CHICAGO - Shereen Peterson worries about her son.

There are the usual worries, sure. Sugar, sleep, tantrums, germs. But the big one - the nagging worry that has latched on to her psyche with all the strength and perseverance of a toddler - is his solitude.

Skylar, 3, is an only child. Shereen and her husband, Kyle, have gone to great lengths to give him a sibling: four rounds of artificial insemination, acupuncture, in vitro fertilization. No dice. Now, they've decided, Skylar will be an only.

"I worry he'll be spoiled," says Peterson, 37. "I want him to know the world doesn't revolve around him. I worry he'll be needy and clingy to Mommy, that when I send him off to school he won't want to go.

"I worry that when he's older, he'll feel lonely," she continues. "He won't have a companion in life - someone in the family he can share every feeling with. Even if you get in fights, there's unconditional family love with a sibling."

Statistically speaking, Peterson's fears aren't likely to come true. Study after study prove only children are no more spoiled, lonely or maladjusted than their peers who have siblings.

They're also, oddly enough, not alone.

The single-child family is the fastest-growing family unit in this country, according to recent Census figures. In 2004, 17.4 percent of women ages 40 to 44 reported having one child, compared with 9.6 percent of women 40 to 44 in 1976. Single-child families now outnumber two-child families (20 percent versus 18 percent), according to the 2003 Current Population Survey.

It's impossible to quantify the various factors fueling this trend, but experts point to a number of circumstances that aren't likely to diminish any time soon - if ever.

"Secondary infertility, women working, a high divorce rate, the expense and difficulty of adoption, the high cost of raising and educating children, terrible parental leave policies, child care that's not up to snuff, an increased number of single parents," lists Susan Newman, social psychologist and author of "Parenting an Only Child: The Joys and Challenges of Raising Your One and Only." "Parents want to do the best job of parenting, and with all these factors working against them, they're often stopping at one."

But for all their strength in numbers, only children (and their parents) still shoulder a hefty load of stigmas - many dating back to 1896, when psychologist G. Stanley Hall said "being an only child is a disease in itself."

"The stereotypes are still there," says Carolyn White, editor of Only Child magazine. "That they're unable to socialize well or have close friendships or be in relationships that are secure and bonded. That they don't think of others as well as themselves."

Never mind that 30 years of research, conducted mostly by social psychologist Toni Falbo, proves the opposite is true.

"In many respects, only children tend to be more well-adjusted," says White. "They learn to socialize very well because they know that if they don't, they're not going to have any pals. They really have to get out there."

Onlies are usually resourceful, independent, gregarious and extremely driven, White says, and they tend to outperform their peers with siblings on academic achievement tests.

"That extra attention from parents can have a very positive effect," she says.

The Petersons are in the midst of moving from their condo in the city to a house in Glen Ellyn, Ill., close to Shereen's parents and siblings.

"When he grows up we'll tell him, 'Mommy and Daddy tried everything we could to give you a sibling, and that's why we moved to Glen Ellyn so you could be surrounded by your cousins and my very fertile brother,"' Shereen says. "We're settling into a groove now. … I just have to think about what matters and everything we have instead of what Skylar's not getting."