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Cooperation among relatives is often regarded as evidence of kin selection. Yet altruism not requiring shared genes can also evolve among relatives. If characteristics of relatives (such as proximity, familiarity, or trust) make kin preferred social partners, the primary causes of nepotistic biases may reside principally in direct fitness payoffs from cooperation rather than indirect fitness payoffs acquired from aiding collateral kin. We consider the roles of kin selection and reciprocal altruism in maintaining nepotistic food transfers on an Ache reservation in northeastern Paraguay. Households do not primarily direct aid to related households that receive larger comparative marginal gains from food intake as we would predict under kin selection theory. Instead, (1) food transfers favor households characterized by lower relative net energy production values irrespective of kinship ties, (2) households display significant positive correlations in amounts exchanged with each other, suggesting contingency in food transfers, and (3) kinship interacts with these positive correlations in amounts households exchange with each other, indicating even stronger contingency in sharing among related households than among unrelated households. While kin are preferred recipients of food aid, food distributions favor kin that have given more to the distributing household in the past rather than kin that would benefit more from the aid. Such discrimination among kin accords better with reciprocal altruism theory than with kin selection theory.

Behavioral studies demonstrate that individuals in small-scale societies preferentially aid close kin over more distant kin and nonkin (e.g., Betzig, 1988, Betzig and Turke, 1986, Chagnon, 1981, Chagnon and Bugos, 1979, Flinn, 1988, Gurven et al., 2000b, Hames, 1987, Hawkes, 1983 and Patton, 2005). Such nepotistic biases are often cited as evidence that indirect fitness payoffs (Hamilton, 1964 and Maynard Smith, 1964) have shaped human social interactions. Kin selection theory is so elegant and appealing that theorists often attribute instances of nepotism to inclusive fitness benefits without a careful consideration of alternatives. However, pathways to altruism not requiring shared genes can lead to increased levels of cooperation among relatives over nonrelatives if kin possess characteristics that are preferred in social partners. We examine the roles of indirect fitness impacts and reciprocal exchanges in maintaining nepotistic food transfers among reservation-living Ache forager-horticulturists of northeastern Paraguay. We previously reported that Ache households give preference in food distributions to recipient households that contain at least one close relative (Gurven, Allen-Arave, Hill, & Hurtado, 2001). This nepotistic bias in food transfers follows lines of genealogical relatedness rather than lines of Ache social kinship terminology (Allen-Arave, Gurven, Hill, & Hurtado, 1999). Theorists have used similar results from other populations to argue for the importance of indirect fitness payoffs in patterning human social interactions. Yet, our previous report also reveals that even among households linked by a close kinship tie, the amount of food any household D (donor) transfers to any household R (recipient) is correlated with the amount household D receives from household R ( Gurven et al., 2001). We expect such a result if returns from reciprocation provide the adaptive payoffs of the transfers but not if nepotistic investments in indirect fitness benefits provide the adaptive payoffs of the transfers. The presence of both nepotism and correlated amounts of food transferred between related households challenges us to disaggregate the relative contributions of indirect fitness impacts and reciprocal benefits in maintaining nepotistic Ache food transfers. The present paper presents new analyses to (1) examine the direction of imbalances in food transfers between households and (2) consider the difference in net caloric production between households. 1.1. Kin selection theory Researchers commonly predict from kin selection theory that altruistic aid will positively correlate with the degree of relatedness between interactants. Yet, kin selection theory does not presume that individuals should always act altruistically toward all relatives, nor should they necessarily share mainly with close relatives. Mathematical models illuminate that natural selection can favor nepotistic acts when the benefit to the recipient, B, discounted by the coefficient of genetic relatedness, r, is greater than the cost to the provider, C: Br>C ( Hamilton 1964). Whenever a household can obtain higher inclusive fitness payoffs by hoarding resources rather than providing them to relatives, kin selection theory suggests that no transfer will occur. Likewise, when distant relatives obtain a much larger positive fitness impact than close kin from assistance, kin selection theory predicts higher rates of transfer to distant kin than to close