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Keywords:

  • human behavioural ecology;
  • human social behaviour;
  • inclusive fitness;
  • kin selection;
  • marriage;
  • monogamy;
  • paternity;
  • polygyny;
  • social norms;
  • strategic behaviour;
  • wealth inheritance

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information

The majority of human societies allow polygynous marriage, and the prevalence of this practice is readily understood in evolutionary terms. Why some societies prescribe monogamous marriage is however not clear: current evolutionary explanations—that social monogamy increases within-group co-operation, giving societies an advantage in competition with other groups—conflict with the historical and ethnographic evidence. We show that, within the framework of inclusive fitness theory, monogamous marriage can be viewed as the outcome of the strategic behaviour of males and females in the allocation of resources to the next generation. Where resources are transferred across generations, social monogamy can be advantageous if partitioning of resources among the offspring of multiple wives causes a depletion of their fitness value, and/or if females grant husbands higher fidelity in exchange for exclusive investment of resources in their offspring. This may explain why monogamous marriage prevailed among the historical societies of Eurasia: here, intensive agriculture led to scarcity of land, with depletion in the value of estates through partitioning among multiple heirs. Norms promoting high paternity were common among ancient societies in the region, and may have further facilitated the establishment of social monogamy. In line with the historical and ethnographic evidence, this suggests that monogamous marriage emerged in Eurasia following the adoption of intensive agriculture, as ownership of land became critical to productive and reproductive success.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information

Background

Eighty-three percent of human societies allow polygynous marriage (Murdock & White, 1969; Murdock & Wilson, 1972). In humans, as in other sexually reproducing species, the lower investment in gametes by males leads to the higher potential rate of reproduction of individual males relative to individual females. This, coupled with proximate constraints such as internal gestation and lactation, produces the typical mammalian pattern of polygynous breeding, characterized by high male investment in mating effort and high female investment in parental effort (Trivers, 1972; Clutton-Brock & Vincent, 1991). Extension of this paradigm to human social systems is used to explain the cross-cultural prevalence of polygynous marriage (e.g. Low, 2003, 2007; Marlowe, 2003). In some societies that allow polygynous marriage the majority of men may be each married to a single wife, because few command sufficient skill or resources to marry polygynously (White, 1988). This marriage pattern, sometimes referred to as ‘monogamy’ (e.g. Marlowe, 2003), is common among foragers and likely evolved because of the benefits of biparental care to offspring survival (Low, 2003, 2007).

This is distinct from the system of social monogamy found in the remaining 17% of societies, in which polygynous marriage is forbidden or disapproved (Murdock & White, 1969; Murdock & Wilson, 1972). Current evolutionary explanations view this marriage strategy as a mechanism of reproductive levelling (e.g. Alexander et al., 1979; Alexander, 1987; Bowles et al., 2003). A system of ‘socially imposed monogamy’ (Alexander et al., 1979, p. 420) would reduce within-group competition by suppressing differences in reproductive success among men. Because of the attendant increase in within-group co-operation, societies adopting this strategy would have an advantage in competition with other groups. This would enable the cohesion of increasingly larger societies, ultimately leading to the formation of large nations (Alexander et al., 1979; Alexander, 1987). However, social monogamy long predates the establishment of large nation states (Herlihy, 1995): while the diffusion of norms prescribing monogamous marriage is commonly attributed to the spread of Christianity, restrictions on polygynous marriage appear in the earliest historical records (Westermarck, 1921). For instance, Babylonian men were legally entitled to an additional wife only under special circumstances, such as illness or infertility of the first (as documented by the Codex Hammurabi, early second millennium bce); strict monogamy is the only legally recognized form of marriage documented for ancient Greece and Rome (Herlihy, 1995; Scheidel, 2009).

