Despite its impressive history, developmental psychology has not provided a clear answer to the question of why organisms grow and age. Some have tried to obviate the problem by declaring that time itself may serve explanatory functions. This means nothing else than to say, for instance, 4-yearolds have a certain height because they are 4 years old. Others have been satisfied by (commonly overappreciated) attempts to reduce psychological development to changes in nonpsychological conditions, particularly biological factors, and to substitute phenotypical by so-called genotypical descriptions. Such an interpretation is implied, for example, when we explain growth in size by changes in the endocrine system. For many purposes such explanations will be fruitful. But they merely delegate the problem to another area of study where the same question arises again and no satisfactory answer is provided as to why organisms grow and age.