Unlike guardianship or other systems designed for the care of the young, adoption is intended to effect a permanent change in status and as such requires societal recognition, either through legal or religious sanction. Historically, some societies have enacted specific laws governing adoption; where others have tried to achieve adoption through less formal means, notably via contracts that specified inheritance rights and parental responsibilities without an accompanying transfer of filiation. Modern systems of adoption, arising in the 20th century, tend to be governed by comprehensive statutes and regulations.
Identity is defined both by what one is and what one is not. Adoptees born into one family lose an identity and then borrow one from the adopting family. The formation of identity is a complicated process and there are many factors that affect its outcome. From a perspective of looking at issues in adoption circumstances, the people involved and affected by adoption (the biological parent, the adoptive parent and the adoptee) can be known as the "triad members and state". Adoption may threaten triad members' sense of identity. Triad members often express feelings related to confused identity and identity crises because of differences between the triad relationships. Adoption, for some, precludes a complete or integrated sense of self. Triad members may experience themselves as incomplete, deficient, or unfinished. They state that they lack feelings of well-being, integration, or solidity associated with a fully developed identity.
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. 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%. 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.