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Health

How Siblings Fuel Each Other's Worst Impulses

New research suggests bad behavior could be contagious between brothers and sisters.

If you have a brother or sister, you may have each other to blame for whatever trouble you once got into, according to a new study by researchers at Florida Atlantic University. Parents have long blamed troublemaking on delinquent friends and bad genes. But it turns out, when one kid starts making bad choices, the likelihood increases of the other following suit.  Researchers looked at twins' behavior during the prime malleable ages of 13 to 15. "Children don't yet have a firm identity going into adolescence and look to others for guidance when encountering new situations," says lead study author Brett Laursen, professor of social developmental psychology at Florida Atlantic University. Laursen's team found that if one sibling started drinking or doing drugs, the other was more likely to adopt the same rebellious behavior. The smaller the age gap, the more susceptible one kid is to the other. We spend the most unsupervised time with our kin, especially at an age when parents feel more comfortable leaving us in the care of an older brother or sister. (And thus, more likely to join them in picking the lock on the liquor cabinet.) If you don't have a close brother or sister, the same theory applies to an especially close friend. We're more likely to copy personalities that we feel are similar to ours, says Susan M. McHale, professor of human development and family studies at Pennsylvania State University.

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And if everyone blames you for making the most trouble as a teenager, your brother or sister still isn't off the hook. Once ignited by your sibling, the flame of wrongdoing can quickly become a wildfire, Laursen says. It's not just expected transgressions like drinking and having sex, either. Siblings influence seemingly benign acts of rebellion. "It's so easy to get into a cycle of collusion," McHale says. "Parents are perceived to be too strict, so siblings egg one another on to defy mom and dad's authority, plot behind their back, raise their eyebrows, and roll their eyes."

These habits can domino into adulthood. If you're trained at 12 to mock and defy power, that attitude may carry over to when you're working with an uptight boss years later. "There are longterm consequences to the habits we develop in adolescence," McHale says. In fact, a study McHale once authored in the Journal of Family Issues found that if you and your sibling often rough-housed, argued, or teased one another, you're more likely to practice these habits with future romantic partners. However, your beloved siblings aren't all trouble. The flip side is also true: They can take credit for your positive qualities—some of them, anyway. "The twin with fewer problems can also serve as a role model, figuratively pulling on the leash," Laursen says.