More importantly, the ‘socially imposed monogamy’ model rests on the assumption that monogamous marriage significantly reduces the variance in male reproductive success (Alexander et al., 1979). However, the historical and ethnographic evidence show that dominant individuals invariably attain extraordinary reproductive success even where marriage is strictly monogamous (Herlihy, 1995; Low, 2003; Scheidel, 2009). Ancient Rome is a case in point: despite the fanatical prescription of monogamous marriage, wealthy men fathered children by large numbers of slave women (Betzig, 1992a,b; Herlihy, 1995; Scheidel, 2009). Consistently, across data for 18 modern populations collated by Brown et al. (2009) we found no significant difference in variance in male reproductive success between societies practising monogamous marriage (n = 6, median: 10.0, range: 2.3–23.6) and societies practising polygynous marriage (n = 12, median: 10.4, range: 8.1–24.4) [Mann–Whitney U = 27.00, z = −0.84, n.s., r = −0.20. We coded societies on marriage strategy based on information in the original references in Brown et al. (2009), or references therein; our coding corresponds to the mating system coding in Brown et al. (2009), except for the Pimbwe, Dobe !Kung, and Ache, which we coded as practising polygynous marriage].

This evidence suggests that monogamous marriage may have evolved as a form of ‘monogamous transfer’ of a man's resources rather than as a form of monogamous mating.

Objectives and rationale

Here we address the question of the function of marriage strategies, that is, of their adaptive value in terms of differential reproduction, and show that the evolution of monogamous marriage can be understood within the framework of inclusive fitness theory (Hamilton, 1964a,b). At this ultimate level of explanation, we can ask evolutionary questions about cultural behaviours—that is, behaviours that are acquired through social transmission (Richerson & Boyd, 2005)—without reference to the underlying mechanism of transmission (Dunbar & Barrett, 2007). How a given behaviour is transmitted, whether genetically or through social learning, is a proximate question (West et al., 2007).

We proceed in three steps. In the remainder of this section, we identify two candidate factors that can make social monogamy, as a form of ‘monogamous transfer’ of resources, advantageous over alternative marriage strategies. Next, we develop a game-theoretic model of the strategic behaviour of males and females in the allocation of resources to the next generation to show that these factors can indeed result in monogamous marriage as a stable evolutionary strategy. Finally, we discuss previous anthropological observations on the history and cross-cultural distribution of marriage strategies in the context of the model, and briefly outline specific predictions to be tested against the archaeological, historical, and ethnographic data.

Evolutionary accounts of marriage strategies typically assume that male reproductive success is constrained by access to females. However, in traditional human societies where individuals hold rights to property, inherited wealth is a key determinant of reproductive success, and reproductive opportunities may be constrained more by ownership of resources than by access to mates. In these societies, individuals are expected to transfer resources across generations in ways that maximize the effect of the resources on their inclusive fitness (Rogers, 1990; Hrdy & Judge, 1993). To the extent that there is a trade-off between transmitting genes and transmitting wealth to the next generation (Rogers, 1990), in some cases the optimal strategy may be to concentrate resources in a limited number of heirs. By definition, social monogamy channels a man's property to the offspring of a single wife; additionally, unigeniture (e.g. primo- or ultimogeniture) may be used to avoid partitioning resources among them. In contrast, the property of a polygynous man is typically divided among his wives’ offspring (although unigeniture may apply within sets of siblings by the same mother) (Gray, 1964; Mair, 1971; Goody, 1976). This suggests that social monogamy may be advantageous where partitioning of resources causes a depletion of their fitness value.

