✔ 最佳答案
Typically it is out of fear that the child won't love the parents as much if he or she knows about his or her adoption. This is not true, but some parents believed it.
Also, especially in which you and I were adopted, adoptive parents were told that infants were "blank slates" and that we would never want to know about nor search for our original families. It was a common theory at the time and parents were led to believe if they never mentioned it, it wouldn't ever matter.
參考: Adoptee and Adoptive Mom.