In this chapter we step back from the numerous analyses described in previous chapters in order to gain some perspective on what we know and what we need to learn about the origins of individual differences during infancy and early childhood. As presaged in Chapter 1, the ratio of what is known to what is not known is small; however, the ratio is more impressive when we consider how few studies have addressed the issue of the origins of individual differences in infancy and childhood.

Principles

Rather than summarizing the preceding chapters, we have attempted to abstract some principles that outline what is known about nature and nurture in infancy and early childhood. In our book on infancy, several principles were drawn from the infancy results of the Colorado Adoption Project (Plomin & DeFries, 1985a). These principles involve some general points, such as the following. The etiology of individual differences in infancy includes heredity, variations in family environment are related to individual differences in infancy, and the relative extent of genetic and environmental influence varies for different characters. We have no doubt that these general principles hold for early childhood as well as for infancy.

One other general principle should be added to our earlier list: Individual differences among children are substantial and reliable.