A 1996 study reported in Child Development found that all the children studied “reported positive levels of self-esteem, curiosity about their birthparents, and satisfaction with the openness situation” regardless of whether their adoptions were closed, semi-open, or open. What this seems to mean is that the child's sense of security in his adoptive family is more important than contact with the birth family.
In all adoption searches, it is uncommon to find both the birth mother and father at the same time. A separate search, if desired, can be done afterwards for the father. Since males seldom change their surnames, and the mother might have additional information, it is usually easier than the initial search for the birth mother. In many cases, adoptees are able to do this second search for their birth father by themselves (or they try before paying for assistance).