Egalitarian Sharing Explains Food Distributions in a Small-Scale Society

Introduction

Small-scale societies where individuals and families meet their daily subsistence needs by consuming wild foods and cultigens usually place a strong cultural emphasis on the virtues of giving to those in need. Among people as diverse in geographical location and cultural background as the Batek of Malaysia (Endicott 1988), the !Kung of Botswana (Lee 1979), and the Yanomami of Venezuela (Hames 2000), sharing of food is understood as an obligation of the giver and a right of the receiver, and those who refuse to share are invariably faced with accusations of stinginess and other forms of diffuse social pressure to grant the request. Such food-sharing practices may be understood as egalitarian to the extent that their leveling outcomes arise from the interactions of individuals proximately driven by leveling concerns to keep in check the asymmetric accumulation of valued resources within the group. Social anthropologists generally concur that food-sharing arrangements in small-scale societies are part and parcel of the broader repertoire of leveling strategies that prevent would-be dominants from exercising authority over potential subordinates (Boehm 1999, 2012; Clastres 1989; Endicott 1988; Fried 1967; Lee 1988; Wiessner 1996; Woodburn 1982). In the words of Boehm (1994, p. 180), such arrangements “would not be likely to work if decisive political power remained in the hands of a few”, while if valuable food items were not shared “it could be difficult to equalize power”.

In their empirical studies of human food sharing, however, evolutionary anthropologists rarely contemplated egalitarian motivations, drawing inspiration from sociobiological models in which sharing behavior is driven by various dispositions toward reciprocal risk-reduction, maximization of kindred benefits or status-seeking through signaling of individual qualities (see Gurven 2004; Hawkes et al. 2018; Kelly 2013 for reviews). The problem with these theories is that they ignore social role asymmetries: assortativity in cooperation is just as likely to include dominant as subordinate members of the social hierarchy (Gavrilets 2012). Consequently, they fail to accommodate the prevalent anthropological view of food sharing as a power leveling mechanism.

As a response to the insufficiency of traditional sociobiological models of cooperation, recent years have seen fundamental advances in evolutionary research that copes with the particular mechanisms leading to the evolution of egalitarian behavior and derived traits in humans (Boyd et al. 2011; Gavrilets & Richerson 2017; Hill et al. 2011; Silk & House 2016). Two prominent approaches are now well-established in the literature. The first suggests that dominant behavior can be suppressed to the extent that individuals are able to build and sustain reputations for their skills, knowledge and generosity that may bring about favorable relationships with social partners. If adaptive benefits such as social support, advantageous alliances, political influence or access to mating opportunities are more effectively obtained through freely conferred deference of followers rather than violent coercion over subordinates, dominance hierarchies may be expected to at least partly subside in favor of more stable status hierarchies (Cheng 2020; Henrich & Gil-White 2001). Reputation-based mechanisms for the evolution of cooperation are well-studied analytically (Gintis et al. 2001; Panchanathan & Boyd 2004), and associations between measures of social status and reproductive success have been consistently found in nonindustrial societies (von Rueden & Jaeggi 2016). A recent study among Tsimane forager-horticulturalists of Bolivia (von Rueden et al. 2019) provides the best example to date of how cooperative assortativity through status-seeking can play a role in the maintenance of relatively egalitarian relationships within a small-scale social group.

