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]

Evidence about the development of adoptees can be supported in newer studies. It can be said that adoptees, in some respect, tend to develop differently from the general population. This can be seen in many aspects of life, but usually can be found as a greater risk around the time of adolescence. For example, it has been found that many adoptees experience difficulty in establishing a sense of identity.[126]

Infertility is the main reason parents seek to adopt children they are not related to. One study shows this accounted for 80% of unrelated infant adoptions and half of adoptions through foster care.[79] Estimates suggest that 11–24% of Americans who cannot conceive or carry to term attempt to build a family through adoption, and that the overall rate of ever-married American women who adopt is about 1.4%.[80][81] Other reasons people adopt are numerous although not well documented. These may include wanting to cement a new family following divorce or death of one parent, compassion motivated by religious or philosophical conviction, to avoid contributing to overpopulation out of the belief that it is more responsible to care for otherwise parent-less children than to reproduce, to ensure that inheritable diseases (e.g., Tay–Sachs disease) are not passed on, and health concerns relating to pregnancy and childbirth. Although there are a range of possible reasons, the most recent study of experiences of women who adopt suggests they are most likely to be 40–44 years of age, currently married, have impaired fertility, and childless.[82]

Foster care adoption: this is a type of domestic adoption where a child is initially placed in public care. Many times the foster parents take on the adoption when the children become legally free. Its importance as an avenue for adoption varies by country. Of the 127,500 adoptions in the U.S. in 2000[83] about 51,000 or 40% were through the foster care system.[84]
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]
Private domestic adoptions: under this arrangement, charities and for-profit organizations act as intermediaries, bringing together prospective adoptive parents and families who want to place a child, all parties being residents of the same country. Alternatively, prospective adoptive parents sometimes avoid intermediaries and connect with women directly, often with a written contract; this is not permitted in some jurisdictions. Private domestic adoption accounts for a significant portion of all adoptions; in the United States, for example, nearly 45% of adoptions are estimated to have been arranged privately.[83] 

Psychologists' findings regarding the importance of early mother-infant bonding created some concern about whether parents who adopt older infants or toddlers after birth have missed some crucial period for the child's development. However, research on The Mental and Social Life of Babies suggested that the "parent-infant system," rather than a bond between biologically related individuals, is an evolved fit between innate behavior patterns of all human infants and equally evolved responses of human adults to those infant behaviors. Thus nature "ensures some initial flexibility with respect to the particular adults who take on the parental role."[96]