But are they strange? Are they badly socialized? Do they whine all day? These are sensible questions, because only children do miss some of the rough and tumble that comes from having brothers and sisters. In the first place, though, most modern children spend large parts of every day in nursery school or day care, surrounded by other children -- they are hardly only children. And here again the studies speak quite clearly. Only children have as many friends, they're picked as quickly for teams, they score lower on indexes of anomie and resentment. They may even be better at sharing than other children, partly because they're used to the idea that their turn will come. When they grow up, they make as much money and they are slightly less likely to find themselves at the shrink.

As for self-esteem, the chief concern of any right-thinking modern parent, only children are smack in the middle of the pack. Surveys show that firstborns have the highest self-esteem, because they compare themselves with their smaller brothers or sisters. Last-borns score the lowest, because they're always the littlest, unable to do as much as their elders. Middle-borns and onlies score in between, but for different reasons: middle-borns because they're bigger than some siblings and smaller than others, and onlies because ''their self-esteem development process is unaffected by comparison with others.'' They are, in other words, who they are.

It's a thesis confirmed, albeit pretty much by accident, in Frank J. Sulloway's 1996 best-selling analysis of birth order, ''Born to Rebel.'' Sulloway was trying to show that older children, with a monopoly on their parents' attention, are more likely to be conservative, while younger children try to earn more parental affection by breaking the mold, becoming more inventive. His methodology has come under fire from some critics, but on only children his results couldn't be clearer. ''Singletons tend to be more variable,'' he writes. They are ''the least predictable subgroup in my family dynamics model, precisely because they had no siblings.'' Hence Leonardo da Vinci, Nancy Reagan, Elvis Presley, Jean-Paul Sartre and Brooke Shields.

Falbo stresses that the differences between onlies and children with siblings are slight: ''It's 1 or 2 percent on the scores. If you're sitting next to someone on an airplane, you can't tell if he's an only.'' And that's really the point, she says. ''I've done this research up one side and down the other. Believe me, my career would have taken off -- it would have been Nobel Prize stuff -- if I'd found that only kids were sick, sick, sick. But they're not. The conventional wisdom is wrong.''

It's interesting to guess why we're so attached to this stereotype. Partly, I think, it's because parenting is so much fun that there's an undeniable (and biological) urge to see what the next child will be like, and the next, and the next. And most of us had brothers or sisters, so questioning how we would have done without them may seem disloyal. And large families have traditionally been extolled, from the pulpit, in the pages of books like ''Cheaper by the Dozen'' and on screens large and small. If the Brady Bunch is the ur-program of the TV generation, it's no wonder many of us feel a little odd with just one child.

And there is no denying that life as an only child is different. Though psychologists caution that many sibling relations are damaging, many more are wonderful. Siblings are the only people you can share your life with, beginning to end. Though only children may be just as happy and satisfied in their youth, there's more chance for loneliness late in life. Parents need to balance the potential of those very real sadnesses against the realities of their own time and energy, and also against the larger realities of a crowded planet that strains to support even its present population.

As I lay in bed this morning, I could hear my 5-year-old daughter out in the living room, singing happily to herself as she worked a puzzle. A few hours later, we both suffered through a noisy timeout. Tonight we snuggled on the couch and read Roald Dahl. She's not an only child, damn it. She's a child.