Although it offers a plausible alternative for the explanation of human egalitarianism, the reputational approach should be taken with a grain of salt. Theoretically, it seems to share some of the basic issues of earlier sociobiological accounts of cooperation. Status-seeking is at best an indirect motivation underlying the leveling of power asymmetries, which fails to account for the widespread view that egalitarian social relationships are primarily sustained by conscious choice of leveling strategies in small-scale societies. Besides, none of the available mathematical models of reputation-based cooperation is framed on an explicit representation of asymmetric social roles. Empirical research, on the other hand, repeatedly failed to find support for status-seeking accounts of human cooperation in egalitarian settings. The major proxy of social status in hunter-gatherer studies – hunting reputation – is probably too costly and noisy to serve as an effective reputational index (Stibbard-Hawkes et al. 2018). More generally, status-seeking strategies require that favorable relationships accumulated by well-reputed individuals be relatively stable in time. However, longitudinal data from a field experiment suggest that stable partnerships and dispositions have little to do with how cooperation is maintained in an egalitarian society (Smith et al. 2018); rather, cooperative assortativity in this social setting depends on the cooperativeness of one’s current residence group, which suggests a role for local norms in sustaining cooperation. Finally, in at least two well-studied egalitarian societies no evidence was found to support the notion that skillful, knowledgeable or well-reputed individuals accumulate adaptive advantages. Kraft et al. (2019) found that measures of foraging performance, prosociality and kin presence fail to predict lifetime reproductive success among the Batek of Malaysia; and Smith (2019) reported that being a preferred social partner or having stronger reciprocal ties with partners is not associated with higher reproductive success among the Hadza of Tanzania. All these findings agree with observations on the successful leveling of status-seeking behaviors in hunter-gatherers (Wiessner 1996), which suggests that human egalitarian tendencies do not easily yield to reputation-based accounts of cooperation.

A more promising approach emphasizes the role of leveling strategies in supporting the capacity of human groups to enforce strong egalitarian norms among its members and punish antisocial behaviors like dominance, aggrandizing, and hoarding (Boehm 1993; Erdal & Whiten 1996; Power 2009). A rich set of mathematical models now provide a comprehensive picture of how cooperation can evolve in socially asymmetric settings either through third-party punishment performed by leveling coalitions (Boyd et al. 2010) or second-party punishment such as in shunning and withdrawing from cooperative relationships with dominant individuals (Hooper et al. 2021). Most notably, the role of food sharing as a power leveling mechanism that could lead to the disruption of primate-like dominance hierarchies has been mathematically modeled (Gavrilets 2012). Considering the theoretical power and empirical plausibility of this approach, it is surprising that it remains unexplored in quantitative empirical studies of human food sharing.

In this paper, I begin by discussing an evolutionary theory that makes full sense of the anthropological view of leveling behavior as the main driver of hunter-gatherer sharing interactions. Next, I use an agent-based modeling approach to constructively test three theoretical predictions about the institutional architecture of decentralized food sharing systems. The agent-based model uses quantitative data on Hadza hunters’ population and individually heterogeneous food production patterns to track the aggregate results of spatially explicit sharing interactions across several empirically relevant dimensions. By means of a set of evolutionarily plausible sharing interaction rules, I show that Hadza patterns of inter-household food distributions can be accurately predicted under the assumption that agents selectively support and carry on sharing interactions in ways that maximize their income leveling potential within the group. Finally, I discuss model results in relation to previous evolutionary accounts of food sharing and cooperation in small-scale societies and consider opportunities for improvement in its theoretical basis and construction.

Evolution of the Egalitarian Social Instinct

The rationale behind the idea that sharing constitutes an intentional leveling device is pretty straightforward. Given that leveling of political power is a fundamental aspect of social organization in most small-scale human groups and that power leveling requires leveling of material resources that may be used strategically to disrupt the balance of power among individuals and families, then it makes sense to explain distributions of all such strategic resources based on the egalitarian hypothesis that sharing individuals are intentionally committed to the goal of reducing political inequalities that may arise from resource inequalities. Food sharing understood as consumption leveling is of course just a particular application of this rationale.

A more difficult challenge is to understand the evolutionary trajectories that could have lead to the emergence of egalitarian sharing behavior in the human species. Among non-human primates, most certainly, intentional leveling of food consumption is nowhere to be found. It would seem natural therefore to hold culture accountable for this radical innovation (Boehm 1993; Knauft 1991). But it so happens that sharing constitutes part of a common core of leveling devices found in very disparate cultural settings within an environment that is likely to have characterized early human evolution (Boehm 2012; Shultziner et al. 2010; Whiten & Erdal 2012). Besides, the very functionality of those leveling institutions seems to depend on flexible cognitive capacities and motivational dispositions that go beyond what is merely required by norm conformity or cultural adaptation. As Lee (1990) persuasively argues, the institution of sharing cannot simply amount to a set of cultural norms stipulating the ceiling of accumulation above which one cannot rise and the floor of destitution below which one cannot sink; rather, the idea is that ceiling and floor are closely articulated through a series of decentralized interactions that yield consumption leveling as their social outcome. For this process to be set in motion, individuals should be able to gather and process relevant information available in their social environment in order to correctly gauge the extent to which decentralized food distributions promote consumption leveling in the group as a whole. More than conformity to cultural norms, then, hunter-gatherer food sharing depends upon evolved psychological dispositions toward egalitarian behavior.

