According to parental investment theory (Trivers, 1972), it is beneficial for individuals to invest time and other resources in their genetically related children because this may increase the investors’ genetic fitness. Child adoption is often viewed as an evolutionary puzzle, involving substantial investment in children to whom the adoptive parents are not genetically related (Daly & Wilson, 1980; Hrdy, 2009; Silk, 1990). When people think of adoption, they often imagine adopting an unrelated child from another country, a relatively recent phenomenon that began to grow after World War II (Mignot, 2019; United Nations, 2009). In reality, child adoption is more diverse than many might think.

Generally, there are three main categories of child adoption: kin adoption (i.e., when a relative adopts a child), intrafamilial adoption (i.e., when a stepparent adopts a child), and stranger adoption (i.e., when the child is unrelated to the adoptive parents). Different evolutionary logic tends to apply to each category (Daly & Perry, 2020). First, a child may be adopted by relatives (kin adoption), which is consistent with inclusive fitness theory, as individuals can increase their fitness by investing in close kin (Hamilton, 1964). Second, adoption can occur within a family, such as when a child is adopted by a stepparent (intrafamilial adoption). Intrafamilial adoption may be attributed to “mating effort” rather than parental investment, i.e., individuals may adopt stepchildren primarily because they aim to be supportive and caring spouses to their partners (Daly & Perry, 2020). Third, individuals can adopt an unrelated child (stranger adoption) and these adoptions could be either domestic or international (Davis, 2019). Stranger adoption often arises from couples facing infertility, allowing adoptive parents to fulfill their desire to have a child and being the “second best” option (Franklin & Volk, 2021; Jennings et al., 2014). It has been argued that stranger adoption may be a by-product as in ancestral societies, adopted children were almost always adoptive parents’ close relatives (Volk, 2011).

Regarding kin adoptions, intrafamilial adoptions, and stranger adoptions, several hypotheses (H) about the structure of child adoption can be derived from evolutionary theories. In the case of kin adoptions, due to paternity uncertainty and the “nepotistic value of mothers,” maternal kin are expected to invest more in children compared to paternal kin (Perry & Daly, 2017; Perry et al., 2014). Therefore, according to the matrilateral effect, it can be predicted that maternal relatives will adopt children more often than paternal relatives (H1). Furthermore, maternal grandmothers have usually been shown to be the most important alloparents for young children (Daly & Perry, 2017; Danielsbacka et al., 2011), suggesting that they may adopt children more frequently than other kin (H2).

In the case of intrafamilial adoptions, the greater parental investment from biological mothers compared to biological fathers implies that the stepfathers, in particular, can strengthen their relationships with stepchildren’s mothers by investing in stepchildren (Daly & Perry, 2020). Divorced mothers may also be more likely than divorced fathers to select a new partner based on the partners’ willingness to invest in stepchildren (Anderson, 2011). Thus, according to the mating effort hypothesis, stepfathers can be expected to adopt their spouses’ children more often than stepmothers (H3). Moreover, as women typically display greater empathy and provide more care, even to children who are not genetically related to them, compared to men (Hrdy, 2009), it can be predicted that partners in female same-sex families may adopt their spouses’ children more often than partners in male same-sex families (H4). Finally, women’s tendency to be, on average, more caring toward children than men may also influence the structure of stranger adoptions. Based on this sex effect, it could be hypothesized that single women adopt children more often than single men (H5).

Adoption is regulated by laws and policies, and who is approved to adopt a child varies both between countries and over time (Selman, 2023; United Nations, 2009). Thus, not everyone who wishes to adopt may be able to do so due to adoption regulations and processes that determine who is considered suitable as adoptive parents. This may also influence the structure of child adoption by either strengthening or weakening evolutionary tendencies.

Here, we use data from Finland covering all child adoptions between 1999 and 2021. The Finnish Adoption Act stipulates, for instance, that if the adoptive child is a minor (i.e., under the age of 18), the applicant cannot be over the age of 50. This may result in some older adults, such as grandmothers, not being approved to adopt a child due to age restrictions. Moreover, same-sex couples (female or male) living in a registered partnership were granted the right to adopt their partner’s child in 2009, meaning that such family types did not exist before then. Finally, the Maternity Act, which came into effect in 2019, allows a child born through fertility treatments to have two mothers legally recognized even before the child’s birth. This may mean that after 2019, fewer partners in female same-sex families will adopt their spouse’s child, likely leading to a decrease in these family types.