Evaluates theory and methodology underlying previous tests of the developmental proposal that self-concept becomes increasingly multidimensional with age. Develops and provides strong support for a new differential distinctiveness hypothesis, demonstrating that increasing distinctiveness--substantial age-related declines in correlations among latent factors--varied systematically depending on the extent of association between first-order factors in hierarchical models. (Contains 61 references, 4 tables, 2 figures, and 2 appendixes.) (GCP)