For Teen Twins, Bad Behavior Can Be Contagious

Bad behaviors rub off on siblings, but not always directly. Universal Images Group/Getty Images

As if parents don't worry enough about their children, a new study shows that it isn't always a delinquent friend who leads an adolescent astray. In fact, the bad apple could live in the same house.

The study looked at identical and fraternal twins ages 13-15 who are part of the Quebec Newborn Twin Study, which follows twins born in Montreal between 1995 and 1998. It found that siblings are a key factor in the escalation of behavior problems and substance use among their brothers and sisters.


Researchers at Florida Atlantic University (FAU) and universities in Quebec, Canada, surveyed 151 male twin pairs and 163 female twin pairs, asking questions about drug and alcohol use as well as delinquent behaviors, such as stealing from stores, bullying and cheating on exams. They found that over the years, adolescents with a sibling who participated in delinquent behaviors or substance use were more likely to do so themselves.

Here's where the results get a little tough to explain. You might have expected that problem behaviors in twins would be linear. For example, if one twin used drugs, the other would also. But that's not exactly what this study found. Instead, the problem behaviors seemed to spread along one of two indirect paths. The first path showed, for example, Twin A delinquency led to Twin B delinquency, which then led to Twin B substance use. Along the second path, Twin A delinquency led to Twin A substance use, which then led to Twin B substance use.

Because they set up controls to screen out behavior influences like genetic effects and friend and parenting behaviors, the researchers said in the study, they are confident "twin siblings uniquely contribute to the spread of problem behaviors during adolescence."

"The hypothesis that we were testing is that ... the more delinquent one sibling is the more different problems the other sibling has," says Brett Laursen, lead author and FAU psychology professor, in a press release. "This turns out not to be the case. Instead, we found that problems spread between siblings within problem behavior domains — one sibling's delinquency affects the other sibling's delinquency."

This study builds on previous ones that show siblings are unique in their influence on one another, and that adolescents with a delinquent sibling are more likely to misuse drugs and alcohol than those without one, by explaining how these problems spread. The researchers also found that delinquency is more apt to predict later substance use than the reverse.

There is a bit of good news for concerned parents. Because the same problems spread between siblings, the study may provide a window into new ways to try to stem undesirable behaviors in adolescents. Interventions can be targeted at a specific behavior, like problem drinking, rather than at trying to prevent all bad behaviors or at trying to disrupt the relationship between siblings.

"The key take-home message from this study is that intervention programs needs [sic] to be targeted at specific problem behaviors and not the relationship itself," says Laursen. "It is insufficient and impractical to try to keep siblings apart, advice we often give when we try to separate adolescents from their problematic friends."