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Tanzania, a young adult male was brutally attacked by eight members
of his own group (Nishida et al. 1995). The authors speculated that this
young male may have been victimized because he did not conform to
social rules”he did not defer to higher-ranking males and launched
unprovoked attacks on adult females. The problem with these observa-
tions (and other anecdotal observations) is that they are based on a sin-
gle event. In these cases, conspicuous aggressive responses to unusual
types of behavior may be more salient than occasions in which unusual
behaviors were ignored. Without systematic analyses of the conse-
quences of aberrant behaviors, it is dif¬cult to be certain that violations
of social norms are consistently punished.
The only systematic evidence of third party punishment comes from
an experimental study on rhesus macaques conducted by Hauser and
Marler (1993a, 1993b, Hauser 1997). Rhesus macaques give characteris-
tic calls when they discover food items (Hauser and Marler 1993a).
Taking advantage of this situation, Hauser and Marler (1993b) con-
ducted an experiment in which observers surreptitiously dropped
handfuls of coconut or monkey chow and waited for monkeys to ¬nd
it. When monkeys found the food, they sometimes called and some-
times remained silent. Calling had little effect on the likelihood of be-
ing detected after ¬nding food, but calling signi¬cantly reduced the
likelihood of being harassed after discovery by other group members.
Monkeys who discovered food and subsequently called were less likely
to be supplanted, chased, or attacked than monkeys who remained
silent after they found food. In the published report, the authors did
not control for the relative dominance of the original possessors and
the discoverers, even though macaque females rarely initiate aggres-
sion toward more dominant animals. However, subsequent reanalyses
of the data (Hauser personal communication) indicate that noncallers
were more likely to be harassed when they were discovered by higher-
ranking animals than callers were. Apparently, these rules apparently
apply only to females. Males virtually never call when they ¬nd food
and are rarely punished (Hauser and Marler 1993b; Hauser 1997).
These data provide intriguing evidence that rhesus macaques punish
group members who violate social norms. However, the weight of this
conclusion is limited by the fact that these results have not been repli-
cated, and no other observers have reported similar ¬ndings in other
groups or species.
The Evolution of Cooperation in Primate Groups 63



2.8 Prospects for Finding Strong Reciprocity in Primate Groups

For primates, cooperation is bounded by kinship and reciprocity and
involves pairs of animals who have long-term social bonds. Most
primatologists have assumed that reciprocal altruism is ultimately re-
sponsible for reciprocity within dyads, an assumption that is bolstered
by experimental evidence that cooperative behavior is contingent on
the nature of previous interactions. However, de Waal (2000) has sug-
gested that reciprocal exchanges in capuchins may be the product of
attitudinal reciprocity”a tendency to mirror the predispositions of
their partners. If he is right, then we have reason to believe that strong
reciprocity is rooted in the behavior of nonhuman primates. However,
it is also possible that the monkey™s initial attitude toward its partner
re¬‚ects the quality of their social relationship, and this is based on a
long series of cooperative exchanges over time.
Good evidence of punishment would provide support for the idea
that strong reciprocity operates in primate groups. Presently, system-
atic evidence for punishment rests on a single experiment. These data
are quite provocative, but their signi¬cance will not be established
until these experiments are replicated and extended to other species.
To understand the role of strong reciprocity in primate groups, we
need to know more about the proximate factors that motivate coopera-
tive behavior. Strong reciprocity in humans seems rooted in a deep
sense of fairness and concern for justice that is extended even toward
strangers, but we have no systematic evidence that other animals have
similar sensibilities. Even those who have argued most forcefully for
the emergence of moral sentiments in monkeys and apes have drawn
their evidence from the interactions of close associates with long-term
social bonds, not interactions among strangers (de Waal 1996; Flack
and de Waal 2000).
The idea of strong reciprocity emerged from carefully designed ex-
perimental studies on humans that revealed surprisingly high levels of
altruism in one-shot interactions with strangers. It is hard to imagine
obtaining comparable data on interactions among strangers in non-
human primates. Most primates live in stable social groups where
they restrict peaceful social interactions mainly to known group
members. Close associations with strangers are fraught with tension,
generating aggression and avoidance, not cooperation. Aversions to
strangers extend to captive settings. It might be possible to adapt
de Waal™s experimental studies of capuchins to assess cooperative
64 Silk



behavior with anonymous partners, but it is not clear whether capu-
chins or other primates would tolerate this protocol.
In conclusion, the literature suggests that primates reserve coopera-
tion mainly for kin and reciprocating partners, but punishment is
apparently uncommon. While we know a lot about what nonhuman
primates do, we know very little about what motivates them to do it.
The patterning of cooperative interactions among nonrelatives could
be the product of reciprocal altruism, but the same patterns could also
arise from strong reciprocity. To identify the proximate mechanisms
that generate cooperation in primate groups, we need to develop ex-
perimental procedures that allow us to assess the tendency to co-
operate in one-shot interactions with strangers. The ability to interact
peacefully in one-shot interactions with strangers may prove to be one
of the most remarkable traits of our own species. We also need to
know more about other primates™ propensity to punish violations of
social norms. Work addressing these issues in nonhuman primates is
needed to assess the evolutionary roots of strong reciprocity.

