The influences of personal, socioeconomic, and sibling characteristics on the sex-role differentiation of preschool children were investigated. Subjects included 446 lower- and middle-income boys and girls ranging in age from 3 to 5 years. The Peabody Picture Vocabulary Test was administered to each child to obtain a measure of IQ. Information regarding the children's other personal characteristics (age and sex), socioeconomic characteristics (social class, ethnicity, intactness of family, maternal and paternal employment, occupational status of mother's and father's jobs), and sibling characteristics (sibling sex, sibling constellation, total number of children, ordinal position) was obtained from school records and school staff members. A battery of sex role measures (Draw-A-Person, Nadleman Recall, Toy Preference, Occupational Preference, and Brown's IT tests) was administered to each child individually. Results showed that sex-role differentiation was already present in the 3-year-olds and that differences between the sexes increased with age. Findings also showed that sex-role differentiation was unrelated to IQ or to maternal employment variables. The hypothesis that socioeconomic factors are important determinants of sex-role differentiation was not supported. Data indicated that sex-role differentiation was most strongly affected by siblings, who had different effects depending upon their own sex, as well as by the sex and/or age of the child. Ordinal position and number of children in the family also influenced sex-role differentiation. (Author/JMB)