asked to circle three pictures of children that matched four different behaviour descriptions, such as "Tells others not to play with a child" (social aggression) and "Gets into fights" (physical aggression). Each twin received a physical and social aggression score from both teacher ratings and peer selections. Any peer selections on the social or physical aggression descriptors were also recorded.
Results
The findings revealed important differences in the genetic and environmental contributions to different types of aggression:
Physical aggression: MZ twin pairs showed a much higher correlation in ratings for physical aggression compared to same-sex DZ twin pairs. This pattern was consistent in both teacher and peer rating scores, suggesting that physical aggression may be caused by genetic factors.
Social aggression: Scores for social aggression were roughly equally correlated in MZ and DZ twin pairs. These findings suggest that social aggression may be better explained by shared environmental factors rather than genetic inheritance.
Regarding the second aim, a correlation was found between physical and social aggression in the children that was best explained by genetic factors rather than shared environment. This indicates that whilst aggressive tendencies in general may result from genetic factors, the way these tendencies are expressed may be determined by environmental factors such as exposure to other people's aggressive behaviours.
For the third aim, the data suggested that physical aggression may lead to social aggression, but not the other way around. The researchers concluded that the expression of aggressive tendencies changes as children develop. Young children may only be able to express aggression physically, but as their language and cognitive skills develop, they gain the ability to demonstrate aggressive behaviour in new ways, including socially acceptable forms.
Conclusion
Key Findings
The research demonstrates a strong genetic component to physical aggression but not social aggression, which appears more influenced by environmental effects. Children who were physically aggressive were also more likely to display social aggression, probably due to an interaction between genes and environment. As children grow, they tend to become more socially aggressive because of social conventions regarding physical violence and developing different ways to express themselves.
Evaluation: Strengths
Multiple sources of data: The study benefits from taking measures of the twins' aggressive behaviour from two different sources - both teachers and peers. This validates the findings by examining different sources of information, which should eliminate or highlight any bias. The high agreement between teachers and peers adds to the validity of the findings, suggesting neither group was giving a seemingly biased view of the aggressive behaviour of the individuals.
Practical applications: The research findings can be used to prevent the development of social aggression. If children are showing physically aggressive tendencies, addressing this early may prevent them later expressing this aggression socially. This could be used as an early indicator to parents or teachers that a child's behaviour could become problematic later, allowing early intervention rather than waiting until the behaviour has become established. It is much easier to challenge aggressive behaviour in a small child whilst they are still learning, than to change the behaviour of an older child when they have already developed a strong sense of their own personality and may be more resistant to change.
Good control of variables: The samples were well matched through statistical tests across several measures such as nicotine use, revealing no differences between them other than their history of substance abuse. The researchers also ensured that the twins were well known by those providing ratings of their behaviour by collecting data in the spring term.
Evaluation: Weaknesses
Small sample size and limited generalisability: The sample size is small and limited to males from only one culture, suggesting that generalisability beyond this group may be unreliable. However, the results are consistent with other studies testing different brain areas that show similar differences in the reward centres of the brain associated with heroin use. One key criticism is the small sample sizes when examining the different groups being compared. This makes generalisation difficult because the chance of the sample being representative of the entire population would be very low.
Time-consuming methodology: The reason for the small sample was a consequence of the way the study was conducted. Asking 6-year-old peers to provide ratings before they could read and write themselves meant that researchers had to record the data with each child individually, which was a time-consuming process and resulted in the small sample achieved.
Age-specific findings: There could be many possible extraneous variables in the lives of this specific group of twins that may explain the aggressive behaviour shown. The researchers justified this criticism by noting that the costs in time and effort associated with individually measuring the twins' behaviour across 409 different classrooms meant that selecting an assessment of moderating factors would have been extremely difficult. Another issue with generalising from this sample is that the age group being studied is very specific and it would be impossible to assume that aggression in other age groups will have the same cause. Research has found that physical aggression reduces when children start school (Nagin and Tremblay, 2001) while social aggression does not fully develop until later in childhood around the age of 8 years (Bjorkvist, Lagerspetz et al., 1992). This suggests that aggression in children from an older age group may be characterised very differently.
Difficulty isolating brain activity: Using scanning to measure activity in one specific brain area is difficult as other areas will also be active and could cause the analysis to be less reliable. However, the background 'noise' from other brain areas would have affected both groups equally, so the difference that was observed could be regarded as a genuine effect.
Remember!
Key Points to Remember:
- Social aggression involves manipulative behaviours like spreading rumours or withdrawing friendship, whilst physical aggression involves overt physical acts
- MZ twins showed higher correlation for physical aggression than DZ twins, suggesting genetic influence
- Social aggression showed equal correlation in MZ and DZ twins, suggesting environmental factors are more important
- Physical aggression appears to lead to social aggression as children develop, not the other way around
- The study's strengths include using multiple sources (teachers and peers) and practical applications for early intervention, but weaknesses include small sample size and age-specific findings that limit generalisability