Similar mechanisms appear to be at work in the physical development of adoptees. Danish and American researchers conducting studies on the genetic contribution to body mass index found correlations between an adoptee's weight class and his biological parents' BMI while finding no relationship with the adoptive family environment. Moreover, about one-half of inter-individual differences were due to individual non-shared influences.[115][116]
Adoption is the legal process through which a child joins a family different from his or her birth parents. Adoption is a permanent, lifelong commitment to a child. In CPS cases, adoption becomes an option if CPS and the child's birth parents cannot resolve issues that made it unsafe for the child to live at home. Then, CPS may ask the court to end the parents' rights to the child and place the child with another family permanently. A child can also become legally free for adoption if both birth parents agree to give up their parental rights.

Other studies provide evidence that adoptive relationships can form along other lines. A study evaluating the level of parental investment indicates strength in adoptive families, suggesting that parents who adopt invest more time in their children than other parents and concludes, "...adoptive parents enrich their children's lives to compensate for the lack of biological ties and the extra challenges of adoption."[94] Another recent study found that adoptive families invested more heavily in their adopted children, for example, by providing further education and financial support. Noting that adoptees seemed to be more likely to experience problems such as drug addiction, the study speculated that adoptive parents might invest more in adoptees not because they favor them, but because they are more likely than genetic children to need the help.[95]

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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]
The practice of closed adoption (aka confidential or secret adoption),[74] which has not been the norm for most of modern history,[75] seals all identifying information, maintaining it as secret and preventing disclosure of the adoptive parents', biological kins', and adoptees' identities. Nevertheless, closed adoption may allow the transmittal of non-identifying information such as medical history and religious and ethnic background.[76] Today, as a result of safe haven laws passed by some U.S. states, secret adoption is seeing renewed influence. In so-called "safe-haven" states, infants can be left, anonymously, at hospitals, fire departments, or police stations within a few days of birth, a practice criticized by some adoption advocacy organizations as being retrograde and dangerous.[77]
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]
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