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The genetic and environmental etiologies of sex-typed behavior were examined during the preschool years in a sample of 3,990 three- to four-year-old twin and nontwin sibling pairs. Results showed moderate genetic and significant shared environmental influence for boys and substantial genetic and moderate shared environmental influence for girls. For both boys and girls, twin-specific shared environmental effects contributed to twins' similarity in gender role behavior and accounted for approximately 22% of the shared environmental variance. These findings extend previous research conducted with older samples by showing not only important genetic contributions to gender role behavior but also an important role for shared environment. The inclusion of nontwin siblings showed that some of the shared environmental influence is specific to twins.