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3 The Natural History of
Human Food Sharing and
Cooperation: A Review
and a New Multi-
Individual Approach to the
Negotiation of Norms

Hillard Kaplan and Michael
Gurven

3.1 Introduction

Humans share food unlike any other organism does. Many other ani-
mals, including eusocial insects (bees, ants, termites); social carnivores
(lions, wolves, wild dogs); some species of birds (e.g., ravens) and
vampire bats actively share food. However, the patterning and com-
plexity of food sharing among humans is truly unique. Unlike other
mammals, for which food sharing between mothers and offspring is
limited largely to lactation during infancy, human parents provision
their children until adulthood.1 Moreover, the sharing of food between
human parents and their children continues bidirectionally until death
in most traditional non-market societies. Additionally, marriage is uni-
versal among human societies, and husbands and wives regularly
share food with one another throughout their marriage. Food sharing
within human families is based upon a division of labor in subsistence
effort by age and sex, where tasks are divided and the proceeds of
work are shared. In fact, within-family transfers of food are so univer-
sal among humans that they are largely taken for granted and have
rarely been systematically studied. This gap in anthropological re-
search is ironic, since the sexual division of labor and the concomitant
sharing of food between spouses and between parents and offspring
have ¬gured prominently in several models of hominid evolution
(e.g., Isaac 1978; Lancaster and Lancaster 1983).
In addition to within-family food transfers, food sharing sometimes
extends beyond the nuclear family in many societies. Most recent re-
search on food sharing has focused on food transfers among adults
living in different households. The majority of this research has been
conducted in small-scale societies, particularly hunter-gatherers and
groups that combine simple horticulture with hunting and gathering
76 Kaplan and Gurven



(forager-horticulturalists). There are two reasons for this focus. First,
interfamilial food sharing is pervasive among hunter-gatherers and
many forager-horticulturalists; they are often referred to as ˜˜egalitarian
societies.™™ Second, hominids lived as hunter-gatherers for the vast ma-
jority of their evolutionary history (which has lasted over two million
years). Agriculture originated only about 10,000 years ago and has
been practiced by the majority of the world™s population for only two
or three millennia. Since most hunter-gatherers share food on a daily
basis, studies of food transfers among foragers may provide important
insights into the evolutionary basis of human food sharing and more
generally, about the origins of human hyper-sociality.
This chapter has three principal objectives. The ¬rst is to provide a
brief overview of existing theory and research about food sharing in
small-scale societies for nonspecialists. In the ¬rst part of the chapter,
we outline the principal hypotheses proposed to account for variation
in food sharing and evaluate available evidence pertaining to these
hypotheses. The second objective is to present evidence regarding why
we consider it necessary to rethink existing approaches to food shar-
ing. In this second part of the chapter, we argue that intrafamilial re-
source ¬‚ows are critical to the understanding of interfamilial sharing
and that neither the human life course nor human intelligence could
have evolved without long-term imbalances in ¬‚ows of food between
families. We suggest that future research on this topic should consider
small-group decision processes and the emergence of institutionalized
sharing norms. In the third part of the chapter, we review several case
studies of food sharing in different societies and across contexts within
societies as a preliminary step towards building a theory of how these
norms may correspond to local ecological conditions. The chapter con-
cludes with a discussion of new directions for research and some major
unresolved questions that should be addressed.

3.2 Part I: Theories and Empirical Evidence

Six different theories have been proposed to explain the existence and
patterning of intragroup food sharing.

I. Reciprocal altruism
Several investigators have proposed that reciprocal altruism (Trivers
1971), where food is exchanged at one point in time for food at some
later time, may explain many or most cases of human food sharing
The Natural History of Human Food Sharing and Cooperation 77



(Kaplan and Hill 1985; Winterhalder 1986; Smith 1988). The pervasive-
ness of reciprocal food sharing among humans is commonly explained
in terms of the kinds of foods they acquire and their inherently ˜˜risky™™
nature (Kaplan and Hill 1985; Winterhalder 1986; Smith 1988). Human
hunter-gatherers tend to specialize on the largest, highest-quality, most
nutrient-dense foods available in their environments (Kaplan et al.
2000), and as a result, they experience high variance in foraging luck
due to the dif¬culty in acquiring these items. For example, individual
Ache hunters return empty-handed on 40 percent of the days they
hunt, but on some days return with several hundred thousand calories
of meat (Hill and Hawkes 1983). Hunting success is even more spo-
radic among large-game hunters, such as the Hadza, who only acquire
meat on about 3 percent of their hunting days (Hawkes, O™Connell,
and Blurton Jones 1991).
Since there are diminishing returns to consumption of large quanti-
ties of food (especially in environments where spoilage is a problem)
and because food portions are very valuable to hungry individuals, re-
ciprocal sharing can signi¬cantly reduce variation in day-to-day con-
sumption and maximize the intertemporal utility of food. Reciprocal
altruism therefore allows people to devote time and energy to the pur-
suit of large, asynchronously acquired, high-quality packages. Trade is
a special form of reciprocal altruism where the return bene¬ts of giving
are in another currency, such as meat for sex, food for labor, or ¬sh for
yams. However, when the return bene¬t is a non-food currency, such
as increased mate access, such sharing does not serve the goal of risk-
reduction.
II. Cooperative acquisition and byproduct mutualism
Sharing may also enable individuals to achieve gains from cooperative
pursuits of food. The acquisition of dif¬cult-to-acquire foods, espe-
cially wild game, often requires the coordinated efforts of several indi-
viduals. However, usually only a single individual is identi¬ed as the
owner of the acquired resource, determined by cultural-speci¬c norms
of ownership (e.g., the hunter who makes ¬rst lethal shot, the ¬nder,
the killer [Dowling 1968]). In many groups, sharing among task group
members occurs as an initial wave of sharing (e.g., Pygmies [Bailey
1991; Harako 1976]). Owners may reward nonowners for their current
cooperation by giving them shares of the resource, but this sharing
may also act as a means of insuring future cooperation in similar food
production activities. Thus, sharing is a form of trade-based reciprocal
altruism, where labor is rewarded with food.
78 Kaplan and Gurven



