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Peter B. Gray Ph.D.
Peter B. Gray Ph.D.

Adoptive Fathers

Tales of male monkeys, wolves, and humans becoming caregivers of unrelated young

I'm a dog (from one of Darwin's books). Will you adopt me?

Among mammals, the iconic image of altruism is maternal care. A mom gives her time, love, and milk to nurture her next of kin. From an evolutionary perspective, those forms of maternal sacrifice can be understood from the standpoint of kin selection. A mom keeping her young alive and prospecting for future reproductive success also aids the future outlook of the genes those offspring possess. But what about cases in which males step up to a caregiving plate—not to care for their own kin (which also invokes kin selection), but to adopt unrelated young? Wouldn’t that leave the late Bill Hamilton scratching his head?

Observations of wolves in the wild suggest males sometimes adopt unrelated young, a process that has also been replicated in captivity ( But scholars have long noted that wolves reproduce in family groups, involving long-term bonds between mates and paternal investment. That means wolf males may be prepared to care for offspring in ways that the vast majority of mammalian species are not. But in the nonhuman primate world, many surprises await. In the wake of Ebola outbreaks and more typical causes of death among wild chimpanzees at Tai forest in the Ivory Coast, some 36 chimpanzee young were left orphaned during over a quarter century of study. Remarkably, 18 of those orphans gained regular long-term caregivers, with 8 of those adopted by unrelated males. These adoptive males might offer support during aggressive encounters and await their younger charges while ranging. Chimpanzee males do not provide paternal care, making such observations all the more unusual.

In various other multi-male, multi-female primate species in which males do not typically provide parental care, males have nonetheless been observed caring for orphans. These examples include rhesus macaques, Japanese macaques, stump-tailed macaques and yellow baboons. In some cases, this may represent paternal behavior (e.g., tending to genetic offspring), perhaps with preferential mating access to an offspring’s late mother serving as a social cue to paternity. Among non-paternal cases, however, proximate psychological processes (responding emotionally to needy young) or functional ends (exploiting an orphan to curry favors or reduce agonistic encounters with other group members) may be at play.

Let’s bring humans into the fatherly fray. Paternal care is a regular, if quite plastic, feature of human reproductive behavior. Human males may thus have some predisposition toward care of young. Studies of adoption in humans have pointed to the seeming paradox of why males might invest tremendous resources and time in unrelated individuals, without mating effort serving as a potential cause (as in the case of caring for stepchildren). In cross-cultural scope, most adoption of other children is actually fostering—becoming a long-term caregiver for more distant, dependent relatives such as nieces or nephews. In those cases, kin selection appears to help account for otherwise puzzling behavior. In the U.S, however, adoptions of unrelated offspring invite direct attention. In a study of over 10,000 families of kindergarteners and first graders, some 161 consisted of two-parent adoptive parents. Researchers relied on several measures of parental care, including economic and interactional items such as how often a parent helped with homework. Results indicated that adoptive mothers and fathers invested more in their offspring than did parents in other family types. However, adoptive parents were also better educated and had higher incomes. When adjusting for such potential confounding variables, adoptive parents invested at similar levels as households with two genetic parents, and more so than other family types.

An interpretation of these findings is that adoptive fathers were, at face value, engaging in caregiving behaviors that can’t be accounted for by kin selection or other fitness-enhancing behaviors. Instead, the emotionally rewarding aspects of caring for dependents (proximate psychological processes), perhaps amplified among some adoptive parents who had unsuccessfully tried having genetic children, may help account for adoptive caregiving. In another interesting U.S. study, parents invested more in coresidential unrelated adopted children than genetic children. However, the author’s interpretation of that association was that “the squeaky wheel gets the grease,” meaning that the adopted children both needed and received more care than their parents’ genetic children.

Suggesting that we can extend the discussion even further, let’s go back to those wolves—or at least the descendents of some of their Eurasian cousins: domesticated dogs. Why do people invest time and resources not just in some unrelated offspring, but in some unrelated dependent of another species? As the American Pet Products Association points out, expenditures in the pet products industry continue to show recession-proof increases. A good chunk of the approximately $50 billion spent this past year on pets in the U.S. was on dogs. While dogs can serve various functions—in protection, as beasts of burden, as food, as garbage disposals—they increasingly serve as emotional surrogates of young children. Caring for a pet dog can be emotionally rewarding, perhaps activating some of the same physiological mechanisms that evolved in the expression of direct childcare, even if it may not enhance one’s direct fitness. Interestingly, male and female adoptive dog owners (who acquired a pet dog on their own accord) reported greater attachment and investment in a pet dog than did “step-dog owners” (who took on that role after, say, moving in with someone who already had a pet dog).

In summary, adoptive fathers, whether human or some other species, raise various questions from an evolutionary perspective. A host of factors, including activation of proximate psychophysiological mechanisms, may help account for what appears to be puzzling behavior. But in a season when a jolly bearded fellow supposedly gives out toys to millions of unrelated children, we might need even more explanation to inquiring young minds.


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

Gibson, K. (2009). Differential parental investment in families with both adopted and genetic children. Evolution and Human Behavior 30, 184-189.

Gray, P. B., and Young, S. (2011). Human-pet dynamics in cross-cultural perspective. Anthrozoos 24, 17-30.

Hamilton, L., Cheng, S., & Powell, B. (2007). Adoptive parents, adaptive parents: Evaluating the importance of biological ties for parental investment. American Sociological Review 72, 95-116.

Muller, M. N., & Emery Thompson, M. (2012). Mating, parenting, and male reproductive strategies. In. J. C. Mitani, J. Call, P. M. Kappeler, R. A. Palombi, & J. B. Silk (Eds.), The evolution of primate societies (pp. 387-411). Chicago: The University of Chicago Press.

Riedman, M. L. (1982). The evolution of alloparental care and adoption in mammals and birds. Quarterly Review of Biology 57, 405-435.

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

Steiner, E. T., Silver, N. C., Hall, P., Downing, C., Hurton, D., & Gray, P. B. (2013). Raising canine: Cross-species parallels in parental investment. Human-Animal Interaction Bulletin 1, 38-54.

About the Author
Peter B. Gray Ph.D.

Peter B. Gray, Ph.D., is an associate professor of anthropology at the University of Nevada, Las Vegas. He is the co-author of Fatherhood.

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