Nonetheless, a few research groups have creatively applied the concepts underlying the cross-fostering design to naturally occurring phenomena in humans, using natural experimental designs (Rutter, Pickles, Murray, & Eaves, 2001). The first type of natural cross-fostering approach is the twins reared apart design. The Minnesota Study of Twins Reared Apart (Bouchard, Lykken, McGue, Segal, & Tellegan, 1990) and the Swedish Adoption/Twin Study of Aging (Pedersen et al., 1991) are the two best-known studies in this category. In both studies, each twin was reared in a separate adoptive home, and interviewed as adults. The major strength of the twins reared apart design is that it controls genetic similarities across twin pairs, especially in the case of monozygotyic (MZ) twins. This design is powerful because the MZ twins share genes but not environments, and it can thus be assumed that differences in the behaviors of the MZ twin pairs are likely due to environmental influences. A second type of human study extends the twins reared apart design by incorporating non-twin siblings who were placed in separate adoptive homes. One existing study examined
