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Genetic studies show that children's reading achievement is in part genetically influenced, and intervention studies show that reading achievement can be increased by environmental interventions. However, correlational and mean-level analytic strategies are rarely integrated into achievement research, potentially leading to misinterpretation of results. The parent-offspring adoption design offers a novel opportunity to examine the independent and joint roles of genetic and rearing environmental contributions. The sample included 344 adopted children in first grade and their biological and adoptive parents. Results indicated that adoptees' reading scores were correlated with their biological parents' scores, but not with their adoptive parents' scores, suggesting genetic influences. In addition, examination of mean scores indicated that adoptees' scores were significantly greater than their biological parents' (p's < 0.001) for all subtests, suggesting promotive effects of the rearing environment. This pattern was present even when biological parents scored >1 standard deviation below the biological parent mean on achievement.

A plethora of studies with parents and children who are biologically related has shown that the family environment plays an important role in child development. However, scientists have long known that a rigorous examination of environmental effects requires research designs that go beyond studies of genetically linked family members. Harnessing the principles of sibling comparison and animal cross-fostering designs, we introduce a novel approach: the siblings-reared-apart design. Supplementing the traditional adoption design of adopted child and adoptive parents with a sample of the adopted children's birth parents who raised their biological child(ren) at home (i.e., biological siblings of adoptees), this design provides opportunities to evaluate the role of specific rearing environments. In this proof of concept approach, we tested whether rearing environments differed between adoptive and birth families. Using data from 118 sets of adoption-linked families, each consisting of an adoptive family and the adoptee's birth family, both of whom are raising at least a child in each home, we found that compared with families in the birth homes, (a) adoptive families had higher household incomes and maternal educational attainment; (b) adoptive mothers displayed more guiding parenting, less harsh parenting, and less maternal depression; and (c) socioeconomic differences between the two homes did not account for the behavioral differences in mothers. We discuss the potential of the sibling-reared-apart design to advance developmental science.

The mechanisms explaining how parental depression compromises healthy child development are complex and multifaceted, with genetic and environmental pathways intertwined. Reexamination of whether and how maternal and paternal depression serve as environmental risk factors is important because such an investigation can be helpful to identify modifiable mechanisms that are accessible to interventions. We review studies that have employed designs that isolate the effects of the environment from genetic influences, including adoption studies and children of twins studies. Findings indicate that maternal depression is an environmental risk factor for the emotional, behavioral, and neurobiological development of children. Although more studies are needed, preliminary findings suggest that paternal depression appears to be a weaker environmental risk as compared to maternal depression, at least during infancy and toddlerhood. Implications for theory and future research are discussed.