Arecent large-scale study of Finnish adolescent twins (Pulkkinen et al., 2003) reported that individuals from opposite-sex twin pairs were more socially adaptive than individuals from same-sex pairs or singletons. This finding raised questions about the social learning effects of being an opposite-sex twin. The current article predicted on the basis of this finding, and evidence from singleton populations, that having an opposite-sex twin would yield social advantage. It sought to examine the social competencies of opposite-sex twins and compare them with same-sex twins and singletons. The study focused on the preschool years (age 3 to 6), a period in which the majority of children encounter their first large group, nonfamilial social experiences. The study obtained reports from parents and teachers of children aged 3 to 6 years: 72 children (36 pairs) who were dizygotic opposite-sex twins (DZOS), 50 children (25 pairs) who were dizygotic same-sex twins (DZSS), and 85 singletons of the same age and sex as the twins, who had at least one sibling. Reports were made using standardized measures of social competencies, behavior problems, language development and friendships. The main effects found were of differences in social competency between twins and singletons. Twins had lower social competency scores. No differences between same-sex and opposite-sex twins were found. The findings did not support the hypothesis of social advantage for opposite-sex twins in early childhood.