The interaction of the person and the time facet is of utmost relevance in human development and personality. Theories of human development explain normative change and differential change (Bakes, Reese, & Lipsitt, 1980). Normative development is defined as age differences that generalize across individuals. The time facet and the person facet do not interact. By contrast, differential change reflects interindividual differences in intraindividual change and thus an interaction of the time and the person facets. As a consequence, later individual differences cannot be well predicted, if at all, from earlier individual differences (Bloom, 1964).
Trait models neglect person x time interactions by assuming that individual differences remain relatively constant across age (Carr & Kingsbury, 1938). Although longitudinal studies have supported this notion in the domain of personality (Costa & McCrae, 1980), ability (Deary, Whalley, Lemmon, Crawford, & Starr, 2000), and attitude (Alwin, Cohen, & Newcomb, 1991), person x time interactions are relevant for two related reasons.
First, traits are not the only meaningful attributes for describing the personalities of individuals. Unstable personality differences, called states, are no less important than traits for understanding and predicting behavior (Nesselroade & Bartch, 1977; Steyer, Ferring, & Schmitt, 1992). The state-trait distinction is common in lay personality theory and represented in language (Chaplin, John, & Goldberg, 1988). It dates back to, at least, Cicero (Eysenck, 1983).
Secondly, and more directly related to the focus of this handbook, person X time interactions are important because assessment methods differ in their sensitivity to intraindividual change. Consequently, person X time x method interactions can be expected. Some assessment methods measure stable individual differences whereas others measure, in the same psychological domain, individual differences that change with time. Small changes in instructions may be sufficient for generating a person x time x method interaction. Asking individuals how they feel at the moment will more likely measure an emotion state than an emotion trait, whereas asking individuals how they feel in general will more likely measure an emotion trait than an emotion state (Eid, Notz, Steyer, & Schwenkmezger, 1993).
These findings again demonstrate the crucial role of theory in the construction of methods. If intraindividual changes in a psychological phenomenon like emotion can be expected on the basis of theory, methods for assessing this phenomenon must be sensitive to intraindividual change. Using a measure for enduring individual differences instead results in an underestimation of change. Exploring person x time x method interactions is therefore crucial for both the advancement of theory and the improvement of methods.
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