Contemporary Study: Brendgen et al (2005)

Aim

(AO1)

To investigate aggression in MZ and DZ twins to discover the extent to which physical and social aggression are explained by genetics and environmental influences

Who were the participants?

234 sets of twins

• 44 pairs of male MZ twins

• 50 pairs of female MZ twins

• 41 pairs of male DZ twins

• 32 pairs of female DZ twins

• 67 pairs of mixed DZ twins

Procedure

(AO1)

1. Kindergarten teachers rated the social and physical aggression of the children on a three-point scale ranging from never, sometimes and often

   • A prompt to rate social aggression was “Says mean things and spread rumours”

2. Levels of aggression but also recorded from the children’s peers

3. Each classmate was shown three photos of their classmates

4. They nominated three children who fit the description the best

   • A prompt to gage physical aggression was ‘hits, bites and kicked others’

Findings

(AO1)

• Social Aggression was prevalent in twins with non environments 60% and 54% of the time

• Physical aggression was due to genetic factors 63 and 54% of the time

Conclusions

(AO1)

• some genetic factors predisposed some children to be physically aggressive whereas social aggression is more likely due to the environmental effects (e.g. copying parental behaviour)

Strength of Brendgen’s study

(AO3)

I: Ratings from teachers and the children’s peers were used

J: If there was a large difference between the ratings, this would highlight the potential biases. As the two sets of ratings are very similar, it provides inter-rater reliability. This means the measurement of aggression was efficient leading to internal validity.

Weakness of Brendgen’s Study

(AO3)

I: The study cannot conclude a cause and effect relationship between genes and aggression.

J: Since MZ twins are identical one twin might be stereotyped based on the behaviour of another twin. Teacher and peer ratings may have been influenced and both twins may have been given the same rating regardless of the behaviour

E: This reduces the validity of the result that physical aggression is genetically disposed