Such dispositions are predicted by Gavrilets (2012) in his evolutionary account of the egalitarian social instinct. The author starts from the assumption that human egalitarian tendencies evolved in a context of dyadic costly disputes over resource ownership. He describes the fitness \(w_{i}\) of a focal agent \(i\) is a function of resources \(R_{i}\) accumulated by \(i\) as well as resources \(R_{j}\) accumulated by every agent \(j\) who has a stake in the conflict:

\[ w_{i} = \frac{f(R_{i})}{\sum_{j}f(R_{j})}\]

\[(1)\]

so that that an individual’s fitness depends not only on resources she can accumulate but also on the outcome of interactions in which she does not participate directly. Given the assumption that function \(f(R)\) grows faster than linearly, this formula implies that agent \(i\) faces an incentive not only to maximize her own wealth (greater numerator) but also to minimize wealth asymmetries among other agents (lower denominator).

Individual behavior optimized by natural selection under these assumptions leads to a couple of compelling evolutionary scenarios. First, in the absence of coalitions, modeling dyadic ownership disputes as a hawk-dove interaction (Hamilton 1981) generates a stable dominance structure in which weaker individuals usually give up their resources without fighting and only few males father most of the group offspring. Consistent with dominance hierarchies generally observed in non-human primates, this initial scenario provides an acceptable baseline from which to explore other evolutionary possibilities. Gavrilets expands the basic model by allowing doves to receive help from a leveler who incurs helping costs without deriving any direct benefits from cooperation. Even in this restricted coalition of only two members, it is shown that the population of levelers can become stable and generate substantial increases in group-level equality regarding both food distributions and male reproductive success. Key to this egalitarian outcome is that coalition synergy – the strength of coalitions above and beyond the sum of the strengths of individual members – be sufficiently high, while group size and strength assessment error by parties involved in the dispute be sufficiently low. As long as these conditions are fulfilled, levelers can successfully explore interdependent gains from intervening in costly disputes as third-parties.

This evolutionary account of the egalitarian social instinct obviously cannot be directly extrapolated to the ritualized, culturally elaborate, consciously intentional food sharing practices of evolved human societies. However, available empirical evidence suggests that its main assumptions succeed in capturing culturally-invariant regularities in food-sharing behavior observed in small-scale societies. First, the theoretical assumption that self-interested psychological motives are not easily suppressed by the egalitarian drive to share is well documented by ethnographic reports. Intense arguments and jealousies over food distributions, as well as occasional attempts at stealing and cheating, attest against any simple predisposition to share (Erdal & Whiten 1996). Second, that these recurring self-interested behaviors are successfully suppressed by coordinated action of group-members directed at the leveling of power and resource asymmetries within the group is a widely held view among anthropologists (Boehm 1999, 2012; Clastres 1989; Endicott 1988; Fried 1967; Lee 1988; Wiessner 1996; Woodburn 1982). In their descriptions of the hunter-gatherer "sharing ethos", ethnographers have long observed that command of social support is paramount and that interactions are framed in terms of entitlement of those in need to receive shares. Refusing to share food is usually not an option, since individuals can help themselves to it and take resources from unwilling givers without violating property norms (Endicott 1988; Peterson 1993). Even so, those who acquire a reputation for stinginess tend to elicit the anger of other group members and become subject to various forms of social pressure that can go from rough jokes to more serious sanctions like ostracism.

If such a combination of individual self-interest in resource accumulation and successful coordinated action toward its suppression were drivers of the egalitarian equilibrium in early human societies, at least three predictions about the general normative structure of sharing interactions in extant egalitarian hunter-gatherer groups should hold:

• Entitlement to receive shares. Social support should be granted to poorer individuals in sharing interactions based not on considerations about their level of destitution, but rather on the extent to which helping them contributes toward maximizing group-level equality.
• Giver choice. Poorer individuals who are socially supported in sharing interactions should preferentially seek to receive shares from the wealthiest among potential givers, thereby maximizing not only direct gains to themselves but also indirect gains to others upon which their social support relies.
• Equality. Because any resulting income asymmetry is detrimental to every interested party except for the eventual hoarder, the outcome of a sharing interaction should tend toward the pooling and equal division of the total amount of resources held by all those directly involved.