An alternative interpretation of the same phenomenon is that engag-
ing in group production when there is sharing provides participants
with higher per capita returns than if they produced food by them-
selves. Thus, group production may represent a form of byproduct
mutualism (Clements and Stephens 1995; Dugatkin 1997; Alvard and
Nolin 2002). Once rigid sharing norms exist in a population, the deci-
sion to participate in, say, a group whale hunt or cooperative monkey
hunt should depend only on the higher per capita return rates relative
to those that could be gained in solitary subsistence activities (see
Alvard 2002). Thus, an advantage of strong sharing norms is that they
act to transform the payoff structure of group food production strat-
egies from that of a Prisoner™s Dilemma to that of mutualism.
III. Tolerated theft or scrounging
Building on the same insights regarding large asynchronously ac-
quired food packages and diminishing returns to consumption of large
food quantities, others have proposed that much apparent voluntary
sharing may actually be ˜˜theft™™ or scrounging of food from food
acquirers by individuals who have little or none (Blurton Jones 1984,
1987). This hypothesis is based upon the assumption that asymmetries
between individuals in the marginal value of additional food can lead
to contests over packages. The hungry person is more motivated to
¬ght, while the person with more should relinquish some food because
the lost food value is not worth the ¬ght (Blurton Jones 1987; Winter-
halder 1996). When power or ˜˜resource holding potential™™ is equal
among contestants, a simple prediction of tolerated scrounging is that
distributions will be highly egalitarian”such that any additional food
portions have the same marginal value for each contestant (Winter-
halder 1996).
Proponents of this view have pointed out that tolerated theft in the
context of large, highly variable foods raises a secondary problem.
Why do people spend time foraging for large packages if they know
that much of what they acquire will be taken from them? Scrounging
of large packages may effectively reduce their per capita consumption
return rate below several other food production options in the envi-
ronment, especially the pursuit of small packages (Hawkes 1993). To
answer this question, Hawkes (1991, 1992, 1993) proposed that the
pursuit of large packages, particularly hunted foods, is very sex-biased
and that men acquire large packages to ˜˜show off™™ and garner atten-
tion. Men focus their efforts on acquiring large packages, precisely be-
cause others will scrounge from them. As a result, these men will
The Natural History of Human Food Sharing and Cooperation 79



gain the attention and support of scroungers, many of whom will be
women. The payoffs to this attention presumably come in the form of
increased access to mates and an increased number of offspring.
IV. Costly signaling
Costly signaling is an extension of the show-off hypothesis that may
explain why certain individuals (usually men) pursue dif¬cult-to-
acquire foods that often yield suboptimal caloric return rates (Smith
and Bliege Bird 2000; Gurven, Allen-Arave et al. 2000; Bliege Bird,
Smith, and Bird 2001). The costliness of the signal insures the honesty
of the information being advertised (Zahavi and Zahavi 1997; Grafen
1990; Johnstone 1997). The signal might provide information about
phenotypic quality (such as disease-resistance) or about intentions to
cooperate in the future. Recipients of the signal confer bene¬ts on the
generous donor not as payback for food given, but because informa-
tion about the donor™s phenotypic quality makes that donor a desir-
able partner, mate, or ally. Costly signaling differs from showing off
because it does not rely on tolerated theft to explain food transfers.
Additionally, because the honesty of the signal makes the signaler an
attractive partner, costly signaling avoids the second-order“collective
action problem of who should pay prestige back to good hunters.
V. Nepotism based on kin selection
Because biologically related individuals share genes by descent, any
behavior that suf¬ciently bene¬ts kin can be favored by natural selec-
tion. According to models developed by Hamilton (1964), natural se-
lection will favor altruism to kin when the bene¬ts to the recipient,
discounted by Wright™s coef¬cient of genetic relatedness between do-
nor and recipient, outweigh the costs of giving. A simple prediction is
that, all else being equal, close kin should receive shares either more
frequently or in greater quantities than distantly related and unrelated
individuals (Feinman 1979). It has also been argued we should ex-
pect to ¬nd greater imbalances in quantities given and received among
close kin than among nonkin or distant kin (Hames 1987; Feinman
1979), who, presumably, would only share reciprocally. However, this
might not be true if close kin are also reciprocity partners and if recip-
rocal altruism is an important factor in¬‚uencing food transfers among
kin (Gurven, Hill et al. 2000).
VI. Trait-group selection
Selection among groups has also been proposed in order to explain co-
operation and food sharing within human groups (Wilson 1998; Boyd
and Richerson in press; Boehm 1993). In group selection models, the
80 Kaplan and Gurven



relative ¬tness of altruists is lower than that of sel¬sh individuals
within groups, but the average ¬tness of individuals in groups contain-
ing more altruistic members is greater than those in groups containing
fewer altruists. Group selection could favor costly food sharing if the
increased absolute ¬tness of altruists among groups in a metapopula-
tion outweighs the decrease in relative ¬tness within groups, where
˜˜group™™ refers to any congregation of individuals (Wilson 1990, 1998).
While the conditions favoring trait-group selection are much less strin-
gent than those of older group selection models, its overall in¬‚uence is
still limited by grouping patterns and migration, and ultimately may
be no more revealing than egoistic models (Harpending 1998; Krebs
1987). However, given the con¬‚ict between group and individual
interests, cultural means of encouraging individuals to share food may
increase the frequency of giving within groups (Simon 1990; Boyd and
Richerson in press), leading to socially enforced egalitarian behavior
(Boehm 1993).

