Generally, those studies showed that singletons aren’t measurably different from other kids — except that they, along with firstborns and people who have only one sibling, score higher in measures of intelligence and achievement. No one, Falbo says, has published research that can demonstrate any truth behind the stereotype of the only child as lonely, selfish and maladjusted. (She has spoken those three words so many times in the past 35 years that they run together as one: lonelyselfishmaladjusted.) Falbo and Polit later completed a second quantitative review of more than 200 personality studies. By and large, they found that the personalities of only children were indistinguishable from their peers with siblings.

“For most people, this still hasn’t sunk in,” she tells me after a meeting of her graduate seminar in social psychology…