Hard to imagine how to design such study: (close to) 100% of people that adopt kid WANTED to have kid, and planned for them. Some XX% number of people having kid via pregnancy wasn't planning for them, and some even smaller YY% number doesn't want the kid at all.

Not to mention that unless the adoption of kids are done blind by parents, there has to be some biases there. As an example out of my ass, kids with behavioral problems might have way less adoption rate, and potentially those are the one with below median outcome in any case.