The second naturalistic experimental design that has been applied to humans is the siblings reared apart design. Drawing from the plant and animal literature, this approach can be characterized as a naturalistic cross-fostering design. In this paradigm, a newborn is removed from the biological parent home around the time of birth and is reared by genetically unrelated parents. In addition, the biological parent is parenting a biological child, who is the sibling to the child reared in the adoptive home. The unique feature of this design is that while genetic influences on a phenotype are shared among siblings (because they have the same biological mother and/or father), the rearing environment to which each sibling is exposed differs (See Figure 1). In non-human animal and plant studies, the cross-fostering approach is considered one of the gold standard research paradigms for studying the effects of the rearing environment and gene-environmental interplay (e.g., Meaney, 2001). This is because the cross-fostering design allows clear separation of environmental and genetic effects and, thus, examination of the interplay between the two on resulting behaviors is feasible. However, for obvious ethical reasons, the application of randomized cross-fostering designs to human populations is limited.
