The developmental course of pleasure and joy is revealed by decades of solid research on infant smiling, which focused on its occurrence, its progressive changes in form, and its evolving meaning. Perhaps more than is the case for any other problem area, research focused on the nature and significance of infant smiling and laughter has revealed fundamental developmental processes. These processes are central to understanding normative affective development, individual emotional life, and the ties between the two. One cannot, for example, understand that at the core of smiling is arousal regulation and miss the importance of this process for studying individual differences.

There are no doubt several reasons for the important contribution made by research on smiling. First, much of the research carried out in the 1960s and 1970s was faithfully descriptive, a necessary cornerstone for any body of meaningful developmental research. Second, studies were addressed to ages across the span of infancy, often employing a longitudinal format.