Paternity confidence and social obligations explain men's allocations to romantic partners in an experimental giving game

https://doi.org/10.1016/j.evolhumbehav.2019.10.007Get rights and content

Abstract

Paternal care in humans is facultative, with investment decisions responsive to socioecological context. In particular, paternity confidence is thought to have a significant impact on men's provisioning. However, various aspects of the relationship a man has with his partner can also influence the way he provides for his children. Previous papers have tended to focus either on these kinds of relationship dynamics or on the impact of paternity confidence. However, these categories are often intertwined and parsing their contributions can be conceptually and methodologically difficult. To better understand how paternity confidence and relationship dynamics impact men's investment decisions, we used a series of pictorial vignettes to assess the resource allocation strategies of Himba men. We focus on three traits: mate fidelity, partner type (marital or non-marital), and relationship status (current or former). Results suggest that men prioritize mate fidelity and current reproductive partners in investment decisions, but social obligations to past and current partners and the presence of other male investors also influence decisions. Himba men appear to be balancing social norms related to marriage and fatherhood with individually-driven incentives to invest in current and more faithful partners.

Introduction

Human fathers are rare among mammals for the level of care they provide to their mates and offspring. This care, however, is facultative (Geary, 2015), varying depending on their ability to affect offspring fitness, the opportunity costs of allocating energy to parenting over mating effort, and the risks of misallocating investment in extra-pair offspring (Clutton-Brock, 1991; Gray & Anderson, 2010; Trivers, 1972). In addition to these factors, which are typical predictors of paternal care, human fathers face an additional set of decisions related to paternal investment, which appear to be quite rare in other species. Men not only have extra-pair mates and offspring, they invest in them. They also invest in spouses and children from previous relationships, for example after a divorce. This means that men are facing three kinds of decisions about how to allocate care: 1) how much to invest in a faithful partner versus an unfaithful one; 2) how much to invest in a formal (marital) versus an informal partner; and 3) how much to invest in a current versus a former partner. In each individual case there is a clear and intuitive prediction (e.g. men should invest more in faithful than unfaithful partners); but investment decisions often occur within a complex social milieu, where these categories are intertwined.

One of the simplest predictions about variation in paternal care is that males will titrate their investment in both mates and offspring based on certainty of paternity (Buss & Schmitt, 1993). However, in both humans and non-humans, the evidence for this is decidedly mixed. Experimental manipulations (Neff, 2003; Sheldon & Ellegren, 1998) and within-individual comparisons (Dixon, Ross, & O'Malley, 1994) provide some of the clearest evidence, showing that males adjust their care based on cues of perceived paternity. Correlational studies of paternal care and paternity certainty provide further evidence across a range of species, including in arthropods (Hunt & Simmons, 2002; Zeh & Smith, 1985), fish (Rios-Cardenas & Webster, 2005), birds (Møller & Birkhead, 1993) and mammals (Huck, Fernandez-Duque, Babb, & Schurr, 2014) but can suffer from methodological shortcomings (Kempenaers & Sheldon, 1997; Sheldon, 2002). More interestingly however, there are numerous cases where males do not reduce their care in the face of uncertainty. For example, across >50 bird species, it was found that while males do adjust their investment in offspring provisioning, other areas of care such as nest-building and incubation are often unaffected by the rate of extra-pair paternity (Møller & Birkhead, 1993). Another large study looking for trends across taxa found that in order for care to be adjusted, both the cost of caring and the risk of nonpaternity must be high (Griffin, Alonzo, & Cornwallis, 2013). Males also provide more help to faithful females (Matysioková & Remeš, 2013). In contrast, in some species where extra-pair mating is frequent, significant male care still occurs (Campbell et al., 2009; Fietz & Dausmann, 2003; Randall et al., 2007). In humans, where paternal resemblance is typically used as a proxy for paternity confidence, some studies show biased investment toward those who look similar (Alvergne, Faurie, & Raymond, 2009; Apicella & Marlowe, 2004), but other studies point to alternative explanations, including a reverse in causal order with more paternal care leading to a greater perception of resemblance (Volk, Darrell-Cheng, & Marini, 2010) or confounding factors (Dolinska, 2013). Men have also been shown in some studies to prefer long-term mates who show greater propensity for being faithful (Minervini & McAndrew, 2006; Mogilski, Wade, & Welling, 2014; Thiessen, Young, & Burroughs, 1993), and to be more upset than women by sexual infidelity, which carries the risk of nonpaternity (Buss, 2018; Scelza et al., 2019).

