The stereotype of the lonely, spoiled only child has been so thoroughly embedded in popular culture that contradicting it requires a certain willingness to be contrarian. The research, accumulated across decades of study in multiple countries, is fairly consistent: only children are not, on average, less happy, less socially skilled, or less well-adjusted than children with siblings. On some measures, they are doing slightly better.

The academic literature on only children is complicated by selection effects — families choose to have one child for a variety of reasons, some of which (economic constraint, late parenthood, parental health) are negatively correlated with outcomes, while others (high educational aspiration, intensive parenting investment) are positively correlated. Studies that control for these confounds tend to find that only-child status itself has minimal negative effect and modest positive effect on outcomes like educational achievement and verbal ability.

The happiness finding is the most interesting. Survey research in multiple countries finds that only children report slightly higher life satisfaction than children with siblings, an effect that appears to be mediated by closer parent-child relationships. Without siblings competing for parental attention and resources, only children receive more focused adult engagement during the formative years when such engagement appears to have lasting effects on emotional security and cognitive development.

The social skills concern has also been addressed by longitudinal research. Only children who have regular contact with peers — through school, neighborhood, extended family — develop social skills at rates indistinguishable from children with siblings. The sibling relationship provides social practice, but it is not the only source of such practice, and substitutes are readily available.

None of this means that having siblings is bad. It means the old stereotype was wrong, and parents who have one child by circumstance or choice can set aside the guilt that the culture has encouraged them to feel about it.