Brendgen et al 2005

• Brendgen (2005) aim: To see if social aggression could be caused by genes or the environment
• Brendgen (2005) aim: To see if social aggression shared the same causes as physical aggression
• Brendgen et al (2005) Aim: To see if one type of aggression leads to another type.
• Brendgen et al (2005) Results: Physical aggression- high correlation between the ratings of MZ twin pairs compared to same-sex DZ twin pairs.
• Brendgen et al (2005) Results: Social aggression- scores for MZ & DZ twin pairs were roughly correlated.
• Brendgen et al (2005) Results: Physical aggression may lead to social aggression but not the other way round.
• Brendgen et al (2005) Results: Correlation found that physical and social aggression better explained by genes.
• Brendgen et al (2005) Conclusion: Strong genetic component to physical aggression,  but not social aggression. Social aggression more likely caused by environment factors.
• Brendgen et al (2005) Sample: Longitudinal study, focused on behaviour of 6 years. Included 234 twin pairs (44 MZ male pairs,50 MZ male pairs, 32 DZ female pairs, 41 DZ female pairs, 67 mixed DZ pairs)
• Brendgen et al (2005) Procedure: Data consisted of teacher and student ratings. Teacher ratings; Based on agreement with statements from questionnaire (preschool social behaviour scale). Peer ratings; Children were given booklet with every child in the class, asked to circle 3 pictures of children, that they thought matched.
• (strength) High validity with two different sources that measured aggression - Teachers and children.
• (Strength) Application- results can be used to help prevent forming social aggression
• (Weakness) Low ecological validity as it was conducted in preschools so may not apply to older age groups or other settings.
• (weakness) Small sample size which limits generalisability of findings.