How do only children differ from other children?

Developmental outcomes of only and non-only children, categorized by birth order and by family size, were investigated. Multiple hypotheses based on meta-analyses of the only-child literature (Falbo & Polit, 1986) were tested with independent comparison techniques. Results indicated that developmental outcomes of only children were similar to outcomes for firstborns and children from two-child families but dissimilar to outcomes for later borns and children from larger families. Furthermore, outcomes were more positive for only children, firstborns, and children from two-child families than for all other comparison groups. In agreement with Falbo and Polit, only-child deprivation and only-child uniqueness explanations for outcome differences were not supported in favor of an explanation that emphasizes qualities of the parent-child relationship. Results suggest that future comparisons to only-child outcomes should preserve the independence of contrast results and expand the only-child category to include data from firstborns and others from two-child families.