Age and sex effects in multiple dimensions of self-concept were examined in responses by 3,562 preadolescent students (grades 2 to 6) to the Self Description Questionnaire (SDQ). A factor analysis of responses clearly identified the seven facets of self-concept which the SDQ is designed to measure: physical ability, physical appearance, peer relations, parent relations, reading, math, and general-school. Significant age and/or sex effects were found in each of the SDQ scales, but the size and direction of the effects varied with the scale. The largest sex effects were for physical abilities (favoring boys) and reading (favoring girls), but the sex difference was small for the sum of all scales. For every scale there was a linear decline in self-concept with increasing age. This decline is explained by cognitive development. Very young children have unrealistically high self-concepts due to their egocentrism. Self-concepts decline or become more realistic as children incorporate more external information. (Author/BS)