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In the wake of the Boston bombings, many of us have been thinking a lot about siblings, particularly how older siblings can shape the lives of younger ones.

NPR's Alix Spiegel decided to take a look at some of the research and filed this report.

ALIX SPIEGEL, BYLINE: Patricia East is a developmental psychologist who began her career working at an OB/GYN clinic in California. Thursday mornings at the clinic were reserved for pregnant teens. When East arrived, the waiting room was packed with them. And it was in this space, she says, that she discovered her life's work: an accidental discovery that came from the casual conversations staff had with their younger patients.

PATRICIA EAST: The nurses and the doctors there, you know, would bring a teen back for her prenatal visit and they would say, hey, aren't you Maria's younger sister? And the young woman would say, yeah, I am. And they would say to another patient, you know, haven't I seen you before? And she would say, yes, I was here for my older sister when she was pregnant.

SPIEGEL: Over and over, East heard variations of this conversation.

EAST: Aren't you Rebecca's sister? Aren't you Anna's younger sister?

SPIEGEL: Until it came to the point that when she saw a younger sibling sitting next to her sister in the waiting room, an involuntary thought would flash across her mind.

EAST: It's almost like you're watching the younger sister just get pregnant.

SPIEGEL: When her older sister gets pregnant?

EAST: Right.

SPIEGEL: And so, East decided to do a study to figure out if having an older sister who got pregnant really did affect the likelihood that the younger sister would find herself in the same position. She compared a large number of sister pairs, all with the same socioeconomic and life circumstances, and found that the behavior of the older sibling did often change the trajectory of the younger.

EAST: The younger sisters are five times more likely to get pregnant, as other young women who have an older sister who hasn't been pregnant.

SPIEGEL: Five times. Now, until pretty recently, the role that siblings play in determining the trajectory of each others lives hasn't been a particularly hot topic in psychological research. Psychologists, very, very understandably, have focused on the influences that they see as more important - parents and peers and genetics. But in the last decade or so, that's been changing. Psychologists interested in how siblings affect one another are taking a new look at all kinds of behavior, including really troubling anti-social behaviors.

RICHARD RENDE: Damaging property, breaking and entering, stealing, getting into physical fights that are serious.

SPIEGEL: This is Brown University professor Richard Rende listing off some of the anti-social behaviors this research has covered, some of which really challenges the idea that parents are the most important thing. Consider the research which compares how much a parent who smokes influences the younger sibling, to how much an older sibling who smokes influences a younger sibling.

RENDE: Both can have an effect. But in a lot of studies, they found that the effect older sibling smoking has is greater than the effect parental smoking has.

SPIEGEL: Older siblings seem more influential. Now, Rende says you can see older siblings' influence in all kinds of families, whether they're rich or they're poor. But their power is especially magnified in the subset of families he studies: families that are psychologically unstable. There, the power of an older sibling is much greater because parents are often not around, and so siblings are spending a lot of time together.

As part of his research, Rende gives sibling pairs electronic devices, like cell phones, that every half-hour prompts them to say what they're doing. And he says you can literally see their behaviors ghosting each other.

RENDE: When one sibling is smoking in real time, they're reporting that they're having a cigarette, the other sibling is very likely to also report smoking at the same time.

SPIEGEL: In fact, when one smokes, the other is 25 percent more likely to smoke at that moment. With drinking, they're 36 percent more likely to drink. And Rende believes that the reverse of this is also true. Good behaviors in older siblings can be as contagious as bad. It just seems that particularly when families are struggling, the fate of the kids can be more tethered to each other than we originally thought. For good, Rende says, and also unfortunately for bad.

Alix Spiegel, NPR News, Washington.

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