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.