None of these predictions is easy to test against quantitative data collected in the field. Nevertheless, they can be jointly implemented within a modeling environment designed to simulate food-sharing patterns like those reported in field studies. In the next section, I describe a spatially explicit agent-based model of food distributions in which sharing interactions follow a plausible set of rules derived from each of the predictions above.

Model Design

To investigate which distribution patterns can be generated from a set of egalitarian sharing interaction rules, an agent-based model incorporating key aspects of an extant hunter-gatherer population was developed (see Model Documentation). The model consists of three components, the first of which sets out the main structural features of an agent population, while the others are designed to track changes in caloric incomes of each individual agent as they acquire wild foods and then engage in give-and-take of acquired resources with group co-residents from other households. Together, the three model components allow us to investigate aggregate distribution patterns that emerge from structured, rule-governed interactions among individuals given the most salient features of their social and economic environment, which include group size, individual heterogeneity and variability in acquisition. A full run of the model comprises a number of time-steps, each corresponding to a “foraging day” in which caloric returns are stochastically assigned to agents before spatially explicit sharing interactions take place. See Appendix B for a detailed model description.

Population

The main simulations and analyses presented below revolve around a small local group of seven agents representing their respective households. The size of this population is not arbitrary, but reflects the average number of married hunters studied by Wood & Marlowe (2013) across seven different Hadza camps. Because individual heterogeneity in food acquisition among these agents is key to understanding their sharing behavior, the group was further divided into three types consisting of one top, three regular, and three poor agents. This division is intended to take advantage of Wood and Marlowe’s data on food production and distribution relative to a sample of the best, median and poorest married Hadza hunters from each camp. The distribution of agents across types is the result of an educated guess based on average group size together with the fact that hunting success follows a right-skewed distribution among the Hadza.

Foraging

In groups like the Hadza, where hunting is predominantly a solitary activity, the returns of an individual hunter generally depend on his level of skill and luck, both of which are closely associated with the particular resource types he targets. Accordingly, total daily acquisitions for each agent are recorded anew at the beginning of every time-step of the model. These values are defined as the sum of calories each agent obtains according to his success probabilities and average returns in foraging for two types of resources: large game and other foods. Success probabilities and average returns of an individual agent for both resource types depend on whether he is a top, regular or poor hunter, and are modeled after empirical values recorded for the Hadza. While this approach simplifies day-to-day variability in sizes and amounts of food items targeted by Hadza hunters, it adequately represents individual heterogeneity in success and returns. Table 1 displays the food acquisition data used in the foraging component of the model.

Table 1: Hunters’ acquisition and success rates for different food types.

Daily acquisition rates
	Top hunters	Regular hunters	Poor hunters
Large game	15,800 kcal	1,000 kcal	-
Other foods	1,000 kcal	1,700 kcal	1,350 kcal
Daily success rates
	Top hunters	Regular hunters	Poor hunters
Large game	2 %	2 %	-
Other foods	37%	43%	40%

Sharing

After foods are brought into Hadza camps, members from different households typically approach the households of successful acquirers to receive a share. Large game meat tends to attract the attention of crowds, but individuals often also receive shares from other food types, which they may take to their own households for later consumption or else eat on the spot in a shared meal. Following the spatial organization of Hadza camps, agent households are more or less evenly spaced from each other and are positioned at the center of a round-shaped area where sharing interactions take place. At each time-step of the model, after individual foraging returns are assigned, agents proceed to the sharing of the available bounty following three basic rules that describe the structure of a decentralized hunter-gatherer food distribution system according to the predictions of the theoretical model discussed above.

