What I have seen among some friends and patients is the kid really wants to find out/get to meet the biological parents. They sometimes describe being incomplete etc. I have never encountered parents that feel that way. Certainly not a reason to not adopt. One wonders if this will or will not be an issue in the future with adoption processes being more open these days.
Our agency (the Gladney Agency in Ft. Worth, Texas) encourages open adoptions for those who wish to adopt domestically. They don't require it, but they are frank with potential adoptive parents that the wait typically drops dramatically the more open you're willing to be. My wife and I had to talk about that a lot. Neither of us was keen on having a full-on relationship with a potential birth mother, but we also did not want to wait forever, and we were sensitive to some of the "wanting to know" issues we might encounter with a completely closed adoption.
We ended up with a fairly open adoption. Our birth mother calls on our daughter's birthday, and we provided her with lots of pictures over the last 7 years, usually in one big batch around Christmas time. A couple of years ago, she visited some family in our area and we all had lunch with her and some of her family. Long and short of it is that my daughter knows who her birth mother is and will be able to have whatever relationship she chooses when she's mature enough and capable of making that decision. So far, she does not seem too worried about knowing more about her birth mother. A lot of it is how you frame your answers to questions about them.
All-in-all, we have had a wonderful experience with adoption, and my daughter very much resembles my wife, so unless she (my daughter) wants to tell other people she's adopted, most don't even consider the possibility.
''The enemy isn't conservatism. The enemy isn't liberalism. The enemy is bulls**t.''