A term given a restricted meaning in developmental psychology by Joachim Wohlwill (1928-1987) in his book The study of behavioral development (1973). It is intended to refer to those behaviors most suitable for studying intra-individual differences in development, the key concern of developmental psychology. Two conditions have to obtain: 1. the behavior must be robust enough that age-related changes can be expected to reveal themselves, despite variations in environments and errors of sampling and measurement, and 2. these changes must remain invariant over contrasts in specific experiential conditions so that their expected form (i.e., developmental function) under ‘normal’ conditions of development can be determined. Thus, behaviors requiring highly specific training (e.g., factual knowledge) for developmental change to occur are, in Wohlwill’s view, unsuitable for developmental study. Others like emotionality and aggressiveness are better considered as dimensions of individual differences that are independent of development. Behaviors meeting the requirements include motor abilities, visual-motor coordination, spatial ability, attention, memory and language.
See Developmental function, Intra-individual differences, Longitudinal studies