But in humans, as in other sexually reproducing species, the reproductive interests of individuals in a socially monogamous pair only coincide if the male is the biological father of the female's offspring (Alexander, 1987). Therefore, males need to balance the benefit of investing in closely related heirs with the risk of investing in someone else's offspring. If a man has a low probability of being the biological father of his wife's children, he may be better off investing in his sister's: relatedness to a sister is always certain (through one's mother), as is relatedness to her offspring (Alexander, 1974; Greene, 1978). In fact, the transfer of a man's property to his sister's sons is common across societies with frequent female extramarital sex (Flinn, 1981; Hartung, 1981). We extend this reasoning to incorporate the strategic behaviour of females: if natural selection favours males who allocate resources based on their level of paternity, in turn it may favour females who allocate paternity based on the degree of male investment in their offspring. The resulting trade-off between paternity and investment of resources may lead to social monogamy: males would benefit from increased levels of paternity in their wife's offspring, and females from exclusive investment of their husband's resources in their own offspring. Of course, this mechanism can only operate if males have cues about paternity. In humans, in addition to direct phenotypic cues (see discussion in Geary, 2006), indirect behavioural cues may include the conformity of females to norms regulating their sexual behaviour; such norms are found in the vast majority of societies (Broude & Greene, 1976).

Theoretical framework

  1. Top of page
  2. Abstract
  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information

We capture these intuitive arguments with a game-theoretic model, described in detail in the Supporting Information. We focus on a population in which both males and females marry either monogamously or polygamously, with w ≥ 1 wives for males and h ≥ 1 husbands for females. Males transfer resources to the next generation ‘vertically’ to their wives’ offspring or ‘diagonally’ to their sister's offspring (Fig. 1). Females produce one male and one female offspring; each sibling pair inherits resources δ from the parent generation, with δ = δm + δf = 1 in a monogamous population in which all males transfer vertically, and 0 ≤ δm ≤ 1 and 0 ≤ δf ≤ 1 the male and female contributions to δ. The fitness of each sibling pair is given by δz, with z > 0; for z > 1, the fitness value of δ is depleted when δ is partitioned among the offspring of multiple wives.

image

Figure 1.  Inclusive fitness contributions for a focal male inline image and a focal female inline image. In the parent generation crosses represent marriages, solid lines represent brother–sister relationships (inline image’s husband; inline image’s wife; inline image’s brother). In the offspring generation inline image and inline image each represent a sibling pair (inline image’s offspring; inline image’s offspring). Dashed arrows represent resource transfers from parent to offspring generation. βi represents resources transferred to inline image (β1 if inline image transfers vertically; β2 if inline image is polyandrous and her other husbands, excluding inline image, transfer vertically; β3 if inline image does not marry; β4 if inline image marries and transfers diagonally); β2 is grey to indicate that the resources are transferred by inline image’s husbands other than inline image (which are not shown), rather than by inline image herself. φi represents resources transferred to inline image (φ1 if inline image does not marry; φ2 if inline image marries and transfers diagonally; φ3 if inline image does not marry; φ4 if inline image marries and inline image transfers vertically). See text and SI Text for details.

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The probability p that a male is the biological father of his wife's offspring depends on the behaviour of females, who give their husbands either ‘high’ paternity pH or ‘low’ paternity pL, with 0 < pL < pH ≤ 1. Females obtain an additional generic advantage α from mating with other males besides their husbands, with αL > αH, and αH = 0 for pH = 1. Males can infer their level of paternity from phenotypic or behavioural cues.

Schematically, the inclusive fitness payoff for a focal male inline image in the parent generation is given by the fitness value of resources βi, inherited by the offspring inline image of his inline image wives, plus the fitness value of resources φi, inherited by his sister's offspring inline image, each scaled by the respective coefficient of relatedness (inline image or inline image) (Fig. 1; Table 1; SI Text). The subscript i = 1,…,4 denotes the pathway through which resources are transferred to the heir, as per Fig. 1; inline image for a mutant focal male whose marriage strategy differs from the strategy of resident males, and inline image in all other cases. Resources are divided equally among the male's wives’ offspring. This can be written as

  • image
image

Figure 1.  Symbols used in the inclusive fitness payoffs.

