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 greater risk for aggression compared to nonadoptees, since birth parents giving up their children may have increased rates of disordered behaviors, including substance use, criminal offending, and aggression (Cloninger et al., 1985; Lewis et al., 2001). In the Deater-Deckard and Plomin (1999) study, the adopted children did in fact have higher aggression scores compared to the nonadopted children, consistent with the notion that adoptees may be at higher genetic risk for aggression compared to nonadopted individuals. The elevated levels of aggression in adoptees occurred in spite of the fact that background characteristics of the adoptive families were found to be representative of families with children in the larger Denver area, and that the demographic characteristics of the adoptive grandparents and the adopted children's biological grandparents were similar, with regard to educational and occupational level. Similarly, van der Valk et al. (1998) reported mean differences between adoptees and nonadoptees, with adoptees showing higher mean levels in aggressive behavior. About 75% of