3.2.1 Cross-cultural Evidence
There is a great deal of cross-cultural evidence that sharing is most
common for large packaged resources characterized by high acquisi-
tion variance, especially wild game. Widespread pooling of large
game animals is common among the Hadza (Hawkes 1993; Marlowe
n.d.), Dobe !Kung (Lee 1979; Marshall 1976), G/wi (Silberbauer 1981),
Ifaluk (Sosis, Feldstein, and Hill 1998), Ache (Kaplan and Hill 1985),
Yanomamo (Hames 1990), and Gunwinggu (Altman 1987). While such
pooling can signi¬cantly reduce variation in daily meat consumption,
the outcome of risk reduction is consistent with all six models, even
though risk reduction is only explicitly incorporated as a goal within
the reciprocal altruism and group selection frameworks. This is be-
cause widespread sharing of relatively large sized game items, charac-
terized by high within- and across-individual variability in acquisition,
can be explained by future reciprocation (reciprocal altruism), de-
mands and threats of hungry individuals (tolerated theft), honest sig-
nals of phenotypic quality (costly signaling), and the Pareto-optimal
distribution solution maximizing group bene¬t (trait-group selection).
Because the costs of sharing decrease with increased package size of
the resource (assuming diminishing returns), it is not surprising that
large package size is a signi¬cant predictor of sharing for meat and
other food items such as fruits, cultigens, and market foods (Hames
1990; Gurven, Hill et al. 2000; Gurven, Hill, and Kaplan 2002; Kitanishi
1998).
The Natural History of Human Food Sharing and Cooperation 81



Thus the observation that the sharing of large packages is wide-
spread does not help distinguish between the models discussed in
section 3.2. The empirical ¬ndings relevant to understanding which
models are most appropriate for explaining much of the variance in
sharing within and across groups are generally concerned with three
issues: contingency of giving upon receiving, producer control over
distributions, and imbalances between families in what is given and
received.

3.2.1.1 Contingency Contingency is the feature of sharing relation-
ships that is critical for distinguishing between reciprocal altruism and
other models (Rothstein and Pierroti 1988; Hill and Kaplan 1993). Gen-
eral contingency requires that all giving be balanced by all receiving,
while speci¬c contingency requires that giving to speci¬c others be bal-
anced by receipts from those same individuals (Hames 2000; Gurven,
Hill et al. 2000). Speci¬c contingency is usually estimated as the corre-
lation between the percentage or quantity of food given by A to B with
the percentage or quantity given by B to A over some appropriate sam-
ple period. Contingency can be measured within food categories (such
as meat for meat, roots for roots, and so forth) or for all food, which
includes exchange across food types. In order for reciprocal altruism to
be bene¬cial to a donor, donors should preferentially give to those
who are likely to share with them in the future (speci¬c contingency).
Costly signaling, on the other hand, requires that the prestige-related
bene¬ts from signaling outweigh the costs of producing food that is
widely shared (general contingency). Neither tolerated theft, kin selec-
tion, nor trait-group selection predict that food donations will be con-
ditional upon expected return.
Speci¬c contingency has only been measured in four groups, all of
which are in South America”the Yanomamo (Hames 2000), Hiwi
(Gurven, Hill et al. 2000), Ache (Gurven, Allen-Arave et al. 2000), and
Pilaga (Henry 1951). Correlations which describe speci¬c contingency
for all foods are signi¬cantly greater than zero, but not very high (be-
tween 0.2 and 0.5 [Gurven in press], see discussion of imbalances in
section 3.2.1.3), while within resource categories, contingency is often
highest for cultivated and collected foods. These results are most con-
sistent with reciprocal altruism and least consistent with tolerated
theft, because tolerated theft predicts that there should be no relation-
ship between giving and receiving.2 Among the Ache, however, there
is no evidence of speci¬c contingency for wild game over the dura-
tion of single foraging trips, nor for game items brought back to the
82 Kaplan and Gurven



permanent colony. This is inconsistent with reciprocal altruism, unless
sharing among the Ache rewards group work effort (cooperative ac-
quisition and mutualism, discussed in part III). Nevertheless, further
research is needed to determine whether these signi¬cant positive cor-
relations imply that the time-discounted value of food returns is suf¬-
cient to offset the present costs of giving.
Anecdotal evidence that giving is balanced by future receiving and
that those who do not give do not receive is found in many tradi-
tional societies. As one Maimande explained, ˜˜If one doesn™t give, one
doesn™t get in return . . . Some people are speci¬cally excluded from
most distributions because they never or only rarely give any of their
products to us™™ (Aspelin 1979, 317). Similar anecdotes exist among the
Agta (Peterson 1978; Bion Grif¬n personal communication), Pintupi
´
(Myers 1988), Siriono (Holmberg 1969, 45), and G/wi (Silberbauer
1981, 463). Although there is an emphasis on peoples™ expectations for
future receipt in these and other ethnographies, the extent of time
depth separating episodes of giving and receiving is often unclear.
Indeed, Sahlins™ (1972) use of the term ˜˜generalized reciprocity™™ was
meant to re¬‚ect short-term imbalances, especially among kin, that are
eventually balanced over the span of peoples™ lives.
General contingency or balance has been measured in ¬ve
societies”the Ache (Gurven, Hill, and Kaplan 2002), Hiwi (Gurven,
Hill et al. 2000), Meriam (Bliege Bird and Bird 1997), Pilaga (our
analysis of Henry 1951), and Yanomamo (Hames 2000). These studies
showed mixed support for general balance. While the lack of speci¬c
balance contradicts reciprocal altruism, the presence of general balance
is consistent with indirect reciprocity (Alexander 1987; Boyd and
Richerson 1989) or costly signaling, where the return bene¬t to the
donor is food. If the return bene¬t is in another currency, such as
increased mating opportunities, then a lack of general balance is not in-
consistent with costly signaling.