kin. Thus, an evaluation of kin selection theory must consider not only relatedness, but also the costs and benefits of aid. 1.2. Direction and magnitude of imbalances If nepotistic transfers constitute investment in indirect fitness, the direction and magnitude of imbalances within dyads of related households should attend to (1) the capability of household members to produce food calories, (2) the number of hungry mouths a household contains, and (3) the ages of household residents. All of these factors affect the marginal gains of food intake on household summed reproductive value (Fisher, 1958). Given the reasonable assumption that the curve relating food intake to fitness is negatively accelerated, kin selection theory implies that imbalances in food transfers between related households should favor households that produce less food over households that produce more food, when we hold other factors constant. Holding all else constant, kin selection theory also implies that imbalances between related households should favor households with more mouths to feed over households with fewer mouths to feed. The ages of household members matter as much as the number of residents a household contains for determining the fitness impact a transferred unit of food may have for a household because energy requirements and reproductive values peak in young adulthood. Resting metabolic energy expenditure rates indicate that individuals aged from their late teens to fifties require more energy than younger and older individuals do (National Research Council, 1989a and World Health Organization, 1985). Young adults also possess a larger potential to translate food energy into inclusive fitness gains than other age classes, owing to the greater number of childbearing years likely to await young sexually mature and about-to-mature individuals in the future. Thus, individuals in the middle of the lifecourse can return greater indirect fitness benefits to donor kin from large amounts of food than younger and older individuals can. Despite straightforward theoretical expectations that food flows should favor individuals of high reproductive value, application of this logic to human populations presents complications. Several theorists (e.g., Charlesworth and Charnov, 1981, Hamilton, 1964, Rogers, 1993, Taylor and Frank, 1996 and Trivers, 1971) have noted that the reproductive value of donors and recipients should alter the costs and benefits of giving and receiving aid. However, measures of reproductive value do not provide an adequate estimate of the expected inclusive fitness contribution made by individuals in species, such as ours, with child altriciality and common allocare. While prereproductive and postreproductive individuals cannot produce copies of their genes in the form of offspring, they regularly assist copies of their genes located in other relatives through activities such as baby-sitting (Bock, 1995, Fig. 57; Ivey, 2000, Turke, 1988 and Weisner and Gallimore, 1977), passing on important skills and knowledge (Biesele and Howell, 1981 and Liederman and Liederman, 1977), provisioning during times of need (Hawkes, O'Connell, & Blurton Jones, 1997), or offering protection and support (Chagnon & Bugos, 1979). The expected fitness contribution made by individuals—especially postreproductive individuals—is therefore underestimated by reproductive value measures alone because direct reproduction is not the only way to increase inclusive fitness. Still, food requirements and fertility measures alike indicate that food transfers, which enhance inclusive fitness, should predominately favor households containing young reproductive-aged residents over households containing other age classes, when we control for the number of residents and their production abilities. 1.3. Reciprocal altruism Any valid evolutionary explanation accounting for exchanges between nonkin may also apply to economic interactions between kin. Thus, we should never a priori assume that cooperation among kin results from inclusive fitness benefits to the exclusion of other pathways to cooperation. We now consider the role reciprocal altruism may play in food exchanges among relatives. Reciprocal altruism (Trivers 1971) can evolve as long as the cost of aiding another individual is outweighed by the benefit of receiving aid from that individual later, devalued by the probability that aid will be returned (Boyd, 1990). If the reciprocal exchange is profitable, individual altruists can expect payback in the future from self-interested actors who wish to continue obtaining the benefits that accrue from long-term cooperation. Such cooperation is even more likely to appear when punishment of defectors is possible (Fehr and Gächter, 2002, Ostrom et al., 1992 and Yamagishi, 1986). Nepotism may emerge independent of inclusive fitness benefits if individuals find their relatives more desirable as reciprocal exchange partners than nonrelatives. 