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Life-course studies of human development suggest that birth cohorts become more internally differentiated with respect to personality characteristics as their members age. This article examines possible explanations for this macro-level phenomenon in terms of three micro-level research traditions: (1) individual differentiation, (2) social structure/allocation, and (3) behavioral genetics. We conclude that each tradition can marshal credible empirical support, and that, therefore, all three are needed for a full understanding of older-age heterogeneity processes. A model is developed, synthesizing all three approaches. Study designs in any one of these traditions that fail to take account of the others are likely to draw misleading or erroneous conclusions concerning sources of variation and covariation. Several types of study designs are suggested which would avoid such problems.

Aging and Heterogeneity: Genetics, Social Structure, and Personality