Behavioral genetic theory, methods, and research provide a unique perspective on nature and nurture during infancy and early childhood, that is, on the genetic and environmental origins of individual differences in behavioral development. The words “nature” and “nurture” each have warm associations until they are brought together. One of our goals is to emphasize the conjunction “and” rather than the projective test provided by the dash in “nature–nurture” or the explicit hostility in the phrase “nature versus nurture.” We believe that the perspective of behavioral genetics is as useful for understanding environmental influences in development as it is for exploring the role of heredity, and we hope that this book will convince developmentalists of the importance of both genetic and experiential factors in the origins of behavioral differences during infancy and early childhood. At the simplest level, the components-of-variance approach – which we explore in terms of simple correlations as well as by means of model-fitting analyses – often indicates that genetic variance is significant and invariably shows that nongenetic factors are important.

The decomposition of phenotypic variance into genetic and environmental components of variance is the standard fare of behavioral genetic research. Somewhat newer is an emphasis on the decomposition of the environmental component of variance into two components, one shared by family members, which increases their phenotypic resemblance, and the other not shared; correlations for genetically unrelated children reared together in the same adoptive homes are especially powerful for detecting the “bottom line” influence of growing up in the same family.