The word dmelopmcnt means change and developmental psychology is the study of behavioral changes within individuals at different ages (Wohlwill, 1973). One would expect that developmental psychologists spend much of their time charting behavioral growth as a function of age, describing major changes in behavioral dispositions over the life span, and identifying factors that influence quantitative and qualitative changes over developmental time periods in the behavior of species or individuals. But that is not what developmental psychologists typically do (McCall, 1977). We usually do not conduct longitudinal studies and even when we do, most of them are very short-term, consist of assessments at only two or three ages, and are not aimed at discovering changes but at detecting consistencies across age. For example, if you give many developmental psychologists a set of longitudinal data, they will immediately calculate cross-age correlations that reveal consistencies in individual differences across age, not change. Indeed, failure to find cross-age correlations is typically a disappointment to researchers, and the fact that major concepts (for example, intelligence and aggressiveness) cannot be defined or measured identically from one age to the next is a major methodological problem for most developmentalists.