Pair Bonds in Humans

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Claims regarding the universality of human pair bonds are controversial but this is so because they are often confused with statements about origins. The ethnographic record displays a range of grouping patterns, from small two-adult "family" groups to large multimale/multifemale bands (e.g., Pasternak et al. 1997), but within these formations, specialized relationships emerge, between (usually) heterosexual individuals, typically glossed as "marriage." Although it is widely recognized that these bonds do not map precisely onto sexual relationships (Fox 1967), male sexual access to females is key to the definition of marriage, even if it is given quite different salience across cultures (Bell 1997). Furthermore, despite the well-known

"double standards" in sexuality (Betzig 1989), Jankowiak et al.'s (2002) survey of detailed ethnographic material shows that in all the 66 societies studied, men and women actively mate guard, indicating that sexual propriety is a core component to marital unions, even if much violated. Precise definitions of marriage may be elusive, but in all cases, rights and responsibilities are exchanged (Needham 1962), concerning legitimacy of offspring (Gough 1959), property (Leach 1955) and economics (Murdock 1949). In short, marital bonds are about sexual access, and although additional rights and responsibilities are emphasized in different cultural contexts, these bonds are always identifiable. Societies sanctioning total promiscuity as the principle mating system do not exist (Bell 1997; Kunstadter 1963; Rodseth et al. 1991). Note that this is a claim about the universality of pair bonding in the ethnographic record, not about its more contentious evolutionary origins (as discussed in Knight and Power 2005), to which I now turn.

Several sources of evidence point, at least indirectly, to a long history of pair bonding in our species, for example, relatively limited sperm competition (Birkhead 2000), little sexual dimorphism in size dating back in our lineage to 1.8 mya (McHenry 1996), and a distinctive patterning of testosterone production with pair bond status (e.g., lower levels in undergraduate men in well-established romantic relationships: Gray et al. 2004). Furthermore, a notion of "romantic love" is observed across the vast majority of human cultures (Jankowiak and Fischer 1992), mediated by various neuro-endocrinological systems (e.g., Carter 1998). Much more controversial is the role of male provisioning in the evolution of pair bonded behavior.

Pair bonds evolving from male provisioning were once central to narratives of human origins (Washburn and Lancaster 1968). Bipedal hominins could carry meat (Lovejoy 1981), opening up the possibility for a complementary division of labor in which males provide resources to females encumbered with dependent offspring in return for sexual exclusivity. Modern attempts to unravel the origins of pair bonding attribute very different roles to paternal provisioning. In some formulations, males provision because both parents are assumed to have identical reproductive goals. Thus, Fisher (1989) suggests that divorce rates peak after 4 years, because this is when a typical forager child is safely through the period of dependence, and both parents are free to look for new spouses. Others view the relationship as one in which both cooperation and competition exist. In a carefully argued scenario that links intelligence, longevity, altriciality and diet, as coevolved traits, Kaplan et al. (2000) posit that long lifespan and a cooperative division of labor coevolved as humans moved into a foraging niche where food acquisition (hunting) required great skill and knowledge, and partners could benefit from specialization and exchange (Gurven et al. 2009). In other scenarios, marital bonds are believed to be entirely independent of paternal provisioning. In one version, pair bonds are deemed to have emerged from mate guarding, with males favoring pair bonds to avoid incessant fighting over females (Symons 1979) and females to find refuge from harassment (Blurton Jones et al. 2000) and/or infanticide (Hawkes 2004). A different idea that again relies not on paternal provisioning (rather on female provisioning) is that once females discovered how to increase the diversity and density of food value through cooking, they were worth monopolizing (Wrangham et al. 1999). In the latter kinds of scenarios, paternal care is more likely to have evolved after the emergence of pair bonds in our lineage, and not be a necessary condition for the evolution of pair bonding (Chapais 2008).

