“…Among noninstitutionalized domestic adoptees, no difference was found in relationship quality between adoptees and their non-adopted siblings and two biological non-adopted siblings (McGue et al, 2007). Behaviorally, PI children adopted as sibling groups have higher ages at adoption but have lower or similar rates of behavior problems compared to single adoptees (Boer, Versluis-den Bieman, & Verhulst, 1994;Van den Oord, Boomsma, & Verhulst, 1994), and similar patterns are found for domestic adoptees (Erich & Leung, 2002;Hegar, 2005). This finding suggests a protective effect of having a sibling because older age at adoption is generally related to more, not fewer, behavior problems (Cederblad et al, 1999;Gunnar et al, 2007;Rutter et al, 2010).…”