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Similarly, the inclusive fitness payoff for a focal female inline image is given by the fitness value of resources βi, inherited by the offspring inline image of her brother's w wives, plus the fitness value of resources φi, inherited by her offspring inline image, each scaled by the respective coefficient of relatedness (inline image or inline image), plus any advantage inline image she obtains from mating with other males besides her husbands (Fig. 1; Table 1; SI Text). As in the previous case, the subscript i = 1,…,4 denotes the pathway through which resources are transferred to the heir, as per Fig. 1; inline image for a mutant focal female whose paternity strategy differs from the strategy of resident females, and inline image in all other cases. This can be written as

  • image

The possible combinations of male and female strategies differ in inclusive fitness payoffs; given these payoffs, we can derive evolutionarily stable equilibria consisting of a pair of male and female strategies that cannot be invaded by rare mutants playing alternative strategies (Maynard Smith, 1982) (SI Text).

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information

Social monogamy is a stable evolutionary outcome under two scenarios (SI Text and Table S1); both require ‘suspicious’ males, that is, males who transfer vertically if females are monogamous and provide ‘high’ paternity, diagonally otherwise. In the first scenario females always provide ‘high’ paternity. In the second scenario females are ‘astute’, that is, they provide ‘high’ paternity if males are monogamous, ‘low’ paternity otherwise. Both combinations of male and female strategies result in monogamous marriage, vertical transfer, and ‘high’ paternity.

Figure 2 illustrates the two scenarios for pH = 1. In the first case, monogamy can be advantageous where there is a fitness cost to dividing resources among the offspring of multiple wives (i.e. for z > 1; condition b in Table 2; Fig. 2a). In the second case, because of the strategic behaviour of females, polygynous males suffer a reduction in relatedness to wives’ offspring; consequently, monogamy can be advantageous irrespective of whether the fitness value of resources is depleted through division (i.e. for z > 0; condition b in Table 2; Fig. 2b). Vertical transfer can be advantageous where the benefit to a man of providing extra resources to his sister's offspring is offset by their lower relatedness relative to wife's offspring (i.e. for z below the threshold specified by condition c in Table 2; Fig. 2a and b). Monogamy and vertical transfer become increasingly advantageous as each wife provides a relatively smaller share of the resources inherited by her offspring (i.e. as δm increases and/or wP decreases; conditions b and c in Table 2; Fig. 2a and b). Additionally, in the second case the benefit to monogamy increases as the relatedness between a polygynous male and his wives’ offspring decreases (i.e. as pL decreases; for pL < 1/wP, any potential fitness benefit to polygyny is offset by the reduction in relatedness to wives’ offspring, such that monogamy is stable for all values of δm; condition b in Table 2; Fig. 2b).

image

Figure 2.  Stability of ‘suspicious’ monogamous males against mutant males with wP = 2, wP = 4, or wP = 8, for pH = 1; wP denotes the number of wives for polygynous males, and pH the paternity level of males with females who always provide ‘high’ paternity and of monogamous males with ‘astute’ females. δm represents the relative male contribution to the resources transferred to the offspring generation; z describes the relationship between resources and individual fitness; pL represents the paternity level of polygynous males with ‘astute’ females. See text and SI Text for details. (a), with monogamous females who always provide ‘high’ paternity. a is the condition for stability against monogamous males who transfer diagonally, b against polygynous males who transfer vertically, c against polygynous males who transfer diagonally (see Table 2). Monogamy is stable in the darker area, where all conditions are met. (b), with ‘astute’ monogamous females. Monogamy is stable throughout the volume shown.

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Table 2.   Conditions for the stability of ‘suspicious’ monogamous males for pH = 1.
Notation*Condition†Strategy of mutant males
  1. *Corresponds to the notation used in Fig. 2a. See SI Text for details.

  2. wP > 1 denotes the number of wives for a polygynous male.

