The New Science of Siblings

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Alejandra and Sofia Romero, 5-year-old fraternal twins growing up in New York City, entered the world at almost the same instant but have gone their own ways ever since--at least in terms of temperament. Alejandra has more of a tolerance--even a taste--for rules and regimens. Sofia observed this (and her parents observed her observing it) and then distinguished herself as the looser, less disciplined of the two. Sofia is also the more garrulous, and Alejandra eventually became the more taciturn. "Sofie served as their mouthpiece," says Lisa Dreyer, 39, the girls' mother, "and Alejandra was perfectly happy to let her do it."

De-identification helps kids stake out personality turf inside the home, but it has another, far more important function: pushing some sibs away from risky behavior. On the whole, siblings pass on dangerous habits to one another in a depressingly predictable way. A girl with an older, pregnant teenage sister is four to six times as likely to become a teen mom herself, says Patricia East, a developmental psychologist at the University of California, San Diego. The same pattern holds for substance abuse. According to a paper published in the Journal of Drug Issues earlier this year, younger siblings whose older sibs drink are twice as likely to pick up the habit too. When it comes to smoking, the risk increases fourfold.

But some kids break the mold--and for surprising reasons. East conducted a five-year study of 227 families and found that those girls who don't follow their older sisters into pregnancy may be drawn not so much to the wisdom of the choice as to the mere fact that it's a different one. One teen mom in a family is a drama; two teen moms has a been-there-done-that quality to it. "She purposely goes the other way," says East. "She decides her sister's role is teen mom and hers will be high achiever."

Younger sibs may avoid tobacco for much the same reason. Three years ago, Joseph Rodgers, a psychologist at the University of Oklahoma, published a study of more than 9,500 young smokers. He found that while older brothers and sisters often do introduce younger ones to the habit, the closer they are in age, the more likely the younger one is to resist. Apparently, their proximity in years has already made them too similar. One conspicuous way for a baby brother to set himself apart is to look at the older sibling's smoking habits and then do the opposite.

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