teratogenic exposure) can also account for the resemblance of biological mothers to their offspring, and this can inflate estimates of genetic contributions. In addition, adoption designs assume that the selective placement by adoption agencies of children into the homes of parents who are like them (or their biological parents) does not occur. It is possible to estimate the potential biases introduced by selective placement or prenatal influences, but this is very difficult in most research designs. Twin studies also have certain assumptions: that identical twins do not share a more similar environment than do fraternal twins, and that the development of twin pairs is fairly representative of the growth of children in general. These assumptions, too, have been tested, with some researchers concluding that these assumptions are valid and others disagreeing.
Adoption and twin studies each provide means of estimating quantitatively the proportion of variance in human characteristics that is attributable to heredity and to the environment, and of examining how these influences interact in development. During the past decade, developmental behavioral genetics research has expanded considerably in sophistication and analytic methods, using variations on the basic adoption and twin research designs (sometimes combining these methods) and employing structural equations modeling and other quantitative model-fitting methods for estimating genetic and environmental contributions to behavioral variability. These efforts have yielded important new insights into the heritability of individual differences in cognitive abilities, extraversion, emotionality, self-control, and other characteristics and have shown how inherited propensities to childhood disorders like autism, schizophrenia, attention deficit hyperactivity disorder, and antisocial behavior need to be considered by practitioners (see reviews by Plomin et al., 1997b and Rutter et al., 1999b).
Even more important is how this research contributes to an appreciation of how nature and nurture influence development in concert. A recent study of the development of antisocial behavior in children by Ge, Conger, Cadoret, Neiderhiser, Yates, Troughton, and Stewart (1996) is exemplary. Using an adoption design, these researchers found that when biological parents had substance abuse problems or antisocial personality disorder, their adopted children were much more likely to be hostile and antisocial than were adoptees from untroubled biological parents. Children's inherited antisocial tendency may have been manifested as difficult temperaments, problems with emotional self-control, impulsivity, or other difficulties. It was not surprising, therefore, that children's antisocial tendency was also associated with greater harshness and less nurturance and involvement by their adoptive mothers and fathers. This illustrates how children's inherited characteristics can evoke complementary responses from their parents (called “gene-environment correlation”).
Parents and children in these adoptive families influenced each other.