The biological relationship between a parent and child is important, and the separation of the two has led to concerns about adoption. The traditional view of adoptive parenting received empirical support from a Princeton University study of 6,000 adoptive, step, and foster families in the United States and South Africa from 1968 to 1985; the study indicated that food expenditures in households with mothers of non-biological children (when controlled for income, household size, hours worked, age, etc.) were significantly less for adoptees; step-children, and foster children, causing the researchers to speculate that, instinctively, people are less interested in sustaining the genetic lines of others.[92] This theory is supported in another more qualitative study where in adoptive relationships marked by sameness in likes, personality, and appearance, both adult adoptees and adoptive parents report being happier with the adoption.[93]

Adoptions can occur either between related family members, or unrelated individuals. Historically, most adoptions occurred within a family. The most recent data from the U.S. indicates about half of adoptions are currently between related individuals.[78] A common example of this is a "stepparent adoption", where the new partner of a parent may legally adopt a child from the parent's previous relationship. Intra-family adoption can also occur through surrender, as a result of parental death, or when the child cannot otherwise be cared for and a family member agrees to take over.

Externally focused theories, in contrast, suggest that reunion is a way for adoptees to overcome social stigma. First proposed by Goffman, the theory has four parts: 1) adoptees perceive the absence of biological ties as distinguishing their adoptive family from others, 2) this understanding is strengthened by experiences where non-adoptees suggest adoptive ties are weaker than blood ties, 3) together, these factors engender, in some adoptees, a sense of social exclusion, and 4) these adoptees react by searching for a blood tie that reinforces their membership in the community. The externally focused rationale for reunion suggests adoptees may be well adjusted and happy within their adoptive families, but will search as an attempt to resolve experiences of social stigma.[167]

International adoption: involves the placing of a child for adoption outside that child's country of birth. This can occur through both public and private agencies. In some countries, such as Sweden, these adoptions account for the majority of cases (see above Table). The U.S. example, however, indicates there is wide variation by country since adoptions from abroad account for less than 15% of its cases.[83] More than 60,000 Russian children have been adopted in the United States since 1992,[85] and a similar number of Chinese children were adopted from 1995 to 2005.[86] The laws of different countries vary in their willingness to allow international adoptions. Recognizing the difficulties and challenges associated with international adoption, and in an effort to protect those involved from the corruption and exploitation which sometimes accompanies it, the Hague Conference on Private International Law developed the Hague Adoption Convention, which came into force on 1 May 1995 and has been ratified by 85 countries as of November 2011.[87]
Some adoptees reject the idea of reunion. It is unclear, though, what differentiates adoptees who search from those who do not. One paper summarizes the research, stating, "...attempts to draw distinctions between the searcher and non-searcher are no more conclusive or generalizable than attempts to substantiate...differences between adoptees and nonadoptees."[169]
In sum, reunions can bring a variety of issues for adoptees and parents. Nevertheless, most reunion results appear to be positive. In the largest study to date (based on the responses of 1,007 adoptees and relinquishing parents), 90% responded that reunion was a beneficial experience. This does not, however, imply ongoing relationships were formed between adoptee and parent nor that this was the goal.[170]

Infant adoption during Antiquity appears rare.[4][7] Abandoned children were often picked up for slavery[8] and composed a significant percentage of the Empire's slave supply.[9][10] Roman legal records indicate that foundlings were occasionally taken in by families and raised as a son or daughter. Although not normally adopted under Roman Law, the children, called alumni, were reared in an arrangement similar to guardianship, being considered the property of the father who abandoned them.[11]
During the same period, the Progressive movement swept the United States with a critical goal of ending the prevailing orphanage system. The culmination of such efforts came with the First White House Conference on the Care of Dependent Children called by President Theodore Roosevelt in 1909,[33] where it was declared that the nuclear family represented "the highest and finest product of civilization" and was best able to serve as primary caretaker for the abandoned and orphaned.[34][35] Anti-institutional forces gathered momentum. As late as 1923, only two percent of children without parental care were in adoptive homes, with the balance in foster arrangements and orphanages. Less than forty years later, nearly one-third were in an adoptive home.[36]
In the 1970s, as adoption search and support organizations developed, there were challenges to the language in common use at the time. As books like Adoption Triangle by Sorosky, Pannor and Baran were published, and support groups formed like CUB (Concerned United Birthparents), a major shift from "natural parent" to "birthparent"[182][183] occurred. Along with the change in times and social attitudes came additional examination of the language used in adoption.