In a third cross-fostering research paradigm used with humans, adoptees placed at birth are compared to their biological siblings who remained in the home of origin and who are thus reared by their biological parents. In this design, siblings who remain in the home of origin serve as a quasi-reference group that can provide information about potential outcomes had an individual not been adopted. Examples of this type of cross-fostering design include a Danish adoption study of obesity (Sorensen et al., 1989), a small sample of French children (n = 20 sibling pairs; Schiff et al., 1982), and a study of adult full- and half-sibling pairs compiled using multiple Swedish nationwide registries where at least one sibling was reared by one or more biological parents and the other by adoptive parents (Kendler et al, 2016). In the Kendler et al. (2016) study, results indicated that the adopted siblings had a substantially lower risk for drug abuse than the biologically-reared siblings, suggesting environmental influences on drug abuse.
