At the end of the 19th century, the Industrial Revolution changed many social conditions, which in turn, changed adoption practices. For most of human history the heritage of a child who moved from one family to another had been known to the adoptee as well as to the community.
With industrialization, people moved away from their supporting communities. As women left their families to work in cities, the incidence of pregnancies by single women increased. Children needed families not because their mothers had died but because their mothers were single in a culture that attached enormous stigma to both the unwed mother and the illegitimate child.
Without the support of smaller communities, adoption became institutionalized. It soon became standard practice to close original birth records and adoption records, to protect the adoptee and birth mother from the prying eyes of an unforgiving public. After World War II, records were sealed from everyone, including the birth parent and the adoptee.
Adoptees were not told they were adopted -- to keep the "shame" of their birth from them. Members of the community seldom knew which children had been adopted. In an era when environment was considered more influential to individual development than genetics, people considered a child's origins to have little significance.
The adoptees who grew up in that era learned something different. They have pointed out that despite a legal decree that ignores past connections, it is not possible to sever historical, biological and spiritual ties to one's ancestors. The adoptive family truly becomes the adoptee's family; the adoptive parents become the adoptee's mother and father. But heritage and ancestry can never be altered.
Post-war adoptees began to say this publicly, through books such as Jean Paton's Orphan Voyage and B.J. Lifton's Lost and Found, as well as through advocacy organizations for adoptees like ALMA (Adoptee Liberty Movement Association). At about the same time, people not even connected with the field of adoption began talking about the impact of grief in ways that made sense not only to adoptees but also to birth parents and adoptive parents.