The first rule defines which agents are granted decisive social support to claim shares from others and are therefore entitled to receive shares. Following the first theoretical prediction, entitlement to receive shares is operationalized in terms of an individual’s need relative to the current level of group inequality:

• Let \(S\) be the set including the incomes of all group members. For each agent \(i\) whose income is lower than the highest income in \(S\), let \(S_{-i}\) be the subset of \(S\) formed by excluding \(i\)’s income. Then find the Gini-index of income inequality for the set \(S\) and for its subset \(S_{-i}\). If the former inequality measure exceeds the latter, then agent \(i\) is entitled to receive shares.

Next, each agent who was found entitled to receive shares initiates a sharing interaction by choosing a giver. Consistent with the second prediction, agents preferentially choose to receive shares from the wealthiest among all potential givers. This is reflected in the following income-based probabilistic rule of partner selection:

• For each agent \(i\) who is found entitled to receive shares, let \(A\) be the set including every other agent with income greater than \(i\). Then select one member \(j\) of the set \(A\) randomly with probability given by the ratio between \(j\)’s income and the total summed incomes of all \(A\) members. Last, move \(i\) to the household area of \(j\).

Finally, after all eligible agents are positioned in the household area of their respectively chosen givers, mandatory food transfers take place following a simple rule by means of which equality is maximized among all those involved in the local sharing interaction:

• For each agent \(i\) who belongs to the group, let Ri be the set including every other agent located in the household area of \(i\). If \(R_{i}\) is non-empty, pool the incomes of \(i\) and every \(R_{i}\) member together and divide the total sum equally among them.

After food transfers take place, agents return to their respective households. The cycle repeats itself until no more agents in the group are found entitled to receive shares. See Appendix C below for a numerical example of how this set of sharing interaction rules is implemented.

Results

Wood & Marlowe (2013) datasets for primary distributions of large game meat and complete distributions of other food types can be directly compared to a total of seven different outcome variables of the model describing the following dimensions of Hadza food distributions: sharing depth by hunter type – mean percentage of total kilocalories acquired by individual hunters and shared with other households, disaggregated by hunter type (top, regular, poor hunters); sharing depth by food type – mean percentage of total kilocalories acquired by individual hunters and shared with other households, disaggregated by food type (large game meat and other types of foods); sharing breadth by food type – mean percentage of potential receiving households that actually receive shares from individual hunters, disaggregated by food type (large game meat and other types of foods). Model outputs for the seven outcome variables were averaged over 100 independent model runs of 1,000 time-steps each (see Operational details in Appendix A).

Figure 1 compares Hadza food-sharing patterns and simulation outcomes. The striking proximity between real and simulated values emerging spontaneously from the model’s sharing interaction rules is sufficient to validate the underlying theoretical predictions on the evolution of the egalitarian social instinct. Appendix A shows that there is little variation in outcome variables across independent simulations and that model results are fairly robust to variation in population and other parameter values within plausible ranges. Appendix D introduces random variations to each sharing interaction rule and discusses the results of a total of seven alternative models implementing different levels of randomization to the original model design. These results help to further validate the egalitarian model by showing that each of its interaction rules has independent positive impact on the model’s fit to the empirical data.

Discussion

This study represents the first attempt to test the predictions of an evolutionary model that explains food sharing as a power leveling mechanism in small-scale human groups. The agent-based simulations reported here show that seven key food-sharing patterns described in detail for the Hadza hunters of Tanzania can be simultaneously reproduced with striking accuracy under the assumption that individuals selectively favor and carry-on sharing interactions in such a way as to maximize the equality of income distribution within their residential groups.

The appeal to traditional sociobiological theories of cooperation in the context of small-scale human groups has been frequently criticized for sacrificing ethnographic realism in the name of evolutionary plausibility (Boehm 2012; Henrich 2018; Woodburn 1998). Among the most prominent of the theories originally developed by biologists and then adapted to the food-sharing realities of the Hadza and other small-scale societies, kin selection (Hamilton 1964) explains the evolution of cooperation through the inclusive fitness benefits of cooperating with kin; reciprocity (Trivers 1971) suggests that short-term fitness losses in cooperation can be offset by more substantial long-term fitness gains if individuals are able to favor reciprocators in their repeated interactions; and costly signaling (Gintis et al. 2001; Zahavi 1977) assumes that widespread cooperation works as an honest signal of individual qualities to an audience, thereby increasing the fitness-enhancing social support and attention cooperators receive from others. Among the Hadza, kin selection and reciprocity received limited support from empirical data. Wood & Marlowe (2013) report in their study that Hadza food transfers flow more often to close kin than to non-kin. Based on data collected in a previous field study, it has also been shown that there is modest but significant correlation between meat shares given and received among the Hadza (Jaeggi & Gurven 2013). Apart from consistency with these patterns, neither theory seems to afford an ethnographically plausible pathway to integrate substantial bodies of observations on food sharing among the Hadza or small-scale societies in general.

