Do evolutionary mechanisms apply for adoptive families? – The link between physical resemblance, personality similarity and odor recognition on the parent-child relationship in adoptive families

From an evolutionary perspective, parental investment may be guided by psychological mechanisms which were shaped by natural selection and rely on certain cues of resemblance which could indicate kinship. This study investigates whether certain evolutionary mechanisms also apply for adoptive families. The influence of facial resemblance, personality similarity and odor recognition on the parent-child relationship was examined within 54 families with 76 children. A positive correlation between facial resemblance and time investment for adoptive mothers was found. For personality similarity a link to investment was found for both adoptive mothers and fathers. Increased personality similarity between mother and child was related to more maternal time investment and less use of corporal punishment. For adoptive fathers personality similarity was positively correlated to emotional closeness. Additionally, a relation between odor recognition and maternal investment was found, whereby the ability to recognize the child was linked to the increased use of corporal punishment. Taken together, these results indicate that underlying evolutionary mechanisms may also influence the parent-child relationship between adoptive parents and their children. Dubas, Loomans Do evolutionary mechanisms apply for adoptive families? 3 Do evolutionary mechanisms apply for adoptive families? The link between physical resemblance, personality similarity and odor recognition on the parent-child relationship in adoptive families According to the theory of inclusive fitness, parents care more for their biological children than for their “social” children (Schnettler & Steinbach, 2011). Does this mean that adoptive children receive worse care? According to Gibson (2009) the opposite is true and adoptive children receive more attention and investment from their parents than their biological children. These findings were also found by Hamilton, Cheng and Powell (2007). Which statement is true? Based on evolutionary theory, kinship plays an important role for investment of parents. The theory of kin selection states that individuals contribute genetically to future generations by reproduction and favor kin above unrelated individuals through inclusive fitness to ensure their (genes) survival. It is even suggested that psychological mechanisms have evolved, due to natural selection, to protect parents from investing in unrelated offspring (Daly & Wilson, 1980). Parents would invest more in children they can be sure are theirs, based on certain cues of resemblance. From an evolutionary perspective, adoption appears to be a form of altruism, as it enhances the fitness of the recipient and lowers the fitness of the donor. According to kin selection theory, altruism is dependent of the probability of shared genes (Silk, 1990) and most adoption among animals occurs through relatives. Research confirmed that mammalian parents use a variety of mechanisms, like smell, to recognize their offspring. They are even able to distinguish between the more “fit” of their related offspring and bias investment towards them (Gibson, 2009). However, according to Hartman and Laird (1990, in Hamilton et al., 2007) adoptive parents may increase their efforts to be “good parents” in order to compensate for the fear of shortcomings or to mask the fact that they are not the “real” parents. Some researchers even Dubas, Loomans Do evolutionary mechanisms apply for adoptive families? 4 found that adoptive parents tend to invest more in their children than biologically related parents (Hamilton, Cheng, & Powell, 2007). According to the environment of evolutionary adaptedness theory (EEA), mechanisms behind parental investment may respond to cues in the modern environment and actually work against genetic fitness (Kaplan, 1996). Adoption is sometimes explained as the confusion of evolutionary impulses that once worked in a different context. This implies that the current context in which adoption occurs is so new that evolutionary predispositions may not have yet caught up, leaving adoptive parents vulnerable to the intrinsic desire for children (Hamilton et al., 2007). According to Silk (1990) humans seem to posses innate psychological predispositions that promote an intense desire for children and permit the formation of close relationships with infants and children of strangers. As a result, adoptive parents may invest in their adopted children as if they were their own biological children. The main goal of this research is to investigate if the psychological mechanisms which influence parental investment can also be found to apply for the relationship between adoptive parents and their children. From an evolutionary perspective, parental investment may be guided by psychological mechanisms which were shaped by natural selection and rely on certain cues of resemblance which could indicate kinship. Self-referent phenotypic matching is a mechanism which underlies the resemblance. Parents may use this mechanism to compare their own phenotypes to those of their children to indicate kinship. In this study the focus lies on 3 aspects of kin recognition: facial resemblance, personality similarity and odor recognition. The influence of these three aspects on parental investment is investigated among adoptive parents. The correlation between facial resemblance and parental investment The first focus of this study is on facial resemblance, which may seem unimportant when it comes to adoptive families, but research confirmed that perceived physical similarity Dubas, Loomans Do evolutionary mechanisms apply for adoptive families? 