As mentioned in the preceding chapter, quantitative genetic theory recognizes that genetic and environmental influences change during development, and it proposes new concepts and methods for exploring developmental change as well as continuity. This is the core of a new subdiscipline, developmental behavioral genetics. When the field of behavioral genetics is surveyed from a developmental perspective, it is clear that the relative roles of genetic and environmental influences change during development (Plomin, 1986a). If this were not the case, there would be no need for the field of developmental behavioral genetics – the story in childhood would be just the same as that in adulthood.

The conclusion that the relative magnitudes of genetic and environmental influences change during development is founded primarily on cross-sectional comparisons across studies, for the obvious reason that most behavioral genetic studies are cross-sectional. Although the cross-sectional design can be illuminating, the lifeblood of developmental analysis of change and continuity is the longitudinal design (McCall, 1977; see also Chapter 5). The few longitudinal behavioral genetic studies, discussed below, add disproportionately to the weight of these conclusions because the same subjects are studied at different ages and, at each age, subjects are usually studied within a relatively narrow age band.