Brenda Romanchik may have more sources but one of the most highly cited (I have found) and comprehensive is the Minnesota Texas Project (MN/TX) Adoption Project.
MN/TX Adoption Project
Look to the left and you can find the links to key findings. I'll look for some others (on my PC) a bit later and let you know if I have additional resources. I would also check out the Evan B. Donaldson Adoption Institute link on the right. They are another key source of adoption related studies.
Hope it helps...