Human Adoption From An Evolutionary Perspective

This entry was written by H Roshelle Martin as part of a project done in BIAN 2133 ‘Human Reproductive Strategies’ at The Australian National University in 2019 Semester 2.


Human parenting requires a substantial investment due to the altricial nature of human young. When this investment is in biological progeny there is a clear fitness benefit since it facilitates the survival of the parents’ genes beyond their own lifespan. The evolutionary fitness benefits of parenting indirect descendants or genetically unrelated young are less clear. Some scholars have cited kin selection and reciprocal altruism as being possible evolutionary drivers for adoption. Others have suggested that adoption results from innate human desires, including the biological urge to parent, or the psychological drive to continue existence after death by transmitting non-biological traits.

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Kin selection

Kin selection theory is based on Hamilton’s Rule which predicts that helping behaviour will happen if the cost to the helper is less than the benefit to the recipient. Additionally, as the level of genetic relatedness between the two individuals increases, so does the benefit (Hames, 2015). Kin selection could explain the adoption of indirect descendants who are, nonetheless, genetically related. This has been demonstrated in traditional societies in Oceania and the North American Arctic, where older, childless couples unsuccessful in producing biological children frequently adopt the offspring of their biological relations. The biological parents are willing to relinquish their offspring because they are believe they are unable to provide as high a level of care as the adoptive parents, increasing the changes of their offspring’s survival (Silk, 1990). Among the Himba pastoralists in Namibia, first-born children are frequently fostered to close kin (Scelza & Silk, 2014). This allows the mother to reproduce again in a shorter time than she can if she must care for the infant, enabling a higher quantity of offspring to be produced. The energy that would otherwise be expended in lactation and infant care is redirected towards fertility. The foster mothers are usually post-menopausal women whose own children no longer require care, as their cost is relatively low. However, they may still increase their fitness by caring for a genetically related foster child.

Reciprocal Altruism

Like Hamilton’s Rule, reciprocal altruism predicts helping behaviour when the benefit to the recipient exceeds the cost to the helper. However, the two parties need not be genetically related, but they must remember their transactions and an opportunity for future repayment must exist. Adoption of a genetically unrelated child may be considered an altruistic exercise, since the adoptive parent’s fitness decreases so that the child’s fitness can increase. There is, however, the opportunity for the adopted child to reciprocate through, for example, labour or financial help and physical care in old age (Silk, 1990). Among the Himba, when foster parents requested foster children it was generally to help with domestic chores, and most of those foster relationships had the lowest amounts of genetic relatedness (Scelza & Silk, 2014).
Boesch, Bole, Eckhardt, and Boesch (2010) observed such behaviour in wild chimpanzees, our closest living evolutionary relations, suggesting the drivers for adoption may have evolved in an early common ancestor. Their observations spanned twenty-seven years, in which time males were observed caring for unrelated orphans at considerable cost to themselves. This cost could be repaid after the orphans reached adulthood, as they could become social allies of their adopted fathers. Females were observed investing in high levels of care for the orphaned offspring of their friends. It is possible that such behaviour enhanced those females’ reputations within their social group and, consequently, attracted mates, but this was not conclusively determined. This is analogous to some human societies. Individuals in traditional societies in Oceania and the Arctic are known to have practiced adoption to increase their social standing by strengthening social ties (Silk, 1990).

Desire to Parent

Psychological characteristics seem to have evolved in humans that make them predisposed to adoption. Many are inclined to bond with children and are flexible in how and when they form these bonds. Furthermore, they do not appear to be instinctively aware of who they are genetically related to (Silk, 1990). It is possible that innate kin recognition did not evolve in humans because there was no need for it, since early humans and their ancestors lived in small, closely-related groups. Although many modern adopters are aware of their genetic unrelatedness to adoptees, they may not place much importance on this due to their lacking innate kin recognition. Being flexible in forming attachments, they are able to bond with children after infancy, as is evidenced by the numerous cases of adoption of older children.

The costs to adopting parents in Western nations is often significantly higher than the financial, physical or social benefits they receive, which are often not guaranteed. Moreover, while the number of children available for adoption has steadily decreased, the demand for them has continued to increase. Many people are willing to adopt children who are ethnically different or have special needs and invest considerable resources in their wellbeing (Silk, 1990). This suggests that increasing evolutionary fitness is not always the primary driver for adoption.

Hoppe, Fritsche, and Koranyi (2017) found that individuals were more willing to adopt when faced with circumstances that highlighted their own mortality. They were also more willing to adopt teenagers when they believed their personality could be significantly influenced by nurturing them. They attributed this to a desire for “psychological transcendence” which provides a means for immortality when it cannot be achieved through biological reproduction. This supports they theory that humans are fundamentally motivated to extend their existence, often out of a fear of their own life ending. They are content with cultural and psychological descendants when biological descendants are unobtainable.


Human adoption seems to be driven by biological, psychological and social factors which differ on an individual and societal basis. Evidence supporting the evolutionary models of kin selection and reciprocal altruism has been found in differing societies across the world. While those models reason cite increased genetic fitness, they do not explain all adoption situations. Adoption of genetically unrelated children may be more readily explained by an innate biological drive to parent, or a psychological drive to overcome mortality.

Literature Cited

Boesch, C., Bole, C., Eckhardt, N., & Boesch, H. (2010). Altruism in forest chimpanzees: The case of adoption. PLoS ONE, 5(1), e8901.

Hames, R. (2015). Kin Selection. In D. M. Buss (Ed.), The handbook of evolutionary psychology, Volume 1: Foundations (2nd ed., pp. 1-19). New Jersey: John Wiley & Sons.

Hoppe, A., Fritsche, I., & Koranyi, N. (2017). Self-transcendence as a psychological parenthood motive: When mortality salience increases the desire for non-biological children: Self-transcendence as a parenthood motive. European Journal of Social Psychology, 47(4), 488-500.

Scelza, B. A., & Silk, J. B. (2014). Fosterage as a System of Dispersed Cooperative Breeding: Evidence from the Himba. Human Nature, 25(4), 448-464.

Silk, J. B. (1990). Human adoption in evolutionary perspective. Human Nature, 1(1), 25-52.

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