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Children’s Help and the Pace of Reproduction:Cooperative Breeding in HumansKAREN L. KRAMER

One conspicuous life-history featureof primates generally and humans spe-cifically is the lengthy period afterweaning when young continue to growand develop. Humans take more or lesstwenty years to reach reproductive ma-turity and begin to produce children oftheir own, almost twice as long as otherclosely related primates of similar size(Fig. 2). For children, the transition toeconomic independence is a protractedthough cross-culturally variable pro-cess. For mothers, the long duration ofoffspring dependence coupled with rel-atively short birth intervals and a high

probability of child survival means thatmothers in natural-fertility populationsare often in the position of simulta-neously raising multiple dependents ofdifferent ages. These attributes of hu-man life history—a rapid reproductiverate, mothers having dependents of var-ious ages, and expansion of dependencyinto the juvenile period—contribute tothe prevalence of cooperative breedingto raise young across human popula-tions.

Among nonhuman primates, carry-ing and grooming may continue situ-ationally during the transition fromweaning to full independence.1–3 Moth-ers may assist juveniles, let them foragein close proximity, confer protectionand agonistic support, and help negoti-ate social position. But before a subse-quent birth, nonhuman primate youngare well on their way to, if not accom-plished at, food-provisioning. Humanweanlings, however, depend on some-one else for growth and survival oftenup to and, in some cases, even long aftersexual maturity.4,5

Children’s growth and developmentcan be stratified into four generalstages: infancy, childhood (youngchildren), juvenility and sexual matu-rity. Childhood is defined by slowbody growth rates between weaningand the eruption of permanent molarsand completed brain growth, which

occurs by about the age of seven inmodern populations.6 Because braingrowth is calorically demanding, yetteeth and digestive tracts are imma-ture, children require calorie-rich buteasily digestible and low-volume food.In its broadest definition as a generallife stage shared by other mammals,juvenility is defined as the durationbetween weaning and sexual matu-rity, often marked by the age of firstbirth. As human juveniles develop,they make the transition to eatingadult food, which, depending on hab-itat, they may procure. However, theyare often subsidized by others notonly during childhood but alsothroughout the juvenile period.

Since mothers often have multipledependents and because infants,young children, and juveniles benefitfrom different kinds of investments,mothers are posed with an allocationproblem: how to provide high-qualitychild care without sacrificing eco-nomic activities that feed younger andolder children.7–13 Reflecting this,seminal debates in anthropology havecentered on alternative strategies toprovision young. These debates havefocused, for example, on the impor-tance of grandmothers14–17 and maleparental investment.18–21 This paperadds to this growing discussion by ex-amining those features of human chil-dren and subsistence ecology that fur-ther a demand and an opportunity forcooperative breeding.

Cooperative breeding, a reproductivestrategy and social system in whichnonparental members of a social grouphelp to support offspring who are nottheir own, is relatively rate, but hasbeen documented across diverse taxa,predominantly birds, wild canids, mon-gooses, rodents, and several species of

Karen Kramer is an assistant professor ofanthropology at Stony Brook University.Her recently published book on coopera-tive breeding presents an analysis ofMaya family economics and reproductivedecisions. She is currently working withSouth American foragers and horticultur-alists as a comparative sample for theeffects of economic transition on repro-ductive strategies.

Key words: children’s economic value; reproduc-tive strategies; human juvenility; maternaltradeoffs; natural fertility populations; human lifehistory; human subsistence ecology; Maya

© 2005 Wiley-Liss, Inc.DOI 10.1002/evan.20082Published online in Wiley InterScience(www.interscience.wiley.com).

Because children’s ability to support themselves falls below their consumption,human young are subsidized by others throughout much of their growth anddevelopment. Mothers, however, who often have multiple dependents of differentages, are faced with an allocation problem (Fig. 1). This has led to important debateabout who helps to underwrite the cost of children. This article focuses on thecritical role that children themselves play. Because the human subsistence nicheincorporates a broad diversity of resources that require variable procurement andprocessing costs, dependent children are also important producers, furtheringboth a need and an opportunity for cooperative breeding.

