In historical terms, the legal institution of adoption in the United Slates is relatively new. It was between the mid-1800s and the 1920s that the states began to pass laws providing for the adoption of children. Before then children had been adopted informally and in some instances by individual legislative acts, or they had come to live with other families under indenture contracts or as a result of legislation authorizing charitable organizations to place children. Under these new adoption statutes, initially the court records of adoptions were not subject to confidentiality, and adopted children were not issued new birth certificates. Over time, states began to issue new birth certificates and, gradually, to close adoption records -to the public, to all parties to the adoption except for the adult adoptee, and ultimately (in almost all the states by 1990) to the adult adopted individuals themselves.
Adoption, Encyclopedia of Privacy (William G. Staples, ed., 2007).