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. (PsycINFO Database Record (c) 2019 APA, all rights reserved).