In humans, cultural institutions and social norms further complicate men's decisions. In particular, the institution of marriage is central to contextualizing men's investment decisions. Marriage formalizes obligations and expectations toward partners and children (Gough, 1959). These obligations persist even in the face of paternity uncertainty, as seen in cultures where social and biological fatherhood are divergent. For example, among polyandrous groups, recognition of social fatherhood is intimately tied to responsibility for a child's welfare, even though the practice of polyandry significantly increases paternity uncertainty (Levine, 1987; Starkweather & Hames, 2012). Similarly, in societies that practice partible paternity, men who are denoted as a child's father are routinely expected to provide resources and protection, despite the possibility that they are not the biological father (Beckerman et al., 1998; Walker, Flinn, & Hill, 2010).

Studies of investment by step-fathers provide another useful comparison. Step-fathers have been shown to contribute significant amounts of direct care and resources toward children with whom they share no genetic relatedness, but to whom they have a socially accepted paternal role (Anderson et al., 1999a, Anderson et al., 1999b; Hofferth & Anderson, 2003). While evolutionary anthropologists have attributed this care to mating effort, the formalization of the step-father relationship via marriage and the emotional attachment that comes with lengthy co-residential relationships with a partner's children mean a mix of motivations are likely at play. Framing fatherhood as a biosocial endeavor encapsulates this view (Anderson, 2011; Sheppard, 2018).

At the other end of the spectrum, where a man is the biological father of a child but his social role is limited, he may also provide substantial amounts of care. However, examples of this are much rarer in the literature. Mosuo men, living in traditional matrilineal households, were historically not required to invest in or co-reside with their children (Hua, 2001; Mattison, 2010), and yet these men routinely provide both monetary and direct care, at least in contemporary families (Mattison, Scelza, & Blumenfield, 2014). Nayar fathers in southern India who were in informal, “visiting” relationships with their childrens' mothers had few or no social obligations toward their biological children throughout their lives, but were expected to make some contributions, including paying the expenses associated with childbirth (Gough, 1959). Caribbean scholars, in studying the persistent effects of slavery and indentured servitude, have examined the emergence of matrifocal families where men are more marginalized in family life and formal unions are rare, or occur later in life (Brunod & Cook-Darzens, 2002; Gray et al., 2015; Roopnarine, 2013). Biological fathers in these families are not necessarily uninvolved, but their investments tend to be less reliable than those made by husbands, particularly where partnerships are fragile and short-lived (Carlson & McLanahan, 2010; Tach, Mincy, & Edin, 2010).

Finally, there are the cases when marriages break down. It is a common assumption in the evolutionary literature that divorce is equated with the total desertion of parental duties (Blurton Jones, Nicholas, Marlowe, Hawkes, & O'Connell, 2000; Hurtado & Hill, 1992), often due in part to grouping divorce with other reasons for father absence like death, each of which may affect children differently (Shenk, Starkweather, Kress, & Alam, 2013). However, in many cases, paternal care continues in some capacity after a divorce, and rarely ceases altogether. Systems of alimony and child support formalize, and often mandate, these commitments, but even where they are absent, fathers typically continue to have relationships with their children, and even with their former spouse (Minton & Pasley, 1996). In two parallel studies in Albuquerque and South Africa, Anderson and colleagues show that while divorce or dissolution of formal partnerships may lead to lower levels of direct investment (e.g. spending time, help with school work), after divorce fathers still invest in their children indirectly, by providing money for school, clothing, and other expenditures (Anderson et al., 1999a, Anderson et al., 1999b).

When partnerships are informal, their dissolution tends to result in fewer continuing obligations, especially as the years since the partnership ended increase (Lerman, 2010; Ryan, Kalil, & Ziol-Guest, 2008; Tach et al., 2010). However, the extent to which fathers continue to invest in children from informal unions appears to depend in part on the way that these unions are viewed within the community. For example, declines in investment after the union ends, and particularly after a man enters a new union and has additional children, are less precipitous among African-American fathers, who live in communities where non-conjugal unions and out-of-wedlock births are more socially normative, than among whites or Hispanics (Edin, Tach, & Mincy, 2009).

Aside from the status and trajectory of the union itself, it is likely that men are also considering the context surrounding a partnership when making investment decisions. This can include myriad factors, including the wealth of the man and the woman, their degrees of kin support, the number of dependents each has, and so forth. One factor which may be particularly important is the presence of another male provider. In other species, there is evidence that males adjust their care, not only based on factors like paternity certainty, but on the level of care expected from other males (Briskie, Montgomerie, Põldmaa, & Boag, 1998; Davies & Hatchwell, 1992). Similarly, in humans, we expect that an ex-wife who has remarried will likely be treated differently than one who is single. Similarly, an extra-marital partner who is married and has the support of her husband is in a different position than one who is unmarried and relies on her boyfriend as her primary source of support.

