Cross-fostering studies have been ethically impossible to implement in human populations. We introduce a novel approach to address this barrier: the siblings-reared-apart design. Supplementing the traditional adoption design with a sample of birth families who (a) are linked to adoptive families, and (b) raise their biological child(ren) (i.e., biological siblings of adoptees), this design provides opportunities to evaluate the role of specific rearing environments. Using data from 118 pairs of adoption-linked adoptive and birth families, we found that compared with families in the birth homes, (a) adoptive families had higher household incomes and maternal educational attainment; (b) adoptive mothers more often displayed 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.