3.2.1.2 Producer Control Reciprocal altruism and kin selection re-
quire that producers maintain some control over the distribution of
foods they acquire, whereas tolerated theft assumes no producer
˜˜rights.™™ If producers have no control over the distribution of certain
items, then those food items may act as partial public goods (Dowling
1968; Hawkes 1993). Despite observations of widespread game distri-
butions in some groups (e.g., Ache [Kaplan and Hill 1985], G/wi
[Silberbauer 1981], Hadza [Hawkes 1993], Western Desert Aborigines
The Natural History of Human Food Sharing and Cooperation 83



[Gould 1980]), several lines of evidence indicate that producers often
maintain signi¬cant control over distributions in many”if not most”
cultures. First, as shown in the previous section, there is often a bias in
sharing towards those who shared with the producer. Additionally,
there are clear biases in distributions towards close kin living in other
families at the expense of distant kin and unrelated families (Gun-
winggu [Altman 1987], Copper and Netsilik Eskimo [Damas 1972],
Pilaga [Henry 1951], Hiwi [Gurven, Hill et al. 2000], Kaingang [Henry
1941], Batek [Endicott 1988], Pintupi [Myers 1988], Washo [Price 1975],
Yanomamo [Hames 1990], Machiguenga [Kaplan 1994], Ache [Kaplan
and Hill 1985; Gurven, Hill, and Kaplan 2002], Ifaluk [Sosis 1997],
Basarwa [Cashdan 1985]), and to those participating in the hunting
party (Netsilik Eskimo [Damas 1972], Nyae Nyae !Kung [Marshall
1976], Ifaluk [Sosis 1997], Pintupi [Myers 1988], Washo [Price 1975],
Mbuti [Ichikawa 1983], Aka [Bahuchet 1990; Kitanishi 1998], Efe [Bai-
ley 1991], Lamalera [Alvard 2002], Northwest Coast Indians [Gould
1980]). Among the Hiwi and Ache at the settlement, there are clear kin
biases in sharing, even when controlling for residential distance. Fi-
nally, several ethnographies are explicit about the ownership of shares
after an initial distribution, even if others still have not received any
meat (Efe [Bailey 1991, 100]; Nyae Nyae !Kung [Marshall 1976, 363]).
Second, hunters frequently consume portions of kills (e.g., internal
organs and marrow) at the kill site and usually no one begrudges
them this right (Batek [Endicott 1988]; Hadza [Marlowe n.d.]; Nyae
Nyae !Kung [Marshall 1976]; G/wi [Silberbauer 1981]). Third, pro-
ducers often keep signi¬cantly more than 1/n of the game packages
they acquire, even though others in the camp or village may not pos-
sess any meat (Gunwinggu [Altman 1987]; Yora [Hill and Kaplan
1989]; Yuqui [Stearman 1989]; Yanomamo [Hames 2000]; Hadza
[Hawkes et al. 2001]; Ache [Gurven, Allen-Arave et al. 2001]; Hiwi
[Gurven, Hill et al. 2000]). Finally, the frequent observations of in-
cessant demands for food in many foraging societies (Peterson 1993;
Chagnon 1983) does not mean that producers are powerless to ignore
or reject requests for food made by other group members. There is evi-
dence that nonproducers do not possess automatic claims to shares
among the Pintupi (Myers 1988), the Aka (Bahuchet 1990, 38), Agta
´
(Grif¬n 1984), Pilaga (Henry 1951), and Siriono (Holmberg 1969, 88).

3.2.1.3 Imbalances: Relative Need, Bargaining, and Signaling Sev-
eral ethnographies have reported large short-term between-family
84 Kaplan and Gurven



imbalances, but long-term balance in food transfers, consistent with
generalized reciprocity (e.g., Batek [Endicott 1988, 118]; Kaingang
[Henry 1941, 101]). While short-term imbalances are relatively easy to
measure, the existence of long-term balances in terms of lifelong sym-
biosis is much more dif¬cult to con¬rm. Nepotistic food sharing based
on kin selection can predict imbalances in food sharing, even though
kinship is a symmetrical relationship. Food given to dependent and
unskilled offspring and relatives can have a large bene¬t at a relatively
small cost to a skilled producer (B > C in Hamilton™s Rule). Similarly,
the downward ¬‚ow of food provides useful calories to younger kin of
higher reproductive value (Rogers 1993). Over the course of an indi-
vidual™s lifetime, current evidence suggests that among the Ache, Piro,
Machiguenga, and perhaps other traditional groups, children are net
costs to parents, and thus children™s debt is never repaid directly, but
is instead redirected to grandchildren (Kaplan 1994; see part II of this
chapter). We should expect to ¬nd smaller short-term imbalances in
transfers among kin of similar age. Allen-Arave et al. (n.d.) ¬nd that
among the Ache, imbalances over a four-month sample period are
smaller among kin of similar ages (e.g., siblings) than among kin of
disparate ages (e.g., between older parents and their adult offspring).
Apart from kinship, there is good evidence that large short-term
and long-term exchange imbalances among individuals and families
occur among foragers and forager-horticulturalists. The highest food
producers among the Ache, Efe, Pilaga, and Yuqui consistently gave
away more than they received as compared to low producers (Kaplan
and Hill 1985; Gurven, Allen-Arave et al. 2000; Bailey 1991; Henry
1941; Stearman 1989). Indeed, observations that high producing Ache
and Hadza hunters often do not receive in-kind compensation for their
generosity initially led to the proposition that reciprocal altruism was
an inadequate model of human food sharing (Hawkes 1991, 1993).
Imbalances in quantities transferred across individuals or families
can be interpreted in several ways. First, the short-term nature of most
¬eld studies places an arbitrary horizon on the delayed time for recip-
rocation, and the sampling bias associated with any brief series of
snapshots of interfamily exchange relations is likely to result in some
degree of imbalance. Hames (2000) argues that meticulous score-
keeping meant to ensure balance should be found across pairs of
distantly interacting individuals, where trust is weak (e.g., market
transactions), while imbalances might be quite common among indi-
viduals who interact over extensive periods of time.
The Natural History of Human Food Sharing and Cooperation 85



