Attachment may play a crucial role in the development of human mating and parenting behaviors. Hence, the evolutionary design of attachment could be revealed in relations among risk, resources, and parenting. Del Giudice presents a welcomed and state-of-the-art synthesis of attachment research in evolutionary context. The task ahead is to explore empirical avenues to test and refine predictive models. Data and hypotheses concerning associations between attachment and environmental conditions across populations are particularly scarce, suggesting a fruitful area for future research. Del Giudice's analysis of sex differences in attachment suggests some intriguing questions: What environmental conditions promote boys' and girls' attachment styles, either to diverge into more pronounced patterns of avoidance and anxiety in some populations, or to converge, creating higher proportions of secure individuals in other populations? Is it possible for one sex to show a high proportion of security while the other sex experiences greater insecurity? Do environmental effects on attachment shape patterns of cultural diversity in reproductive behavior? Addressing these questions could enhance our understanding of attachment and its role in larger cultural patterns related to mating, parenting, family, and risk. Here, I focus on sex-biased parental investment and predicted associations with reproductive and family behaviors cross-culturally.
Sex-biased parental care may be expressed as differential parental responsiveness to boys and girls, which could promote divergent attachment styles for males and females. Parental investment theory offers standard models for sex-biased investment (Clutton Brock Reference Clutton-Brock1991). Trivers and Willard's (Reference Trivers and Willard1973) model describes several conditions: (1) one sex has higher variance in reproductive success than the other; (2) offspring's reproductive success is sensitive to the parental care they received; and (3) parental care is positively correlated with parental condition. Under these assumptions, parents in good condition could maximize long-term fitness by biasing their attention toward children of the sex with higher reproductive variance. Conversely, parents in poorer condition should bias attention toward the sex with less reproductive variance. Patterns of parental investment fit this model in some human populations (Cronk Reference Cronk, Cronk, Chagnon and Irons2000). Under Trivers-Willard conditions, we expect to see more secure males and insecure females among relatively wealthy families, and more secure females and insecure males among relatively poor families. The population or culture-level implications of this attachment pattern are intriguing. For example, marriage patterns between the high-status Masai and low-status Mukogodo of Kenya include hypergamy, where Mukogodo families encourage daughters to marry into neighboring, higher-status Masai families. This marriage preference creates a problem for Mukugodo men, who sometimes have trouble finding mates; hence, Mukugodo parents tend to bias investment toward daughters. Daughter bias is evident early in infancy, when daughters are more likely to be seen suckling and being held than are sons (Cronk Reference Cronk, Cronk, Chagnon and Irons2000). This is precisely the kind of sex bias in child care that could enhance divergences in attachment style.
Trivers-Willard fails to predict parental investment in many human and other primate populations, probably because additional factors can affect sex-specific returns on parental investment. Local competition among same-sex siblings is a common complication that can weaken the Trivers-Willard effect. If one sex competes for parental resources, then competition can create lower offspring fitness returns per unit of parental investment. Local competition among brothers is not uncommon cross-culturally (Borgerhoff Mulder Reference Borgerhoff Mulder1998; Quinlan et al. Reference Quinlan, Quinlan and Flinn2005), and it may promote female-biased investment, leading to higher levels of security among girls. Local competition can be quite strong in polygynous societies, in which brothers compete for access to parental resources crucial for accruing multiple wives. In that case, we expect relatively high levels of avoidance among males and higher levels of secure attachment among females.
Note that local mate competition in polygynous societies is probably associated with high variance in male reproductive success – a key element in the Trivers-Willard effect – which can create a series of rather complex parental investment decisions. For example, among the agro-pastoralist Kipsigis of Kenya, polygyny, livestock payments made for brides, and patterns of sibling interaction create a mosaic of parental investment considerations that fit multiple models of parental investment (PI) (Borgerhoff Mulder Reference Borgerhoff Mulder1998). Predicting attachment patterns among groups like the Kipsigis will require close attention to underlying parental investment concerns related to “local fitness currencies”– local resources and relationships that are associated with cultural and reproductive success.
A third pattern of sex-biased parental investment may present additional challenges for evolutionary theories of attachment. Local resource enhancement occurs when offspring of one sex help care for siblings or other relatives, which reduces the costs of parental effort. Parents tend to bias investment toward helpers because a portion of that investment is repaid through work that can enhance parental fitness. In that case, helpers can receive more direct parental care than non-helpers (Quinlan et al. Reference Quinlan, Quinlan and Flinn2005). This empirical pattern is at odds with the proposed attachment style promoting helping at the nest: Helpers are predicted to have insecure clingy styles (Chisholm Reference Chisholm1996); yet biased parental care under local resource enhancement suggest that helpers could be securely attached in many circumstances. The role of anxious attachment style (if any) in promoting helping-at-the-nest requires verification in societies with substantial cooperative childrearing at the household level, which is common cross-culturally but relatively rare in many Western industrial populations.
Sex-specific risks (and saturation points for parental effort) can further complicate parental investment decisions. In some populations, one sex can face greater extrinsic risk than the other. For example, in rural Dominica, boys and men consistently experience greater fitness risks and fewer opportunities than girls and women do. Asymmetry in risk may promote daughter-biased parental investment that includes later weaning (by 5 months in Dominica), greater direct parental care during childhood, and more investment in education in adolescence (Quinlan Reference Quinlan2006). Attention to sex-specific risks will probably prove instructive in the years to come.
Not all parental investment occurs during the sensitive period for attachment. Some parental resources may affect fitness, but they can have little impact on attachment. Family wealth could be channeled toward sons or daughters independent of parental responsiveness in early childhood. Parents in Germany, for example, invest substantial wealth in offspring, but parental care in infancy and early childhood promotes independence and a degree of physical and psychological distance that may seem distressing when viewed from other cultural models of parenting (Levine & Norman Reference Levine, Norman, Moore and Mathews2001).
In sum, patterns of investment and their likely influence on attachment depend on local fitness “currencies” based on prevailing or anticipated economic, ecological, and social factors. Accurately predicting the distribution of attachment styles in and across populations will probably require close attention to multiple local dimensions of parental investment.
