Results

• There was an overlap between social and physical aggression in both the teacher and peer ratings so a child seen as socially aggressive was also seen as physically aggressive and vice versa - this link was seen as moderate
• Much higher correlation between the teacher and peer ratings of MZ twins on physical aggression than there was for same sex DZ twins; implies physical aggression may be genetic, but social aggression is more likely environmental
• Socially aggressive children may not be inherently aggressive but simply reflect the aggressive behaviours they observe in their environment (also no gender differences found in terms of physical and social aggression)
• A correlation was found between physical and social aggression in the children that was best explained by genes; aggressive tendencies could be due to genes, but expression is determined by their environment
• To calculate contribution by genes they took the MZ correlation/concordance rate (0.60) from that of the DZ correlation(0.60-0.40=0.20) and the genetic contribution is 2x the difference (0.20*2=0.40; 40% contribution)
• To find out the contribution from a shared environment, take the MZ correlation (0.60) from twice the DZ correlation (0.40*2=0.80; 0.80-0.60=0.20; 20% contribution)
• Data suggests physical aggression leads to social aggression but social aggression does not lead to physical aggression; children can be more physically aggressive but learn more socially acceptable ways to show aggression
• Young children can only express aggression physically, but as their language and cognitive skills develop, they can show their aggression in other ways; conform more to more acceptable social norms and patterns of behaviour