Identical (MZ) twins, who spend much time with each other and are very much alike, may experience less diversity in their playmates than do fraternal (DZ) twins, who may experience less diversity than do other-age siblings. Therefore, developmental social genetics theory might predict that MZ twins will be more socially withdrawn than DZ twins, who will be more withdrawn than non-twin siblings. Conversely, MZ twins might be either more aggressive because of less experience with others or less aggressive because they are more inhibited. These hypotheses were assessed using the Child Behavior Checklist (CBCL) scales of withdrawn, aggressive, internalizing, and externalizing. Parents of 5-year-old twins and 5-year-old non-twin sibling pairs completed a packet of questionnaires, among them the CBCL. Thirty-two MZ twins, 70 DZ twins, and 54 siblings were rated on the CBCL. A MANOVA was performed on withdrawn, aggressive, internalizing, and externalizing scores, with zygosity group (MZ, DZ, or siblings) as the independent factor. There was a significant group effect for internalizing (F(2,154) = 5.06, p < .01). Siblings' internalizing score was significantly higher than MZ scores. This does not support the developmental social genetics theory that experience with dissimilar peers may be important for reducing withdrawn behaviors for preschool children.