David Magnusson is acknowledged to be the titular head of a small group of psychologists who are performing this intellectual service for those who study the complex problem of variation in human development. In lucid, gracefully written papers, chapters, and books, Magnusson has articulated some of the imperfect assumptions that guide much research in personality development. One of these flawed ideas is the belief that individuals should be compared on a single, continuous dimension and that persons differ from each other only quantitatively, not qualitatively. A corollary of this traditional assumption is that the relations among a set of psychological variables should be similar across individuals of different gender, class, and temperament and the magnitude of a correlation between two variables, for example, social anxiety and school achievement, is a sound basis for inferring how those variables function within every individual. This is rarely the case.