Phylogenetic analyzes of the relationship between mating systems and paternal care shed light on this origins debate. In mammals, Brotherton and Komers (2003) show that monogamy evolved more often in the absence of paternal care than in its presence, and propose that paternal care most likely arose subsequently (although in birds biparental care may have preceded avian pair bonds, Burley and Johnson 2002). Similar conclusions are being reached for nonhuman primates. Since direct paternal care is present only in some species, it is most likely that monogamy is a preadaptation facilitating the evolution of paternal care rather than a consequence (Palombit 1999; van Schaik and Kappeler 2003). In short, despite the apparent universality of pair bonds in contemporary human populations and cogent models that these evolved to subsidize the high costs of reproduction and encephalization (Kaplan et al. 2000; Gurven and Kaplan 2006), there is little clear comparative evidence that pair bonds evolved in mammals to facilitate paternal provisioning, nor is there much evidence from nonhuman primates to support such a claim. Humans, however, with their exceedingly large brain and unusually long lifespan, may be unique in this respect.

To gain more insight into the nature of the pair bond, anthropologists turn to ethnographic materials, both within and between population analyzes. One question pertinent to this debate is whether men's economic activities are best characterized as mating effort or parental effort. There are cases where men work particularly hard when their mates are lactating (Hadza: Marlowe 2003), allowing the latter to do less work at this energetically demanding time (Hiwi and Ache: Hurtado et al. 1992). These data suggest that men's activities are a form of parenting effort. Similarly, there are cases where men's allocation of effort to food production fits closely with expectations derived from the paternal effort model (Tsimane: Gurven and Kaplan 2006); Although Tsimane men invest in mating effort through extramarital relations, they do so early in the marriage when there are few, if any, children to care for, rather than later, when paternal contributions are most needed (Winking et al. 2007). In addition, confronted with hypothetical scenarios, Ache and Hadza men show preferences for hunting groups with good hunters that yield high returns for provisioning but low returns for mating effort, rather than groups of poor hunters where provisioning benefits are small and mating benefits large (Wood and Hill 2000; Wood 2006). These patterns support the parental effort hypothesis, as do demographic and economic data that show how brides are hard to find (Borgerhoff Mulder 1990; Cashdan 1993) and quick to leave (Betzig 1989) when men's provisioning resources are not forthcoming, patterns also observed in the modern US when marriage transitions are analyzed in relation to income (Nakosteen and Zimmer 1997; Burgess et al. 2003). However, in other studies, men's economic activities are better characterized as mating effort, as with Hawkes' (1993) analysis of hunting in the Hadza and Smith et al.'s (2003) study of turtle feasts of Mer islanders in the Torres Straits. In both these cases, it is argued that men specialize on risky resources, ones that are better characterized as public goods to be ostentatiously shared (the show off hypothesis: Hawkes 1991) than as reliable streams of paternal provisioning. Despite strong arguments on each side, it seems most likely that men everywhere exhibit elements of both paternal and mating effort in their economic exploits (Anderson et al. 1999a, b; Gurven and Hill 2009), for example in the Marlowe's analyzes of the Hadza (1999).

Cross cultural data can also be used to address, at least indirectly, the question of whether pair bonding is associated with the provision of paternal care or with the defense (or guarding) of mates. Quinlan and Quinlan (2007) find that marital stability (a low divorce rate) is associated with substantial male contributions to subsistence, an absence of alternative caretakers, and late weaning, all indices of the potential value of male assistance. However, marital stability is also associated with high levels of polygyny, a possible proxy for the difficulty men face in finding new mates. The authors therefore conclude that pair bonds likely evolved in response to multiple selection pressures - a need for male care and a strategy for each sex to deal with intense mate competition. In fact, there may even be two different kinds of human pair bonds - one geared to child rearing and one geared toward male reproductive competition (Quinlan 2008).