Kin selection does not explain widespread distributions of food to genetically unrelated others in small-scale societies. This is especially true about mobile hunter-gatherers, who are notable both for their prodigious food sharing and low intra-group relatedness (Hill et al. 2011; Walker 2014). Besides, pervasive cultural institutions like partible paternity (Walker et al. 2010), reflected in the lack of correspondence between kin terms and genetic relatedness typically observed in human societies (Cronk et al. 2019), also contribute to undermine the theory’s plausibility. Although potentially able to account for cooperation with non-kin, reciprocity cannot accommodate the outstanding imbalances documented in human food transfers. Among the Hadza studied by Wood & Marlowe (2013), best hunters who succeeded in large game hunting shared six times more food than median hunters each day on average (see ESM Table S4). Were this outstanding contribution disparity primarily sustained by reciprocal cooperation, one would expect best hunters and their families to be helped more often and more substantially than other group members on a daily basis. Yet this is likely not the case. For example, there has been found no evidence of nutritional advantage to being or marrying a well-reputed Hadza hunter (Stibbard-Hawkes et al. 2020).

Kin selection and reciprocity theories also beg the question of producer control over acquired resources. In spite of countless ethnographic accounts suggesting the opposite (Endicott 1988; Hawkes et al. 2001; Kent 1993; Woodburn 1982), supporters of both theories and associated hypotheses have often argued in favor of producer control based on the quantitative observation that individuals in small-scale societies tend to keep substantially larger portions of their production than the portions they give to others (Gurven 2004). In a recent contribution, I showed that this argument is misleading (Pinheiro 2021). Because income leveling entails net resource flows from the wealthier to the poorer, the average contribution rate for the population as a whole will tend toward the relatively low rates of the poorer whenever they outnumber the wealthier. Consequently, just about any successful income leveling strategy should be consistent with the observation that individuals keep larger average portions of their production to themselves than the portions they give to others.

Costly signaling theory provides an alternative explanation of why male hunters engage in widespread sharing of meat with unrelated non-reciprocators among the Hadza and other foragers (Hawkes et al. 2001, 2014; Hawkes & Bird 2002). However, signaling motives do not explain why individuals in small-scale societies often share reliable and easy-to-acquire food items that would seem to have little appeal to an audience. For example, what could be the signaling value of various fruits that Hadza hunters share with other households in a relatively modest but still significant proportion, usually in the context of one-on-one interactions removed from the eyes of others? Another difficulty with signaling theory is that interested audiences will be on the look out for both successes and failures of signalers, with the consequence that less capable producers should invest relatively less time and energy in subsistence activities with signaling potential (Stibbard-Hawkes et al. 2018). This is inconsistent with the low inter-individual variance in hunting effort generally observed among hunters. Finally, supporters of signaling theory have not been able to show that their assumed indirect benefits from cooperation are associated with any actual fitness gains. On the contrary, evidence shows that being a preferred social partner is not associated with higher reproductive success among the Hadza (Smith 2019), so that the point of signaling to gain favorable attention of others is moot in this empirical setting.

In contrast, the constructive agent-based modeling approach adopted here integrates a substantial sum of ethnographic observations within a sound evolutionary framework. By deriving its central assumptions from a plausible theory on the evolution of leveling coalitions and operationalizing the notion of entitlement to receive shares, the model makes sense of the prevalent anthropological view of food sharing as leveling behavior contingent upon coordinated group effort rather than the outcome of an individualized willingness to give shares. The agent-based approach developed here is also the first to reproduce patterns of both widespread large-game meat distributions and distributions of other food types based on a single coherent set of sharing interaction rules. Since givers are chosen randomly based on probabilities weighted by their income, receivers tend to flock around the households of large game meat acquirers – whose incomes are outstandingly larger than anyone else’s – even on days when most agents also acquired other foods. Conversely, on days when no large game is available, receivers are more likely to scatter around the households of agents who acquired other foods and whose incomes are therefore relatively undifferentiated from each other. This realistic spatial patterning of food distributions is reflected in the gap between the sharing breadth of large game meat and other foods reproduced by the model.