The complicated nature of men's investment patterns described above makes disentangling the effects of marital status, societal obligations, mate fidelity, and paternity difficult. Here we take an experimental approach to begin to parse these effects. This study was conducted with Himba men living in northwest Namibia. Himba represent an optimal population to conduct this study because it is a culture where concurrent and sequential partnerships are both common, nonpaternity events are frequent, and where notions of social fatherhood are strong (see below for details). We use a multistage vignette questionnaire to understand how competing influences affect men's allocation strategies.

We first test three primary predictions: (1) men will favor faithful over unfaithful partners; (2) men will favor formal over informal partners; (3) men will favor current over former partners. These predictions are not mutually exclusive, and we expect to find support for all of them.

Next, we test predictions derived from three strategies that men could employ in deciding how to allocate their resources. While there is some overlap in what is expected from each strategy, they lead to divergent pictures of men's allocations (Fig. 1).

(a) Social Obligation Hypothesis: If societal norms about monogamy and social fatherhood are driving men's investment decisions, we expect formal partnerships (with wives and ex-wives) to take priority over informal partnerships (with girlfriends), with the largest allocations going to current wives. In addition, in line with the idea that men should invest in the children to whom they are the named father, we expect no differences in allocations to biological versus non-biological children.

(b) Paternal Investment Hypothesis: If men are motivated primarily by concerns of paternal investment, we expect them to titrate their allocations depending on the biological status of the child. Partners with biological children will be favored over partners with non-biological children across partner types.

(c) Reproductive Effort Hypothesis: If men's allocations are driven mainly by the goal of securing as much future paternity as possible, current partners (wives and girlfriends) will be favored over former ones (ex-wives). Among current partners, we expect men to allocate more to informal partners, as there is less certainty that the partnership will continue, and therefore more effort needed for relationship maintenance.

Finally, we address the role of need in men's resource allocations, which may be further complicating the relationship between social norms and mating-parenting trade-offs. When men allocate resources to their partners, they are typically aware of her partnership status (i.e. whether she has a romantic partner other than him), and we expect that this may in part be driving men's decisions about how to invest. Therefore, we predict that when paternity and partner type are equalized, men will favor the partner whose need is greatest. In our study we use the presence of another partner (husband or boyfriend) as a proxy for need.

Section snippets

Study population

Himba are semi-nomadic agro-pastoralists, living in the Kunene region of Namibia. They are an ethnic minority, closely related to Herero, but with greater reliance on a subsistence-based diet, mainly of milk, meat, and maize (Bollig, 2010). Increasingly, these foods are being supplemented by store-bought items like pasta, rice and sugar, although they still make up only a small portion of the diet. Electricity and plumbing are absent in the community save for a few community water pumps that

Task 1: - food allocation

Bayes factor comparison of categories reveals differences in the allocation of food tokens between categories (BF >100). Wives were allocated the most food tokens, and women with biological children (non-omoka) received more than women with non-biological children (omoka) (Fig. S4). Comparison of models with single predictors (Models 1–3) indicate that the partner fidelity model yields the best fit, followed closely by the relationship status model. When multiple predictors were included,

Discussion

Our results show strong evidence for our primary predictions, which are derived from life history theory, and which are fairly intuitive. Fig. 3 demonstrates this most clearly. All else equal, men favor more faithful partners (rows 1, 10, 15), partners they have formal unions with (2, 7) and, to a lesser extent, current partners (4, 9, 11, 14). Our secondary predictions, which bring these factors together into a series of potential strategies, show greatest support for the paternal investment

Conclusion

Understanding the facultative nature of paternal investment in humans requires considering not only the dyadic relationship between father and child, but also the relationship between the father and the mother of that child. As Carolyn Bledsoe wrote, “Because children are symbols of links between adults, resource allocations to children, like the performance of sexual or domestic duties, become barometers of adult relations,” (Bledsoe, 1995, 131). Here we evaluate the impact of predictions from

Acknowledgements

First and foremost, we would like to thank the community of Omuhonga for their ongoing support of the Kunene Rural Health and Demography Project. In particular, we would like to thank the men who participated in this study. John Jakurama, Cancy Louis and Gita Louis worked as research assistants for this study. This work was funded by the National Science Foundation (NSF-BCS-1534682) and the Max Planck Institute for Evolutionary Anthropology.

Data availability

Data and code for this project can be found at: https://osf.io/8bp5z/

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