Sibling Aggression: A Training Ground for Anti-Social Behaviour

Brothers and sisters learn a lot about relationships from their interactions with each other; on the Family Anatomy podcast, Dr. G and I have described the sibling relationship as a testing ground in which kids develop and practice their social skills. But if kids learn about the social world by interacting with their brothers or sisters, what happens when that relationship is a negative one, characterized by coercion and aggression? That's exactly the question that was asked by researchers in a study published in the current issue of Developmental Psychology. The idea that siblings learn about relationships from each other isn't a new one. Twenty-five years ago, Gerald Patterson and his colleagues developed the concept of deviancy training, which proposed that negative exchanges between siblings provide opportunities for them to learn delinquent behaviour, leading to increases in aggression and other problems as the kids get older. Although the deviancy training theory has received attention in recent years, it's tough to test; siblings share genes, an environment, and parents as well as interactions. These commonalities make it difficult to tease apart the influence of one factor over another; studies of sibling interactions rarely take into account genetics, and genetic studies seldom focus on the sibling relationship.

The new study, conducted by researchers at the University of Minnesota, Yale, and Penn State, looked at data collected from a large group of identical and fraternal twins, other siblings that were close in age, and step-siblings. They also examined parenting behaviour. They followed over 700 sibling pairs from two-parent homes for 3 years. By examining pairs that varied in genetic similarity, the researchers were able to take into account the genetic relationships when examining the results.

The researchers found that, after statistically controlling for other factors, sibling aggression had a "modest but significant" relationship with the development of "externalizing problems," which include behaviours such as verbal and physical aggression, as well as rule-breaking. The age difference between the siblings didn't change the results, indicating that the "training" can go both ways - from an older to a younger sibling or vice versa. Externalizing problems have been linked in previous research to lower academic achievement as well as other difficulties - it's not hard to imagine that aggressive, rule-breaking youth might be more likely to get into trouble with the law!

So siblings in aggressive relationships seem to be at greater risk for developing behavioural difficulties. What does this mean for parents? One implication noted by the researchers is that the treatment of aggression and other externalizing problems might be undermined by sibling aggression at home - these findings suggest that the involvement of siblings in treatment, and working to improve the sibling relationship, might be helpful for externalizing youth. It would probably be worthwhile for parents to be alert to chronic negative interactions between their children. If it's possible that a negative sibling relationship could put kids at risk for behaviour problems, it could be helpful to address the difficulties earlier rather than later. Family therapy is one avenue. We also talked about siblings in an early episode of the Family Anatomy podcast; you might find some helpful information there. And I'll recommend a book by two of my favourite parenting authors one more time: Siblings Without Rivalry: How to Help Your Children Live Together So You Can Live Too.

You can read the Developmental Psychology article here.

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