  3. pL = 1 with females who always provide ‘high’ paternity.

az < log 3/log 2Monogamous marriage with diagonal transfer
bwP(δm/wP + δf)zpL < 1Polygynous marriage with vertical transfer
c(2δm + δf + wPδf)z < 3Polygynous marriage with diagonal transfer

Figure S1 shows that these results hold for values of pH < 1. Here pH = 0.5, which is likely an extremely low value of pH: men would attain on average as much reproductive success by other men's wives as by their own in a society with a paternity level of p < 0.5 (Hartung, 1981). For comparison, in contemporary populations men are the biological fathers of their putative children, on average, in 98.3% of cases if they have high confidence of paternity, and in 70.2% of cases if they have low confidence of paternity; actual paternity levels must fall between these values for most societies (Anderson, 2006). Assuming that comparable paternity levels characterized our species’ recent evolutionary past, this suggests that social monogamy represented a stable outcome in the evolution of human social systems.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information

We developed a game-theoretic model to investigate whether monogamous marriage can be viewed as the outcome of the strategic behaviour of males and females in the allocation of resources to the next generation. The model showed that where resources are linked to fitness and are transferred across generations, social monogamy can be a stable evolutionary outcome (i) if dividing resources among the offspring of multiple wives causes a depletion of their fitness value, and/or (ii) if females grant husbands higher fidelity in exchange for exclusive investment of resources in their offspring. In both cases, the benefit to monogamy increases as the relative contribution of resources by females decreases.

These findings suggest that monogamous marriage can be understood within the framework of inclusive fitness theory. In turn, this challenges previous evolutionary explanations for the emergence of monogamous marriage, and for variation in marriage strategies across societies more generally: the former assume the implication of group-level processes, while both assume that male reproductive success is always maximized by polygynous marriage or, equivalently, that variance in male reproductive success is always greater under polygynous than under monogamous marriage (Low, 2003, 2007). The framework we develop makes both assumptions unnecessary. Rather, it shows that where resources are transferred across generations and are linked to fitness, whether monogamous or polygynous marriage represents the optimal strategy for males depends on whether the value of the resources they provide is depleted through division among multiple heirs; some form of division is inevitable if multiple wives are involved.

Further, this framework extends current evolutionary explanations for transfer strategies, which rely on the notion of strategic male behaviour (e.g. Alexander, 1974; Greene, 1978; Kurland, 1979), to incorporate the strategic behaviour of females: if selection favours males who allocate resources strategically, based on their level of paternity, it is also likely to favour females who allocate paternity strategically, based on the level of male investment in their offspring. This simple extension has important implications for analysis of the evolution of marriage strategies, leading to a situation where both males and females stand to gain from monogamous marriage: males benefit from investing resources ‘safely’ in the individuals in the next generation that provide the greatest potential fitness returns, that is, their wife's offspring; females, in turn, benefit from exclusive investment of their husband's resources in their own offspring. In a similar way, this extension is likely to have important implications for analysis of the evolution of other aspects of human social organization that are linked to transfer strategies: for example, the notion of strategic male behaviour in this context underlies current explanations for the evolution of descent systems (see review in Cronk & Gerkey, 2007).

The historical and ethnographic evidence suggest that these mechanisms likely operated in shaping the evolution of human social systems. In the Old World, polygyny prevails among African societies with subsistence economies based on pastoralism or extensive agriculture (Goody, 1976). The relationship between resources and fitness documented for the Gabbra pastoralists of Kenya (Mace, 1996) and for the Chewa horticulturalists of Malawi (Holden et al., 2003) indicates that in pastoralism and horticulture the fitness value of resources is not depleted through division. Among the Gabbra, for example, parents provide on average 10 camels to marry off a son: three as bridewealth to the bride's kin, and seven to the groom for starting an independent household (Mace, 1996). If the division of resources depleted their fitness value, the reproductive success of men owning five camels would be less than half the reproductive success of men owning 10. Conversely, men with five camels have more than half the reproductive success of men with 10 (Mace, 1996). This is likely because in both subsistence systems productivity is constrained more by availability of labour than by ownership of the primary productive resources (Goody, 1976): in pastoralist societies holdings of livestock can easily be increased through husbandry; in horticultural societies the low productivity afforded by extensive agricultural techniques means that land is rarely a scarce resource (Gray, 1964; Goody, 1976).

