Twin Studies into Aggression
Monozygotic (identical) twins share all their genes. Dizygotic (non identical) twins share only 50% of their genes. Researchers compare the similarity for a trait e.g. Aggression between sets of MX twins and sets of DZ twins. If MZ are more similar in aggression this should be due to genes rather than the environment. (Both types of twins share the same environment as each other but MZ are more genetically alike.) Study of aggressive behaviour using adult twin pairs, found that nearly 50% of the variance in direct aggression behaviour (i.e aggressive towards others) could be attrivuted to genetic factors. Coccaro et al (1997)
If a positive correlation is found between aggressive behaviour in adopted children and aggressive behaviour in their biological parents, a genetic effect is implied. If a positive correlation is found between the adoptee’s aggressive behaviour and the rearing family – environment
factor is implied. A study of over 14,000 adoptions in Denmark found that a significant number of adopted boys with criminal convictions had biological parents (particularly fathers) with criminal convictions (Hutchings and Mednick) this provides evidence for a genetic effect.
Miles and Carey Twin and Adoption research
Carried out a meta analysis of 24 twin and adoption studies that had investigated the genetic basis of aggression. Most studies had relied on parental or participants’ self-reports of aggressive tendencies, although some involved observation of aggressive behaviour. Results and the influence of genes increased. The results suggested a strong genetic influence that could account for as much as 50% of the variance in aggression. Variables Age differences were notably important, with both genes and family environment being influential in determining aggression in youth , but at later ages the influence of rearing environment decreased
Problems with assessing aggression
Most studies use either parental or self reports. In the Miles and Carey meta-analysis mode of assessment was found to be a significant moderator of aggressive behaviour in the 24 studies that made up their analysis. They found that genetic factors explained a large proportion of the variance in aggressive behaviour in studies that had used parental or self-reports. However those that had made use of observational ratings showed significantly less genetic contribution and a greater influence of environmental factors.