Dr Matthew McGue is a behavioral geneticist at the University of Minnesota. He and Dr Bill Iacono started an ongoing research project in 1999. The project is called Sibling Interaction and Behavior Study (SIBS) and Research on Adoption. Dr Iacono is interested in:

investigating how genetic and environmental factors combine to influence the development of substance use and related disorders in families that contain twin and adoptive children.

Recently the Washington Post published some results from the study. (see link below) The main objective with the study is to see "the behavior and attitudes of an older sibling represent a major source of environmental influence on adolescent substance use and abuse".  It looks like older siblings have a stronger influence then parents. The secondary objective was to explore the "psychological adjustment of adopted youth".

And it turns out that adopted youth are doing OK in their teenage years. But parents of adopted girls seem to be "hypersensitive". About 27% of the girls in the study "had contact with a mental health professional". But only 18% got a diagnosis. This seems to suggest that maybe there was "parental predisposition to seek help".

I have seen this "parental predisposition" idea show up in different studies. Sometimes it makes the adoptive parents sound crazy. But seriously... If you adopted an abused or neglected child, wouldn't you do EVERYTHING in your power to make sure the child gets the help he/she needs? You just don't know, what you don't know.