Although adoption studies have been criticized concerning selective placement, the ultimate findings of these studies are still

I believe the answer is: somewhat valid

adoption studies is the type of studies that aimed to find out the effect of environment and genetics to a certain phenotype by  matching adoptive parents to adopted children .
 Even though the studies is conducted with proper scientific method (which make it somewhat valid) The critique came because the pre-selection process of the parents and children would make the result of studies couldn't be applicable to most people in general/