Human societies seem to have an innate tendency to divide people up into good/bad, superior/inferior, belonging/not belonging and so on.
In the past we’ve had light skins superior to dark, legitimate births superior to base-born, Aryans superior to Jews, men better than women, intellectuals better than labourers and Christians better than everybody.
Now, in an attempt to be more civilised, many of us are trying to pretend everybody is THE SAME.
You have to be pretty brave nowadays to do research on what might be genuine, measurable differences between different races, different types of intelligence and different sexual preferences and personality types.
And how many people want to examine seriously the consequences of growing up different in a family to which you are not biologically related? (Nancy Verrier in The Primal Wound, of course, but not many others).
There is some recent research into the effects on the brain of early trauma such as being separated from your mother. There is a lot of statistical and anecdotal evidence that many adopted people are walking around feeling "not quite right" but not able to grasp exactly why this should be. The effects of separation from your natural family can be subtle, easy to misinterpret, and easy to sweep under the carpet as if they do not exist.
Researching the psychological effects of adoption on adoptees would undoubtedly upset a lot of adopters and baby brokers, most of whom have a vested interest in believing it makes no difference - but more research will eventually be undertaken, and there will be no shortage of volunteers.