1.4. Contingent reciprocity Since reciprocal altruism provides exchange partners with the temptation to defect by accepting the benefits of their partner's altruism, without later paying any costs of altruistic acts themselves, reciprocal altruists must identify and punish or avoid free riders. For this reason, Hill and Kaplan (1993) argue that reciprocal altruism makes a central prediction of “contingency” in food exchanges. Hill & Kaplan define contingency as giving that is conditional upon expectations of future receiving, where individuals infer expectations of future receiving from prior sharing patterns. In modern societies, despite legal enforcement of reciprocity, contingency is implemented through practices such as credit checks. Only those who have met obligations to repay in the past are provided current goods and services with the expectation of repayment in the future. Contingent reciprocity may prove difficult to detect because reciprocal altruism does not imply perfectly balanced exchanges between individuals. For an exchange to occur, reciprocal altruism theory predicts only that the average utility of the expected return outweighs the utility of the resource given up today. For example, a satiated household may pay little cost in providing (say) 2000 calories now, while benefiting greatly from (say) 500 calories at a future date when household members are ill or hungry. This may be analogous to the logic of insurance coverage in which one may pay premiums at a low utility cost for years in order to cover any high utility needs in the event of a future catastrophic shortfall. Additionally, if households engage in reciprocal exchanges that include several goods and services, an evaluation of food exchanges alone may underestimate the true contingent reciprocity occurring in the society. Despite these complications, we expect to find that the amount of food provided by any household D to any household R will correlate with the amount of food provided by household R to household D if reciprocal altruism plays a role in food transfers over the time scale of observation. Researchers studying other forager-horticulturalists populations have found dyadic correlations in food shares among the Achuar/Quichua/Zapara ( Patton, 2005), Aka Pygmies ( Gurven, 2004), Dolgan/Nganasan ( Ziker, 2005), Hiwi ( Gurven et al., 2000b), Mikea ( Tucker, 2004), Pilaga ( Gurven, 2004), Yanomamo ( Hames, 2000), and Ye'kwana ( Hames & McCabe, 2007). However, amounts of food given to all others does not correlate with amounts of food received from all others for Hadza large game ( Hawkes, O'Connell, & Blurton Jones, 2001) nor Meriam turtle meat exchanges ( Bliege Bird, Bird, Smith, & Kushnick, 2002). 1.5. Reciprocity among kin Individuals may prefer close kin to distant kin and nonkin as partners for reciprocity. Relatives can make ideal candidates for reciprocal exchanges due to factors such as familiarity, trust, proximity, a high probability of future interaction, or an expectation that relatives will cooperate. When choosing among potential reciprocity partners, individuals should generally prefer partners who will provide the highest expected return benefit. Due to additive indirect fitness benefits on top of the direct benefits that collaborators gain from cooperation, reciprocal exchanges with relatives will often yield larger expected return benefits than reciprocal exchanges with nonkin. Familiarity and emotional bonds fostered over time may make close kin easier to “read” and trust than distant kin and nonkin. Would-be transgressors likely experience more guilt from cheating victims with whom they have emotional ties (Frank, 1988). Furthermore, individuals may have good reason to trust close kin over other potential exchange partners because indirect fitness costs make cheating a close relative less profitable than cheating nonkin. If an exchange partner does fail to reciprocate due to deliberate cheating or an inability to repay (as can occur with a move, injury, or death), the loss is not complete for a slighted relative who still receives an indirect fitness benefit from their nonreciprocating relative's gain. Therefore, individuals assume less risk in initiating reciprocal exchanges with relatives than with nonrelatives. Finally, the close proximity that kin often maintain can create more opportunities for exchange and increase the probability of future interaction, which promotes cooperation (Andreoni and Miller, 1993 and Axelrod and Hamilton, 1981). At our study site, the homes of households joined by a close kinship tie tend to be nearer to each other than the homes of households not joined by a close kinship tie (Gurven et al., 2001). The factors discussed above (familiarity, emotional bonds, trust, proximity, and indirect fitness costs and benefits) can promote an expectation among kin that a relative will cooperate, and experimental research has shown that expectations of cooperation promote and stabilize altruistic behavior (Dawes, 1980 and Messick and Brewer, 1983). Thus, we might expect close kin to provide frequent goods and services in a stable arrangement of reciprocal altruism rather than simple kin-directed charity.