The goal of this book is to explore the origins of individual differences in behavioral development during infancy and early childhood. A key phrase is “individual differences.” When developmentalists look at infancy and early childhood, they are usually absorbed by the dramatic changes that members of our species undergo during this fast-moving period of development. For example, Jean Piaget, the most influential figure in developmental psychology since the 1960s, described cognitive changes in terms of the transition from the sensorimotor actions of infancy to the representational abilities of early childhood, seen most clearly in the blossoming of language. However, Piaget was concerned only with average developmental trends, not with differences among children.

In contrast, when we look at children, we see children, not the child. That is, our interest centers on the development of individual differences among children rather than universal or normative (average) aspects of our species' development. A powerful theory of development must be able to explain individual differences, if for no other reason than that such differences exist – individual differences represent a major part of the phenomenon to be explained. There are, however, other reasons for studying individual differences: Descriptions and explanations of normative aspects of development bear no necessary relationship to those of individual differences; questions concerning the origins of individual differences are more easily answered than questions concerning the etiology of normative aspects of development; and the developmental issues of greatest relevance to society are issues of individual differences.