Second, an imbalance may be intentional if sharing is based on
the signaling of phenotypic or genotypic quality. Imbalances in turtle
meat exchanges, where hunters expend a great deal of energy to pro-
vide community feasts, are most likely due to costly signaling (Bliege
Bird and Bird 1997; Smith and Bliege Bird 2000). Similarly, an imbal-
ance is expected if the return bene¬ts of reciprocal altruism or costly
signaling are in other currencies. High-producing Ache hunters may
give away more than they receive, but they obtain greater mating
opportunities and higher offspring survivorship (Hill and Hurtado
1996). Yuqui and Tsimane hunters sometimes trade portions of their
´
kills for garden products (Stearman 1989; Chicchon 1992), while Kui-
kuyu with unsuccessful gardens will trade labor for access to a neigh-
bor™s manioc patches (Carneiro 1983).
As mentioned above, contingency estimates well below 1.0 suggest a
relatively high occurrence of exchange imbalances among pairs of fam-
ilies. It is important to mention that all measures of contingency are
based on quantities of food shared among families. Bargaining theory,
however, can lead to outcomes consistent with reciprocal altruism but
˚
inconsistent with strong balance in food quantities (Stahl 1972; Hill
and Kaplan 1993; Sosis, Feldstein, and Hill 1998; Gurven, Hill et al.
2000). If donors continue giving portions to others as long as the
expected future bene¬ts outweigh the current costs of giving relative
to other options, there is no reason to expect the exchange of equal
quantities over time. The extent of imbalance should be a function of
differential wealth holdings, in¬‚uence, status, and need. One calcula-
tion of contingency that measured the balance in ˜˜value™™ transferred
across families”by incorporating the frequency and sizes of shares”
found a slightly higher level of balance among Hiwi and Ache foragers
(Gurven 2004).
There has been much written on the importance of ˜˜need™™ and the
direction of food ¬‚ows (Woodburn 1982; Barnard and Woodburn
1988; Winterhalder 1996), supporting the notion that ˜˜if there is
hunger, it is commonly shared™™ (Marshall 1976, 357) and possibly the
group selection hypothesis. Among the Ache, Maimande, G/wi, and
Hiwi, shares are often given in proportion to the number of consumers
within the recipient family (Gurven, Hill, and Kaplan 2002; Aspelin
1979; Silberbauer 1981; Gurven, Hill et al. 2000). Batek families with
high dependency tend be net consumers while those with low depen-
dency are net producers (Cadelina 1982). There is additional evidence
that older men, with larger families, preferentially bene¬t from sharing
86 Kaplan and Gurven



networks at the expense of younger men™s labor, especially if one con-
siders brideservice payments (Efe [Bailey 1991], Gunwinggu [Altman
1987], Kutse [Kent 1993], Yanomamo [Ritchie 1996], and Agta [Bion
Grif¬n 1984]). Differential need among families leads to different costs
and bene¬ts of giving across families, and should therefore in¬‚uence
bargaining outcomes and observed levels of balance. We explore this
issue further in part II of this chapter.
Although certain levels of imbalance may be due to differential need,
there is much evidence to suggest that such imbalances are some-
times tolerated only within limits. Those who do not produce or share
enough are often subject to criticism, either directly or through gossip,
and social ostracism. Anecdotes of shirkers being excluded from distri-
butions until they either boosted their production or sharing levels are
found among the Maimande (Aspelin 1979), Pilaga (Henry 1951, 199),
Gunwinggu (Altman 1987, 147), Washo (Price 1975, 16), Machiguenga
(Baksh and Johnson 1990), Agta (Grif¬n 1984, 20), and Netsilik Eskimo
(Balikci 1970, 177). However, other ethnographies report the persis-
tence of long-term imbalances without any obvious punishment, exclu-
´
sion, or ostracism (Chacobo [Prost 1980, 52]; Kaingang [Henry 1941,
101]; Batek [Endicott 1988, 119]), although these anecdotes suggest
that such imbalances are due to a small number of low producers
within the group.
In summary, there is substantial cross-cultural evidence supporting
the view that reciprocal altruism of some sort underlies much food-
sharing behavior. First, in many societies producers appear to exert
control over the distribution of resources. Second, although speci¬c
contingency of giving upon receiving has been measured in only a few
cases, there is evidence over the short term that people form preferen-
tial food-sharing partnerships with high rates of giving and receiving
and share less with those who give less (meat sharing in the forest
among the Ache is one exception, however) (Kaplan and Hill 1985).
There are also a plethora of qualitative reports suggesting that giving
and receiving are contingent in many or most cultures and in different
contexts.
At the same time, persistent imbalances in amounts given and
received between families suggest that strict reciprocal altruism cannot
account for all food sharing between families. Some of those imbal-
ances may be due to kin selection, costly signaling, tolerated theft,
trait-group selection or some combination of these four forces. In the
next section, we sketch the importance of food sharing in the evolution
The Natural History of Human Food Sharing and Cooperation 87



of human life. We show that the evolved life history of humans
required long-term imbalances in food ¬‚ows. We also present a new
way to understand imbalances in terms of multi-individual decision
processes and long-term mutual bene¬t.

