A HUNDRED AND FIFTY years ago, adoption as we know it did not exist. Adoption in those days usually meant one of two things: a child moved in with relatives, or a child lived with an unrelated family in exchange for work. In neither case would the adults become the child's legal parents. Such adoptions were often temporary, and there was no expectation that the adopted child would be treated like the new family's biological children.

Over time, however, adoption became a more and more formal arrangement, increasingly intended to safeguard the interests of the child. By the end of the nineteenth century, most states required that various legal steps be taken before an adoption could proceed. These included an official surrender by the birth parents, issuance of an adoption decree by a judge, and an investigation to make sure the new parents were fit to care for...