In addition to the descriptive and predictive changes discussed in the preceding chapter, developmental change can be seen in terms of etiology – changes in genetic as well as environmental influences. Changes in environmental influences can be explored without behavioral genetics; for example, the effects of prematurity on individual differences in social and mental development tend to diminish during infancy (Kopp, 1983). Behavioral genetics, however, provides a particularly powerful and general approach to the study of developmental changes in etiologies of individual differences.

From a behavioral genetics perspective, three kinds of etiological change can be considered; these mirror the phenotypic changes described in Chapter 5. The most basic phenotypic change that can occur is a change in variance, although changes in the magnitude of phenotypic variance are difficult to interpret because measures are not comparable across ages. The analogous genetic concept – change in heritability – is easier to interpret because it refers to a proportion of phenotypic variance due to genetic differences among individuals rather than to the absolute magnitude of variance.