However, adoption studies suffer from a number of methodological limitations. Adoptive parents and biological parents of adoptees are often not representative of the general population. Adoptive parents tend to be socioeconomically advantaged and have lower rates of mental problems, due to the extensive screening procedures conducted by adoption agencies (Kendler 1993). Biological parents of adoptees tend to be atypical, as well, but in the opposite way. Additionally, selective placement by adoption agencies is confounding the clear-cut separation between genetic and environmental effects by matching adoptees and adoptive parents on demographics, such as race and religion. An increasing number of adoptions are also allowing contact between the biological parents and adoptive children, further confounding the traditional genetic and environmental separation that made adoption studies useful for genetically informative research. Finally, greater contraceptive use is making adoption increasingly rare (Martin et al. 1997). Accordingly, this research strategy has become increasingly challenging, though a number of current adoption studies continue to make important contributions to the field (Leve et al. 2010; McGue et al. 1995, 1996).