A potential objection that can be leveled against the present approach is that it relies on excessively demanding cognitive assumptions. In order to effectively determine who is entitled to receive shares, it is implicitly assumed that agents have accurate information about each other’s holdings, use that information to assess each other’s relative needs in ways consistent with Gini-index computations of inequality, correctly update all the relevant information at each sharing interaction, and do not face serious coordination problems in giving or withholding social support to potential beneficiaries. Although the model is neutral about how these informational, cognitive and organizational processes are proximally implemented, there are a number of reasons to believe they are effective in the context of small-scale egalitarian societies.

First, evidence from neurobiology and developmental psychology shows that inequality aversion is wired into the human brain’s reward circuitry (Tricomi et al. 2010) and is displayed by young children from an early age (Fehr et al. 2008; McAuliffe et al. 2017; Rizzo & Killen 2016), so that the capacity to correctly assess unequal distributions is likely part of the evolved human cognitive repertoire. When development of this capacity is encouraged through social learning, such as is certainly the case in small-scale egalitarian societies (Lew-Levy et al. 2018), individuals may come to age with a robust sense of fairness (Erdal & Whiten 1994) that equips them to make reliable judgments about distributive equality. Second, much of the monitoring costs and coordination problems involved in acquiring and updating information on resource distributions in a small-scale group may be mitigated through linguistic communication and gossip (Smith 2010), which allow the rapid acquisition and diffusion of fine-grained information at a low cost. Third, an interdependent psychology that instantiates common interest in group-level equality among all except the occasional hoarders has the potential to greatly facilitate the coordination of decentralized sharing interactions. Ethnographic accounts of the various gains to cooperation as well as the strong sense of interdependence associated with human small-scale group living suggest that these are privileged contexts for development of interdependent psychologies (Tomasello et al. 2012).

To the extent that food sharing is one from a diverse set of leveling strategies deployed in small-scale societies, it is unsurprising that it requires "a political capacity to operate in large coalitions and a cognitive capacity to arrive at a shared plan of action" (Boehm 1993, 1999). Granting and withholding social support to potential receivers in ways that effectively reduce group-level inequality may be a less daunting task if it is performed collectively rather than individually. Future studies should gain insight on proximate mechanisms that have the potential to engage collectivities in otherwise decentralized interactions and examine the possibilities and constraints related to social scale and organization. For example, future elaborations of the present model could represent the mechanism by which entitled receivers are identified as a process of social problem-solving engaging the collective intelligence of observers (Carletti et al. 2020).

Sharing rules derived from the theoretical predictions explored here do not exhaust the diversity of patterns and correlates documented by anthropologists in their field work among small-scale human groups. Yet one of the advantages of agent-based models is that they can be relatively easily expanded to generate more complex interactions and integrate larger datasets. Future developments of this egalitarian model should attempt to reproduce other documented features of sharing behavior in small-scale societies, like preferential sharing with kin. My provisional take on the matter is that most of the evidence on food sharing left aside in the present elaboration could be elucidated by taking into account the social embeddedness of sharing behavior. In this way, even if individuals preferentially seek high-income resource holders as sharing partners, the fact that they generally interact more often with relatives on a daily basis would suffice to explain a kin bias in their resource distributions. A similar constructive agent-based approach has been successfully applied to the study of patterns of social affiliation in primates (Puga-Gonzalez et al. 2009, 2015). At the ultimate level of evolutionary causation, an expanded theory of fitness interdependence has the potential to account not only for the human egalitarian social instinct but also for evolved patterns of preferential interaction with genetically related and fictive kin as well as friends (Aktipis et al. 2018). Further explorations of the fitness interdependence hypothesis therefore hold great promise to advance the study of sharing and cooperation in small-scale human groups as well as the understanding of decentralized resource distribution systems in general.

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