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FAR SUBTLER--AND OFTEN FAR SWEETER--than the risk-taking modeling that occurs among all sibs is the gender modeling that plays out between opposite-sex ones. Brothers and sisters can be fierce de-identifiers. In a study of adolescent boys and girls in central Pennsylvania, the boys unsurprisingly scored higher in such traits as independence and competitiveness while girls did better in empathic characteristics like sensitivity and helpfulness. What was less expected is that when kids grow up with an opposite-sex sibling, such exposure doesn't temper gender-linked traits but accentuates them. Both boys and girls hew closer still to gender stereotype and even seek friends who conform to those norms. "It's known as niche picking," says Kimberly Updegraff, a professor of family and human development at Arizona State University and the person who conducted the study. "By having a sibling who is one way, you strive to be different."
But as kids get older, that distance from the other gender must, of necessity, close. Here kids with opposite-sex siblings have a marked advantage. Last year William Ickes, a psychologist at the University of Texas at Arlington, published a study in which he paired up male and female students--all of whom had grown up with an opposite-sex sibling--and set them to chatting with one another. Then he questioned the subjects about how the conversation went. In general, boys with older sisters or girls with older brothers were less fumbling at getting things going and kept the exchange flowing much more naturally.
"The guys who had older sisters had more involving interactions and were liked significantly more by their new female acquaintances," says Ickes. "Women with older brothers were more likely to strike up a conversation with the male stranger and to smile at him more than he smiled at her."
If siblings can indeed be as powerful an influence on one another as all the research suggests, are all siblings created at least potentially equal? What about half-sibs and stepsibs? Do they reap--and confer--the same benefits? Research findings are a bit scattered on this, if only because shared or reconstituted families can be so complicated. A dysfunctional home in which parents and siblings hunker behind barricades alongside the ones they're biologically closest to does not lend itself to good sibling ties. Well-blended families, on the other hand, may produce step- or half-siblings who are extraordinarily close. One of the best studies on this topic is being conducted in Britain with a large group of many different kinds of nontraditional families. In general, the researchers have found that the intensity of the relationships closely follows the degree of physical relatedness. No hard rules have emerged, but the more genes you share, the more deeply invested you tend to grow. "Biological siblings just get into it more," says Thomas O'Connor, an associate professor of psychiatry at the University of Rochester Medical Center. "They are warmer and also more conflicted."
• How those early bonds can grow stronger with age