Infancy blossoms into childhood with the dramatic changes of the second and third years of life. These average changes from infancy to early childhood are so marked that one of the founders of developmental psychology, James Mark Baldwin (1894), suggested that, during the first year, infants possess only the properties of lower vertebrates; during the second year, they employ processes of higher vertebrates; not until the third year of life, however, do children begin to use cognitive processes characteristic of the human species. Although Baldwin's ontogeny-recapitulates-phylogeny interpretation of the changes from infancy to early childhood would find few adherents today, no one would deny that the average changes from infancy to early childhood are considerable. It is critical, however, to recognize that what we know about the transition from infancy to early childhood is limited primarily to average age differences rather than to individual differences.

This chapter discusses what is meant by developmental change in terms of individual differences rather than average age differences. Developmental change in terms of individual differences can be quite different from normative change because, as discussed in Chapter 2, the description and explanation of group differences are not necessarily related to individual differences. Indeed, it has been suggested that “there may be an inverse relationship between the suitability of a dimension as an expression of individual differences and its status as a dimension of major developmental change” (Wohlwill, 1973, p. 335).