This is in stark contrast with the intensive agriculture practised in the historical societies of Eurasia, where irrigation and ploughing led to increased productivity, which in turn sustained continued population growth. Combined, increased productivity and population growth caused shortages of land. As land scarcity increased, so did the pressure to keep holdings above the minimum size required to set up a viable productive and reproductive unit (Goody, 1976; Hrdy & Judge, 1993). Under these conditions of habitat saturation, the partitioning of estates depleted their value; in extreme cases the reduction in value was so great that parents commonly designated a single heir, at the expense of all other offspring, through systems of unigeniture (Hrdy & Judge, 1993; e.g. Boone, 1986, 1988; Voland & Dunbar, 1995).

Consistent with our finding that social monogamy can be advantageous where the value of resources is depleted through division, marriage was typically monogamous in the agrarian societies of Eurasia with economies based on intensive agriculture (Goody, 1976). In line with our expectations, the relative contribution of women to production is lower in these societies compared to other subsistence systems (Murdock & Provost, 1973; Goody, 1976). Indeed, across societies access to new land for expansion is a key ecological determinant of polygyny (White & Burton, 1988), and within societies the incidence of polygyny declines with increasing scarcity of land (White, 1988). This raises the possibility that restrictions on polygynous marriage emerged in the ancient societies of Eurasia following the adoption of intensive agriculture, as ownership of land became increasingly critical to economic success, and growing shortages of land imposed greater costs on partibility. Cultural norms promoting high paternity, such as ideologies of honour, virginity, and sexual fidelity, were common in these societies (Mair, 1971; Scheidel, 2009). To the extent that these norms resulted in an increase in average relatedness between a man and his wife's offspring, our findings suggest that they may have facilitated the establishment of social monogamy in this region.

The model generates the following predictions about the cross-cultural distribution and history of marriage strategies, to be tested against the ethnographic, archaeological, and historical data. First, the stability of monogamous marriage requires that men transfer resources vertically, that is, to their wife's offspring. Therefore, we predict the cross-cultural data to reveal an association between monogamous marriage and the transfer of a man's property to his wife's offspring. Second, we expect the archaeological evidence to show that the emergence of monogamous marriage was linked to the development of intensive agricultural techniques, possibly coupled with the establishment of social norms promoting high paternity. Analogous property considerations may help explain historically attested transitions between marriage strategies within societies, such as the recent shift from polygyny to monogamy in several Muslim countries, or the shift from monogamy to polygyny among the Mormons during the 19th century (Cairncross, 1974).

Of course, any model can capture but a small fraction of variation in human social systems, and must overlook the many historical contingencies, such as the diffusion of religious beliefs (e.g. Goody, 1983), that may have influenced their development. Yet placing this variation within an inclusive fitness framework allows us to conceptualize general evolutionary mechanisms shaping the organization of human societies. This finally resolves the crux of anthropological discussions about whether the primary function of marriage is ‘economic and productive’ or ‘sexual and reproductive’ (Goody, 1973, p. 189). In evolutionary terms, the proximate economic determinants of marriage underlie its ultimate reproductive function.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information

We thank A. Grafen, K. Laland, R. Mace, R. Sear, S. Shennan, J. Wettlaufer, and D. Yu for comments on the manuscript, W. Scheidel for access to an unpublished manuscript, G. Brown for discussion of issues relating to the coding of mating systems versus marriage strategies, and C. El Mouden, A. Gardner, and two anonymous reviewers for feedback during the review process. L. F. was funded by the Economic and Social Research Council UK and by the UCL Graduate School. M. A. was supported by the Human Frontier Science Program Organization and by St John's College, Oxford.

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  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Theoretical framework
  5. Results
  6. Discussion
  7. Acknowledgments
  8. Supporting Information

Data S1 Theoretical framework.

Table S1 Summary of the possible strategies.

Table S2 Symbols used in the model.

Figure S1 Stability of ‘‘suspicious’’ monogamous males for pH = 0.5.

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