The U.S. example, however, indicates there is wide variation by country since adoptions from abroad account for less than 15% of its cases. More than 60,000 Russian children have been adopted in the United States since 1992, and a similar number of Chinese children were adopted from 1995 to 2005. 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. Embryo adoption: based on the donation of embryos remaining after one couples in vitro fertilization treatments have been completed; embryos are given to another individual or couple, followed by the placement of those embryos into the recipient womans uterus, to facilitate pregnancy and childbirth. In the United States, embryo adoption is governed by property law rather than by the court systems, in contrast to traditional adoption. Common law adoption: this is an adoption that has not been recognized beforehand by the courts, but where a parent, without resorting to any formal legal process, leaves his or her children with a friend or relative for an extended period of time.
This theory is supported in another more qualitative study wherein adoptive relationships marked by sameness in likes, personality, and appearance, were associated with both adult adoptees and adoptive parents report being happier with the adoption.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 childrens lives to compensate for the lack of biological ties and the extra challenges of adoption." 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.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 childs 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.