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The Sibling Factor The Number And Gender Of Siblings Can Make A Major Difference In Personality Development

One day my newborn daughter and I were at the park, and a neighbor approached to congratulate me. “Now you have a rich man’s family,” he said, smiling.

I had no idea what he meant.

“It’s an old saying,” he explained. “You know, one of each.” He gestured toward the baby and my son, then 5 years old. “Now that you have a boy and a girl, you have it all.”

He may be right. “Having a boy and a girl is considered the ideal American family, according to the research,” says Marcia Summers, Ph.D., associate professor of educational psychology at Ball State University in Indianapolis. “Most people want a child of their own gender - mothers usually want a daughter, and most fathers want a son. When you get at least one of each, everybody’s happy.”

Parents aren’t the only ones who benefit from having a mixed-gender family. The children benefit, too. Most notably, they grow up feeling quite at ease with the opposite sex. “They have a more intuitive understanding of the other sex than kids who grow up in same-sex families,” says Vivian Friedman, associate professor of psychiatry at the University of Alabama. It’s already clear to me that my daughter has a certain ease around boys, thanks to her brother. She doesn’t hesitate to play with them, which contrasts sharply with the behavior of my friends’ daughter, Megan, who has no brothers. Megan tends to respond to boys the same way she treats dogs - as if they’re unfamiliar and maybe a bit scary.

At least one study shows that kids growing up with siblings of the opposite sex are more flexible about their gender roles - they see more options for themselves, even at an early age. In a study of the behaviors of 4- to 9-years-olds in their homes, same-sex siblings were highly traditional in their choices of toys and games. Boys with brothers played with trucks, girls with sisters played with dolls. But among opposite-sex siblings, the choices of activities were largely determined by the sex of the older child. Boys with older sisters played house as much as pairs of sisters did, while boys with older brothers never did.

“For girls, having a brother seems to broaden the definition of what’s acceptable for a girl to do,” says Friedman. “A girl raised with only sisters may be more likely to do only feminine things, whereas a girl who has a brother probably does girl things and boy things, too. It broadens her range of activities.”

There’s also likely to be less sibling rivalry between opposite-sex kids, at least in two-child families, according to Sylvia Rimm, clinical professor of psychiatry at Case Western Reserve University School of Medicine in Cleveland, and author of “Why Bright Kids Get Poor Grades” (Crown). That doesn’t mean brothers and sisters won’t vie for your attention at dinner or battle for first dibs on the new computer game. But rivalry tends to be more benign between opposite-sex siblings because each one has staked out exclusive territory as the only boy or only girl in the family.

Things get more complicated as families get larger. In fact, there’s little conclusive research on the relationship between boys and girls in big families, since birth order and spacing begin to play a larger role in family dynamics as families grow. Says Ball State’s Marcia Summers, “If you have two girls and then have a boy, the boy is more likely to be spoiled than if he’d been born first or in the middle. If you have two boys and then have a girl, the girl tends to become the princess of the family.”

At least one intriguing study has shown that there are substantial benefits to being the only girl in a mixed-gender family. Economists Kristin Butcher of Virginia Polytechnic Institute and State University and Anne Case of Princeton University discovered that the only girl in a family of one or more boys had several advantages over girls in other families. On average, such girls got more schooling and ultimately made more money than women who grew up with one or more sisters (even when there were also boys in the family).

Butcher and Case analyzed data from three different longitudinal studies, involving roughly 45,000 women. In addition to getting more schooling, women raised as the only girl in a family of one or more boys also tended to take more math and science courses in high school than women raised with sisters.

No one knows exactly how to explain this phenomenon. Case theorizes that an only girl may be “lumped together with the boys,” and given similar opportunities with her brothers. It’s also possible that girls with brothers must learn to compete and make herself heard. But once another daughter is born, the sisters may end up in a different category in their parents’ eyes, defined more by gender. “It’s the addition of a second daughter that seems to have a negative effect on education,” says Case.

As rewarding as it is to raise a boy and a girl, all is not always rosy. Children with opposite-sex siblings may sometimes feel shortchanged because there’s no one to share their gender-related interests. Adam, who has not yet mastered the art of diplomacy, sometimes complains that he doesn’t have a brother to play with like his friend Joe. All he has, he insists, is just a “creepy little sister.” My daughter, meanwhile, has asked me on several occasions to have another baby. “And this time,” she says, “make it a girl.”

There also seems to be a lot more teasing in these relationships and it’s usually the boys who do the teasing, often mercilessly for hours on end. That’s certainly the case in my household, where my daughter’s anguished shrieks are invariably followed by her brother’s wicked, triumphant laughter.

But boys don’t always end up the winners in mixed-gender families. Some research shows that boys with sisters are often underachievers. That’s because they may do poorly in school as a way to distinguish themselves from their sisters, who are likely to be teacher-pleasers, at least in the elementary-school years. “Boys often differentiate themselves from their ‘perfect’ little sisters by doing poorly in school. Very rarely will a girl be an underachiever in grade school. But for the boy, underachievement can become a pattern,” says Rimm.

In the end, though, what mothers and fathers often find most fascinating is seeing how extraordinarily different boys and girls can be.

“When my daughter was a baby, I thought, this really isn’t any different from having boys,” recalls Stephanie Williams, mother of two boys ages 12 and 10, and a 7-year-old daughter. But by the time Jana reached 18 months, Williams knew her daughter was different. “It’s everything,” she says. “The way she walks, the way she eats, the way she gestures. It’s all so feminine. Unlike my sons, who are rambunctious, she’s quiet and calm. When my boys have birthday parties, I’m pulling my hair out because they’re so rowdy and loud. My daughter’s parties are the complete opposite. They’re always so quiet. I’m amazed at the difference between boys and girls.”