They can grease a slide into bad behavior. Drinking. Smoking. Delinquency. Some research suggests that siblings' bad habits rub off. "If you have a sibling who is participating in those types of activities, then you're at higher risk for participating yourself," says Katherine Jewsbury Conger, an associate professor of human development and family studies at the University of California-Davis who has studied those effects. Patricia East, a research scientist at the University of California's San Diego School of Medicine, has found that girls were more than four times more likely to become pregnant as a teen if a sister had a baby as a teen, compared with girls whose sisters weren't teenage moms.
They may inspire us to be different from them. Accumulating evidence suggests that while some kids strive to be like their siblings, others do the opposite. She's the pretty one, I'll be the smart one.He's the jock, I'll be the scholar. Mark Feinberg, senior research associate at Penn State University's Prevention Research Center for the Promotion of Human Development, studies this "differentiation" process. He says it's how siblings try to carve out their own identity within a family so that each can be "special" in the eyes of parents. In one study, Feinberg found that siblings who were closer in age—say, a year apart—were more likely to differentiate than siblings separated by a bigger age gap, like four years. "Kids do this to minimize rivalry with one another," says Jeanine Vivona, a psychologist and an associate professor at the College of New Jersey. But there may be consequences: "You lose something, some potential you might have had," says Vivona, who has seen these feelings emerge in patients during therapy.
They may make us more jealous of romantic partners. Earlysibling jealousy may be a precursor to later romantic jealousy, says Amy Rauer, an assistant professor at Auburn University. Young adults who felt their siblings were favored by parents as kids had lower self-esteem and were more likely to report romantic relationship distress than people who felt they'd had a fair deal, Rauer reported in 2007. The former were more likely to be jealous of partners, suspicious of their loyalty, and wary of them interacting with others, she says. "What seemed to predict really good functioning in your relationship was feeling that you had been treated equally to your sibling," says Rauer.
Or they may give a boost to our love life. "Children who grow up with an opposite-sex sibling can be incredibly advantaged," says Susan McHale, a professor of human development at Penn State University, "because they have more direct access to the world of the other sex." In his new book, Strangers in a Strange Lab: How Personality Shapes Our Initial Encounters With Others, William Ickes, a distinguished professor of psychology at the University of Texas-Arlington, reviews some of the evidence from a study he did with college students. Those who were raised with older siblings of the opposite sex "hit it off better" with strangers of the opposite sex than did those raised with younger siblings of the opposite sex; conversation flowed more easily, and they were better liked. "If you've had any success with members of the opposite sex," Ickes says, "you owe some of the credit to your older brother or older sister."