3.3 Part II: Human Life History and Food Sharing

3.3.1 Features of Our Human Life History
The distinctive life history of humans is related to their unique forag-
ing niche relative to that of other mammals (and even primates). Five
distinctive features of the human life course are noteworthy.
1) an exceptionally long lifespan
2) an extended period of juvenile dependence
3) support of reproduction by older post-reproductive individuals
4) male support of reproduction through the provisioning of females
and their offspring
5) a large brain and its associated capacities for learning, cognition,
and insight
Humans have a very ¬‚exible foraging strategy, consuming different
foods in different environments, and this ¬‚exibility has allowed us to
survive successfully in all of the world™s terrestrial environments. In
another sense, however, the human foraging niche is very special-
ized. In every environment, human foragers consume the largest, most
nutrient-dense, highest-quality, and most dif¬cult-to-acquire foods, us-
ing techniques that often take years to learn (Kaplan et al. 2000, Kaplan
1997).
This foraging niche is related to human life history because high
levels of knowledge, skill, coordination, and strength are required to
exploit the suite of high-quality, dif¬cult-to-acquire resources humans
consume. The attainment of those abilities requires time, a signi¬cant
commitment to development, and a large brain to support the learn-
ing, information processing, and planning underlying those skills. This
extended learning phase during which productivity is low can be com-
pensated by higher productivity during the adult period and subsi-
dized by an intergenerational ¬‚ow of food from old to young. Since
productivity increases with age, the time investment in skill acquisition
and knowledge leads to selection for lowered mortality rates and
greater longevity, because the returns on the investments in develop-
ment occur at older ages.
88 Kaplan and Gurven



There are three foraging groups (the Ache, Hadza, and Hiwi) and
two groups of forager-horticulturalists (the Machiguenga and Piro) for
whom quantitative data are available regarding age-pro¬les of food
consumption and production. All of these groups display similar age-
pro¬les of net food production. Children are largely supported by their
parents until about age eighteen (when food production approximately
equals consumption), after which productivity rises steeply through
the twenties until the mid-thirties. The more skill-intensive the task,
the greater is the delay to peak performance and the greater the in-
crease in productivity with ˜˜on-the-job-training™™ (Bock 2002). High
productivity is maintained until the mid-sixties when the deleterious
effects of senescence become signi¬cant. This pattern of development
and aging bears a striking resemblance to modern societies, where
wages depend on education-based capital and the ages eighteen and
sixty-¬ve have similar signi¬cance.
Figure 3.1 shows survival probabilities and net production by age
for wild chimpanzees, our closest living relatives, and modern human
hunter-gatherers living under conditions similar to our evolutionary


Human Survival
1 2000
Chimpanzee Survival
1750
Net Prod. Humans
0.9
Net Prod. Chimpanzees 1500
1250
0.8
1000
0.7 750
Net Production



500
0.6
250
Survival




0.5 0
-250
0.4
-500
-750
0.3
-1000
0.2 -1250
-1500
0.1
-1750
0 -2000
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
Age

Figure 3.1
Survival and net food production: Human foragers and chimpanzees. Adapted from
Kaplan, Lancaster, and Robson 2003.
The Natural History of Human Food Sharing and Cooperation 89



past (see Kaplan et al. 2000, 2001 for details on data sources). It is evi-
dent that the chimpanzee net production curve shows three distinct
phases. The ¬rst phase, to about age ¬ve, is the period of complete to
partial dependence upon mother™s milk and of negative net produc-
tion. The second phase is independent juvenile growth, lasting until
adulthood, during which net production is zero. The third phase is re-
productive, during which females, but not males, produce a surplus of
calories that they allocate to nursing. Humans, in contrast, produce less
than they consume for about twenty years. Net production becomes
increasingly negative until about age fourteen and then begins to
climb. Net production in adulthood in humans is much higher than in
chimpanzees and peaks at a much older age, re¬‚ecting the payoff of
long dependency. More precisely, human peak net production is about
1,750 calories per day, reached at about age forty-¬ve. Among chim-
panzee females, peak net production is only about 250 calories per day
and, since fertility decreases with age, net productivity probably
decreases throughout adulthood. By age ¬fteen, chimpanzees have
consumed 43 percent and produced 40 percent of their expected life-
time calories, respectively; in contrast, humans have consumed 22 per-
cent and produced only 4 percent of their expected lifetime calories! In
fact, the human production pro¬le requires a long lifespan and would
not be viable with chimpanzee survival rates, since expected lifetime
net production would be negative (Kaplan et al. 2000).
These results imply a highly structured life course in which phys-
iological and behavioral processes are coordinated. The greater pro-
liferation of neurons in early fetal development among humans, as
compared to monkeys and apes, has cascading effects, extending other
phases of brain development and ultimately resulting in a larger, more
complex, and effective brain. From a behavioral point of view, al-
though cognitive development is largely complete among chimpanzees
by about eight years of age, formal abstract logical reasoning does not
emerge in humans until age sixteen to eighteen. This is the age when
productivity begins to increase dramatically among human foragers.