5 plays an important role for parental investment (Alvergne, Faurie, & Raymond, 2010; Platek, Raines, Gallup, Mohamed, Thomson et. al., 2004). It is even demonstrated that facial resemblance increases trustworthiness and leads to greater cooperation (DeBruine, 2002). Some brain areas are specifically involved in the recognition of familiar faces (Malone, Morris, Kay, & Levin, 1982). Facial resemblance is a physical visible cue which can act as an indicator for kinship (DeBruine, Smith, Jones, Roberts, Petrie, & Spector, 2009; Maloney & Dal Martello, 2006; Alvergne et. al., 2007). Indeed facial resemblance between individuals (assessed by unfamiliar judges) has been demonstrated to be a reliable indication of genetic relatedness (DeBruine et al., 2009; Maloney & Dal Martello, 2006; Alvergne et. al., 2007). Fathers face uncertainty when it comes to paternity, therefore evolutionary theory suggests that they use physical cues to judge kinship (Alvergne et al., 2010). Research has confirmed that, especially for fathers, facial resemblance seems to act as a key for kin recognition (Platek, 2002). They therefore should invest more in children who resemble them. Adoptive parents do not face parental uncertainty and know that they are not genetically related to their children, so facial resemblance is less likely in this group. However, the evolutionary mechanism of parental investment that is linked to resemblance could also apply for adoptive parents. Even though the average similarity between adoptive parents and their children might be lower, this does not mean that there are no adoptive parents who resemble their children. Variation in resemblance can exist in adoptive families with some parent-child pairs having more resemblance than others. Platek et al. (2004) found that only for males, a significant activation in the left frontal cortex, the part of the brain which is involved in the inhibition of negative responses (Colette, Van der Linden, Delfiore, Degueldre, Luxen, & Salomon In Platek et al, 2004), occurred when they were exposed to self resembling photos of children’s faces (Platek et al., 2004). These results suggest that even on a cerebral level, facial resemblance plays a more important Dubas, Loomans Do evolutionary mechanisms apply for adoptive families? 6 role for males. These perceived facial resemblance could play a role unconsciously and influence the relationship between adoptive parent and child and therefore would be related to parental investment. There is no research yet available, that tested the link between facial resemblance and parental investment among adoptive parents and their children. However, research that tested adoption preference and hypothetical investment found significant effects for males based on children’s self-resembling facial photographs (Platek et al., 2004). In accordance Volk and Quinsey (2002) found a significant correlation between facial resemblance and adoption preferences for males, where participants had to choose which child they would like to adopt, based on (self-resembling) child photographs. This correlation was significantly greater for males than for females. These findings suggest that facial resemblance is more important for males, but a study conducted by Bressan, Bertamini, Nalli and Zanutto (2009) showed the opposite. A significant preference for self-resembling children’s faces (morphed photographs) for women, but not for men, was found. In contrast, a study conducted by Welling, Burris and Puts (2011) found evidence that both men and women preferred self-resemblance in children’s faces. In accordance, DeBruine (2004) also found that children’s facial resemblance increased hypothetical investment for both sexes. The contradictory findings on parental investment and facial resemblance can be a result of methodological differences. Some studies used actual photos (Volk & Quinsey, 2002) where the participants and the photographed child were not even related, others used photos morphed with different methods, to simulate resemblance (Platek et al., 2004; Bressan et al., 2009; Welling et al., 2011; DeBruine, 2004). As the results concerning children’s facial resemblance and hypothetical investment decisions are contradictory, more research is needed to clarify if facial resemblance of the child to self