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.
Infant adoption during Antiquity appears rare. Abandoned children were often picked up for slavery and composed a significant percentage of the Empire's slave supply. 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.