There are several assumptions in both twin and adoption studies that are important to consider when reviewing their findings. In adoption studies, the most important factors are (1) random placement of the adoptees into homes and (2) generalizability. Selective placement or matching (i.e., similarities between adoptive and biological parents) for certain characteristics can lead to inflated correlations between adoptive siblings (and thus overestimated effects of shared environment). Although such matching may occur for physical characteristics (including race), direct selective placement is unlikely to be made for aggressive behavior, per se. (Children with more aggressive or antisocial biological parents would not be placed into homes with more aggressive adoptive parents.) Thus, it is unlikely that the genetic and environmental effects summarized in Table 8.1 are biased in any way as a result of selective placement. In terms of generalizability, it is often the case that adoptive parents tend to be in good health and from higher socioeconomic levels; thus, findings from adoption studies may not always be unquestionably generalized to the entire population (Rutter, 2006). Adopted children may also be at