All in all, studies of contemporary populations do little to resolve whether pair bonds evolved for paternal investment or mate guarding. Why so little progress? First, despite the wealth of empirical studies we have puzzling cases where different data sets from the same population support different models. Second, comparative studies raise multiple problems for interpretation - for example is divorce prevalence really a valid indicator of the importance of pair bonds? A more appropriate variable might be the number of people who actually do marry in the population. Third, contemporary correlates of a trait do not necessarily shed light on its evolutionary origins. While human behavioral ecologists maintain that studies of current behavioral diversity illuminate the flexibility of human nature to different social and ecological triggers (e.g., Smith et al. 2001), extrapolating to evolutionary sequences is much more tenuous. Now that pair bonding is in place, paternal care might be very important in contemporary populations. However, to claim that it was the original selective pressure for pair bonds is an interpretative error, Chapais (2008: 169) dubs the "pitfall of modern family reference" (see also Marlowe 2007). Finally, investigators often talk about men and women as if they had a single sex-specific strategy. In the modern USA, for example, upper strata parents use biparental care to invest in highly profitable education for their children, whereas lower strata women raise children alone and both sexes have serial and simultaneous relationships (Kaplan and Lancaster 2003); multiple strategies characterize many other populations (see, for example, Dickemann 1982). In so far as an individual's optimal mating and reproductive strategy depend on the behavior of both same- and different-sexed conspecifics, multiple strategic spaces emerge. Only recently have theoreticians begun to tackle this, as greater computing power allows model parameters to be generated by individual strategies rather than fixed at a priori levels (e.g., Cotar et al. 2008, and for more general discussion see Kokko et al. 2006).

As such, it is perhaps more productive to think about pair bonds in terms of sexual conflict (Borgerhoff Mulder and Rauch in prep). Sexual conflict theory does not inherently avoid the pitfalls of making generalizations about sex differences, but it provides a more dynamic framework for analyzing variable sex roles. For example, it examines the question of how much a wife and husband should work and when they should break their contract, in terms of broader market (supply and demand) conditions. Economists focus on each spouse's bargaining power, the well-being a lady can expect without the cooperation of a gentleman, and vice versa; differential bargaining power sets each spouse's "threat point" (Manser and Brown 1980), the resource sharing contractual arrangement at which the lady (or gentleman) would be better leaving than staying. Anything that improves an individual's bargaining power with their spouse, such as relatively larger earnings, gender-biased divorce laws, or greater chances of remarriage, increases that person's share of the marital pot (e.g., Lundberg and Pollak 1996). Accordingly, the benefits of a marriage are not expected to be shared equally, but in accordance with bargaining power. The implications of these considerations for marital stability, marital assortment, and equilibrial states of the marriage market can be explored with game theory, as demonstrated in models of the "better options" hypothesis for divorce in birds (e.g., McNamara et al. 1999) where divorce rates are shown to decrease with individual quality (fewer "better options" assuming a good first pairing), and age (no time to recoup the costs of divorce).

A key factor affecting these conflicts is whether spousal labor is complementary or substitutable (Kaplan and Lancaster 2003). If mum feeds and dad protects the baby, parental roles are complementary, in so far as each activity is valueless without the other. If mum (or dad) uses salary to pay rent and school fees, parental roles are substitutable and marriages become more brittle (and subject to corner solutions). Changes in divorce rates and prevalence cross culturally might be usefully examined from this perspective. Rather intriguingly, Quinlan and Quinlan (2007) find that divorce is least common in populations where both sexes contribute approximately equally to household production, suggesting (albeit indirectly) that marriage is indeed a more stable institution where spouses' work is complementary.

Thinking about the marriage as a complementary division of labor raises the interesting question of when and how the benefits of specialization can be offset by positive assortment of skills (Borgerhoff Mulder 2004). When are the public goods produced by a breadwinner and homemaker eclipsed by two extremely successful breadwinners who might bicker over homemaking? In traditional economies, where most tasks are gender-specialized, the benefits of the sexual division of labor are unlikely to be dwarfed by positive assortment for skills. One might expect, however, that in a modern economy, where most jobs can be done by men or women, positive assortment counterbalances the division of labor (corporate executives intermarry and hire an au pair). Positive assortment among mates for various skills is found in many modern economies (e.g., Logan et al. 2008). Indeed a study testing whether US university students show preferences to assort on similar traits (rich men like rich women, and vice versa) or on reproductive potential (high earning men prefer beautiful women, and vice versa) showed that the first pattern is much stronger than the second (Buston and Emlen 2003); this suggests that the complementary marital relationship is disappearing in some western societies. We see here how far an evolutionary-based discussion of marriage in terms of sexual conflict and bargaining theory has moved us away from the simple constraints of mammalian reproduction.

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