It is time to reconceptualize nature and nurture in a way that emphasizes their inseparability and complementarity, not their distinctiveness: it is not nature versus nurture, it is rather nature through nurture. If gene expression is inconceivable apart from the environment, then it is useless and potentially misleading to try to finely distinguish the relative importance of nature and nurture in the course of human development. Nature is inseparable from nurture, and the two should be understood in tandem. Moreover, by contrast with a traditional view that heredity imposes limitations and environments induce change in developmental pathways, research in developmental psychobiology shows that the coactivity of nature and nurture accounts for both stability and malleability in growth. This view is, indeed, one important way of integrating the science of early childhood development, and it is also reflected in recent scientific advances in some of the research fields that are currently generating greatest interest among developmental scientists: developmental behavioral genetics, molecular genetics, and brain development.
In animal species, the importance of genetic influences on behavior can often be studied directly through selective breeding research. In humans, less intrusive procedures are necessary, and for the past several decades developmental behavioral genetics has provided a powerful means of understanding the strength of heritable influences on individual differences in human development, and the environmental contexts in which they are expressed (see Lemery and Goldsmith, 1999; Plomin et al., 1997a; and Rutter et al., 1999a, for overviews of this field). By taking advantage of naturally occurring variation in genotypes and environments, behavioral geneticists seek to partition behavioral variability into its genetic and environmental components and describe their interaction.
They have two primary research strategies for doing so. In adoption research, genetic contributions are estimated by comparing the characteristics of an adoptive child with those of the birth mother (to whom the child is genetically related, but they do not share an environment) and the adoptive mother (who shares the child's environment, but not genes). Sometimes biologically related and unrelated siblings are also studied. The second approach is twin research. Because identical (monozygotic) twins are genetically identical, comparing the similarity of their characteristics with those of fraternal (dyzygotic) twins, who on average share half their genes, is another way of estimating genetic contributions.
Twin and adoption research designs each have assumptions or limitations that can make the interpretation of findings difficult and sometimes controversial. In adoption research, for example, prenatal influences (e.g.,