ABSTRACT
The search for between-individual differences that, by definition, show stability over time and situations characterizes the study of personality and temperament. Indeed, without such stability, knowledge of a person's characteristics in one situation or at one age would be of little use in predicting their characteristics in another situation or at a later age. Stability in this context is the stability of the individual's rank within the group, and though everyone could change in concert on the characteristic in question, stability of the ranking within the group could remain stable. Age-related change, though generally ignored by the personologist, is of central concern to most developmentalists. In short, personologists and developmentalists tend to study opposite sides of the same coin. Such a division of labor can be scientifically productive, but an understanding of the changing structure of temperament and personality from infancy to adulthood will benefit from a knowledge of developmental changes. This lesson can be seen in a core dimension of individual difference in infancy, activity level (AL).
