- Adolescents have just as many friends, whether they have siblings or not.

- The average size of the American family is shrinking, and only children are becoming more common.

- The findings should offer comfort to parents, no matter how many kids they decide to have.

Whether they're only children or one of five, teens and pre-teens make plenty of friends, new research concludes.

The new study should offer comfort to parents that their kids will grow up to be just fine, no matter how many they decide to have. It may be a growing concern: With women having kids later in life and pocketbooks tightening against the economic downturn, the number of families with only children has nearly doubled -- to about 20 percent -- since the 1960s, according to the National Center for Health Statistics.

"People are having smaller families and more children are growing up with fewer siblings," said Donna Bobbitt-Zeher, a sociologist at Ohio State University in Columbus. "What this study suggests is that there really isn't a need to worry for parents who have only children in terms of their social development."

The stereotype of a lonely, spoiled, bossy and maladjusted only child dates back to 1896, when an American psychologist named Granville Stanley Hall did a research paper on the subject. Despite major flaws in his study and fundamental changes to the structure of family life since then (like a shift from isolated farms to urban daycares for 3-month olds), the stereotype has generally stuck around -- even as families have been getting smaller.

To analyze how demographic shifts might be influencing the latest generation of kids, scientists have focused mostly on educational outcomes and test scores. On those measures, studies have shown no advantage for kids with siblings.

In fact, the more brothers and sisters a kid has, the worse he tends to do in school. And kids who are onlies have a slight advantage in their motivation to achieve, said social psychologist Susan Newman, author of "Parenting an Only Child: The Joys and Challenges of Raising Your One and Only."

More recently, researchers have been looking at how family size might affect social skills -- with some evidence that onlies are at a disadvantage, at least early on. A study of kindergarteners, published in 2004 in the Journal of Marriage and Family, found that teachers rated sibling-less children lower on a variety of social skills, including self-control and interpersonal skills.

Only children make just as many friends as those with siblings, research finds. iStockPhoto

To find out how that difference panned out as kids got older, Bobbitt-Zeher and colleague Douglas Downey (one of the authors of the kindergarten study) analyzed data from a long-term study of adolescents. The data set included lists made by each student when asked to name five male friends and five female friends.

The study, which involved nearly 13,500 kids in grades 7 through 12, found that only children were listed just as often on friendship lists as were kids with siblings. The number of siblings they had made no difference, Bobbitt-Zeher reported today at a meeting of the American Sociological Association. Neither did the gender of their siblings.

"It's a huge finding," Newman said, "and a positive one, because one of the concerns parents have when deciding how many children to have is, 'What's going to be the outcome of my child?'"

Plenty of other studies in plenty of countries have scrutinized each aspect of the only-child stereotype and failed to find evidence for any of it, Newman added. Other research, she added, suggests that parents of only children are actually happier than parents with more than one kid.

"I think the numbers are offering comfort," she said. "The reality is essentially that you don't have to live with a norm that doesn't work anymore -- the norm being a mom, a dad and two children."

Of course, having siblings can still be a positive experience, said Laura Padilla-Walker, a Brigham Young University researcher who studies sibling relationships. She recently found that having affectionate siblings helped kids, ages 10 to 14, feel less lonely and depressed and act more generously, especially if their siblings were sisters.

"Siblings provide the training grounds for essential skills that can be learned," Padilla-Walker said. "If parents only have one child, they will just have to work a little harder to give children those opportunities."