Although, when studying any phenomenon, it is an important step to describe age-graded patterns of change and stability, age by itself (that is, time since birth) cannot explain why these patterns occur. Age (and other measures of time) can provide a metric along which change and stability can be plotted, but they are only considered to be markers for the temporally-graded causal factors that give rise to development patterns in the target phenomenon. Hence, explanation differs from description in that it refers to an account of the causes that together are necessary and sufficient to produce the patterns of changes and stability that have been described or observed. If descriptions answer questions like “what?” (i.e., the nature of the target phenomena), “how?” (i.e., the ways in which phenomena can change or remain the same), and “when?” (i.e., the ways in which these patterns appear as a function of age or time), then explanations focus on the “why?” questions: What sets of factors cause, influence, or produce these different patterns of change or stability over time? Explanations of normative development focus on the causes that underlie typical patterns of change and stability. In the example of motivation, explanatory theories and research would focus on the causes of the steady declines in students’ academic engagement and motivation, and why sharper declines are typically evident during school transitions. They would also focus on the causes that maintain engagement or compensate for losses in motivation, and so produce patterns of normative stability. Causal processes can remain the same over development, resulting in what can be called “explanatory continuity,” or different causal processes may be involved in explaining similar phenomena at different ages, resulting in “explanatory discontinuity.”
Explanations are completely different from descriptions. Researchers can compile the most elaborate description of the development of a phenomena, and not have discovered anything about the causes that underlie it. In the motivational area, general consensus exists about normative and differential changes in academic engagement over the school careers of children and youth, but a great deal of lively debate persists about the causes of these developments—neurophysiological, psychological, social, and contextual factors have all been nominated. Of course, description and explanation are linked—the search for explanations are guided by signposts originating in the patterns of development that have been described-- but even when normative descriptions have been ascertained for decades (e.g., the sequence of locomotion from creeping to crawling to walking described by Gesell in the 1930s; Gesell & Ames, 1940), it often takes many more decades for causal accounts to be well-established and accepted (Thelen & Adolph, 1992; Thelen, Ulrich, & Wolff, 1991).