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June 13, 2011

Birds of a feather? Friendships and later aggression in early childhood.

Vitaro, F., Brendgen, M., Boivin, M., Cantin, S., Dionne, G., Tremblay, R.E., Girard, A., & Perusse, D. (2011). A monozygotic twin difference study of friends’ aggression and children’s adjustment problems. Child Development, 82 (2), 617-632.

This study examined the importance of early childhood friendships and their contribution to subsequent aggression.  The aim was to discover whether having aggressive friends in kindergarten leads to a child showing signs of aggression 1 year later.  Aggressive friends could be viewed as an environmental factor contributing to childhood aggression. However, something in an individuals’ genetic makeup may cause them to choose aggressive friends. This would be a case of genetic factors interacting with environmental factors. It can be difficult in such cases to determine whether it is genetic or environmental factors that are contributing to aggression.  Monozygotic (MZ) twins share 100% of their genes and are usually brought up together. Thus, in an MZ twin pair, if one twin shows signs of aggressive behaviour and the other does not, aggression could not be explained by genetic factors or by an interaction between genetic and environmental factors. Instead, the difference must be explained by some factor out with genetics or their common environment.
This study had 3 aims:
  1. To examine whether MZ twins who had different experiences of aggressive friends in kindergarten would show different signs of aggression and depression one year later.  
  2. To examine whether twins who were victimized by their friends showed higher subsequent aggression. 
  3. To test if this effect would be the same for female twin pairs as male twin pairs.

Two-hundred and thirty- three MZ twin pairs (117 female pairs) from Canada took part in the study. Children were assessed at 5, 19, 30, 48, 60, 72 and 84 months. Teachers and classmates were asked to rate the extent to which each twin had shown aggressive behaviour and depressive signs over the last 6 months. Twins were asked who their three best friends in the class were but were not allowed to nominate their own twin.  Twins’ perception of their friend’s aggression towards them was assessed by interviewing each twin and asking questions about their friend’s behaviour. For example, twins were asked how often their friend had said mean things to them since the beginning of the school year.

The results showed that, even after controlling for other factors such as twins being bullied and twins being treated differently by their parents, having aggressive friends in kindergarten made a significant contribution to the difference in aggression between MZ twins one year later. This result was found for male and female twin pairs.

Contradictory to previous evidence, having aggressive friends in kindergarten was not correlated with depressive symptoms one year later. This may be because in the present study the authors did not ask the children themselves whether they experienced any depressive symptoms (they collected this information from teachers and classmates). This meant that they could not assess the thoughts and feelings of each twin. Another possibility could be that friends’ aggression and depressive symptoms correlate due to genetic factors or an interaction between genetic and environmental factors. However, by using MZ twins, the present study eliminated this possibility.

For male, but not female, twins who thought that they were being victimized by their friends were more likely to show symptoms of aggression one year later. This raises interesting questions about how friendship experiences can have different effects for male and female children.

In summary, the present study found a link between early childhood friendships and subsequent aggressive behaviour. This link was found independent of genetic influences, interactions between genetics and environment and other influences from the twin’s unique environment (such as being treated differently by parents and being bullied). Therefore, friendships early on in childhood seem to be of great significance to subsequent development.

This summary was produced by Lynsey Emery, an undergraduate student who is entering her final year. Her dissertation is focussing on the relationships between theory of mind, the hostile attribution bias, and proactive and reactive aggression.

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