3.3.2 The Evolutionary Role of Sharing
A central thesis of this chapter is that the human life course could not
have evolved without long-term imbalances in food transfer within
and among families. First, it is clear from the above ¬gures that if chil-
dren are eating more than they produce for some twenty years, those
de¬cits must be subsidized. Surplus food provided by older people
90 Kaplan and Gurven



and those with few dependents can be utilized to ¬nance this long
developmental period.
Second, those data represent average production and consumption
by age, combining data from both sexes. Men and women, however,
specialize in different forms of skill acquisition with correspondingly
different foraging niches and activity budgets and then share the fruits
of their labor. The specialization generates two forms of complemen-
tarity. Hunted foods acquired by men complement gathered foods
acquired by women, because protein, fat, and carbohydrates comple-
ment one another with respect to their nutritional functions (see Hill
1988 and Hames 1989 for a review) and because most gathered foods,
such as roots, palm ¬ber, and fruits are low in fat and protein (nuts are
an exception). The fact that male specialization in hunting produces
high delivery rates of large, shareable packages of food leads to an-
other form of complementarity. The meat inputs of men shift the opti-
mal mix of activities for women, increasing time spent in childcare and
decreasing time spent in food acquisition. They also shift women™s
time to foraging and productive activities that are compatible with
childcare and away from activities that are dangerous to them and
their children.
There are data on the productivity of adults for ten foraging societies
(see Kaplan et al. 2000 for details). On average, men acquired 68 per-
cent of the calories and almost 88 percent of the protein; women
acquired the remaining 32 percent of calories and 12 percent of protein.
After subtracting their own consumption (31 percent of total calories),
women supply only 3 percent of their offspring™s caloric de¬cit (i.e.,
children™s consumption minus their production), while men provide
the remaining 97 percent! Men not only supply all of the protein to off-
spring, but also the bulk of the protein consumed by women. This con-
trasts sharply with most mammalian species (> 97 percent), where the
female supports all of the energetic needs of the offspring until it
begins eating solid foods (Clutton-Brock 1991) and the male provides
little or no investment.
In addition to specialization among men and women, specializa-
tion in productive activities by age is equally important. Foragers and
forager-horticulturalists typically assign low skill/low strength activ-
ities (such as collecting fruits or fetching water) to children, high
strength/high skill activities (such as hunting and extractive foraging)
to prime-aged adults, and low strength/high skill activities (such as
child care and craft manufacture) to elderly people (Bock 2002; Gurven
The Natural History of Human Food Sharing and Cooperation 91



and Kaplan n.d.; Kramer 1998). In this way, family returns from labor
are maximized. It also appears to be the case that there is some special-
ization in activities within age-sex classes. Although this is less docu-
mented, anecdotal reports suggest that some men spend more time
gathering or farming and others more time hunting (Tsimane [Chic-
´
chon 1992], Yuqui [Stearman 1989]), and there is even specialization in
hunting roles and in prey types pursued.
There are also imbalances between families that support this in-
tensive mothering characteristic of human life histories. Figure 3.2
(adapted from Lancaster et al. 2000) compares the acquisition of calo-
ries and reproductive status of baboons (Altmann 1980) with Ache for-
agers (Hill and Hurtado 1996). Time spent foraging during the day is
presented in relation to reproductive status for female baboons, where
foraging time includes both travel and feeding time. Baboon mothers
are hard pressed to meet the demands of lactation. When they must
produce energy beyond their own maintenance needs, their daily time
budget is stretched to the limit. They cannot afford to increase their


Baboon Females: Time Feeding and
Reproductive Status
100
80
% of Day




60
Feeding
40
Traveling
20
0
Nonlactating Lactating


Ache Women: Caloric Acqusition and
Reproductive Status
3500
Kcals per Day




3000
With Weaned
2500
Dependents
2000
Without Weaned
1500
Dependents
1000
500
Nonlactating Lactating

Figure 3.2
Production and reproductive status. Adapted from Lancaster et al. 2000.
92 Kaplan and Gurven



travel time, which would be energetically costly, especially since they
must carry their infants. Instead they increase their feeding time by
reducing resting and socializing to about 15 percent of the day. Lactat-
ing baboons thus work harder. In fact, female baboons have higher
mortality rates when lactating than when cycling or pregnant (Alt-
mann 1980).
In contrast, when lactating and even when they have dependent
juveniles to be fed, Ache, Efe, and Hadza women reduce their time
spent foraging for food (Hurtado et al. 1985; Ivey 2000; Hawkes,
O™Connell, and Blurton Jones 1997). It appears that human females are
able to reduce time spent in energy production when they are nursing,
even though their caloric consumption must increase to support lacta-
tion. Among the Ache, most of women™s food production is derived
from pounding the pulp of palms to produce starch. About 60 percent
of the starch that women produce on extended foraging trips is shared
outside the nuclear family (Kaplan and Hill 1985) with no bias towards
close kin. Since lactating women produce much less palm starch than
women without a baby, this pattern of sharing means that there are
net food transfers from women to other women over periods of several
months to several years.
Third, and most important for the present discussion, even with
such extensive cooperation within families, additional ¬‚ows of food
between families are necessary to support this life history pattern. The
fact that parental provisioning does not cease when children are
weaned means that the caloric burden on parents increases as they
produce more children. The diamonds show how the net demand on
Ache parents changes with age as they produce additional children
(viewed in terms of the man™s age). Demands peak between forty and
¬fty years of age and remain signi¬cant until age sixty. Even though
food production increases with age to about age thirty-¬ve or forty
and remains high, demands increase faster than food production. The
triangles show how net family food production (calories produced mi-
nus calories consumed by self and offspring) changes with the age of
the man.3 These data show that there must be net transfers from the
families of younger men to the families of older men! Moreover, there
is a great deal of variance among men in both family size and pro-
ductivity. Family size is inherently stochastic, due to both infant and
child mortality and to individual differences in fecundity (see Hill and
Hurtado 1996). There are also large differences in hunting ability
among men. For example, there is a ¬ve-fold difference in the long
The Natural History of Human Food Sharing and Cooperation 93


5000

4000

3000
net production (cals/day)




2000

1000

0

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