Well, here's a headline sure to give adoptive parents a little panic attack:
"Study: Adopted kids more susceptible to mental, physical woes," http://azstarnet.com/news/science/health-med-fit/article_7d188a39-87ac-56e0-b2ad-a3e87e4fed83.html
The researchers determined that adopted kids are three times more likely to have physical and mental-health disabilities than kids raised by their biological parents. BUT... all the kids in the study were adopted from foster care, so likely suffered some abuse and/or neglect in their early months or years, plus the likely trauma of moving from one foster home to another. As I know so well from Quinn, early neglect - even in the fairly benign form of a lack of stimulation - leaves a lasting impact. So are those kids likely to have problems down the line? Absolutely. Would they have those problems whether they were with biological parents, adoptive parents, foster parents or in a group home? Absolutely.
I'm no researcher, but it seems to me that this study measured the long-term impact of a rough beginning, but attributed that impact not to the rough beginning, but to adoption.