Evolutionary Anthropology 14:000 – 000 (2005)

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Figure 1. Pume mother balancing her role as child-care and food provider.53

Figure 2. Comparative life-history variables for select primates showing age at weaning, age at menarche, age at first reproduction, andaverage life expectancy given survival to reproductive age. Error bars show ranges for age at menarche and age at first birth. Sources:Orangutans: age at weaning66; age at first birth, birth intervals, life span.118 Gorillas: age at weaning119; age at menarche, age at first birth,birth intervals120; TFR.72 Chimpanzees: age at weaning121; age at menarche122; age at first birth, birth intervals123; TFR, life span.72 Humans:age at weaning,72 age at menarche124; age at first birth (mean for Ache, !Kung, Hiwi, and Hadza)72; life span,72 TFR (mean for 57 groupsof foragers, horticulturalists and agriculturalists)125; birth interval mean (mean for Ache, !Kung, Amele and Turkana)14; range: lower,upper.126

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primates.1,22–27 Help benefits mothersby redistributing the cost of raising off-spring, which may have a positive effecton reducing birth intervals, raising ma-ternal fertility or infant survival. Al-though the role of children in subsidiz-ing siblings has been well documentedin the fields of economics, demography,and anthropology,10,28–35 several con-siderations are important to place hu-mans in the framework of cooperativebreeding.

This article reviews those concernswith a specific focus on the relevanceof extended offspring dependence tocooperative breeding. An overview ofthe available time-allocation data forchildren illustrates the kinds of assis-tance that children provide. Becausejuveniles in many traditional societiesare both consumers and producers,attention is given to the analytic chal-lenges this presents in determiningthe effect of their contributions on re-productive success. Cross-culturalcomparisons are limited to naturalfertility populations, not only becausenatural fertility was the human evolu-tionary reproductive milieu but be-

cause family planning and economicoptions to raise children, which affectthe need for and cost of help, are verydifferent in market-economy popula-tions where contraception is prac-ticed. Foragers, agriculturalists, andpastoralists are included in the surveyof natural-fertility populations sinceboth the level of fertility36 and thelevel of children’s economic contribu-tions cross-cut these classifications(Fig. 3). The methodological issuesraised here are applicable acrossmodes of production. The last sectionconsiders the implications of chil-dren’s contributions as allomothers totwo key features of the human adap-tation, demographic proliferation andgeographic expansion.

CONSTRAINTS ONREPRODUCTIVE RATE:

BALANCING MATERNALALLOCATIONS TO CHILD CARE

AND FOOD PRODUCTION

Among nonhuman primates, allo-maternal care is associated with accel-

erated infant growth and shorter birthintervals. Help with carrying infants,for example, allows mother to foragemore efficiently and reallocate energyfrom carrying, an energetically de-manding activity, toward lactationand the production of young.37–41

Among humans, however, mothersmay benefit more from help that re-solves the competing demands of car-ing for younger and older childrenthan they do from help that decreasestheir investment in infant care per se.Among the Toba of Argentina, for ex-ample, helpers did not have a signifi-cant effect on the time mothers spentnursing or in other child care activi-ties. However, nursing women wholived with female helpers spent lesstime in domestic work.42 In his semi-nal time-allocation study among theIfaluk, Turke35 showed that girls con-tribute substantially to the subsis-tence economy and that mothers whobore girls early in their reproductivecareers had greater completed fertilitythan did those whose first-born chil-dren were boys (but see Hames andDraper43).

Figure 3. Children at work. Daily hours that children cross-culturally spend in productive activities. Sample limited to natural-fertility subsistencepopulations (a minor portion of calories coming from wage labor). Values reported are daily hours children allocate to foraging, hunting, fieldwork, and domestic tasks taken from published sources (where child care is included as domestic work, it is noted below). Sources: Foragers(bottom to top): !Kung, 4–14-year-olds50; Hadza, 6–13-year-olds.16 Forager/Agriculturalists (bottom to top): Mikea children, weaning to adoles-cence (includes child care127); Machiguenga, 6–13-year-olds128; Agriculturalists: Switzerland, age group not specified129; India, Skoufias, 14–17-year-olds130; Maya, 9–14-year-olds51; Java, 9–11-year-olds54; Java, 10–12-year-olds85; Mayo, age group not specified,131 Nepal, 9–11-year-olds84;Bangladesh, 10–12-year-olds (child care included in domestic work).29 Pastoralists: Ariaal, 6–11-year-olds.132

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Any mother has a finite time andresource budget out of which variouscompeting expenditures are funded:taking care of herself, caring for in-fants, providing specialized food foryounger children and adult food forolder children. She therefore makesdecisions about how to allocate hertime. The time mothers allocate tochild care shows surprising cross-cul-tural similarity (Table 1). This regu-larity may, in part, indicate that noone can substitute maternal timespent nursing, and that mothers candownwardly adjust time allocated tochild care only so much, regardless ofthe availability of helpers. However,mothers may readjust time spent inactivities that support older children.

Among the Ye’kwana, neotropicalagriculturalists, time-allocation datashow that mothers are challenged tocare for infants while simultaneouslymaintaining a normal amount of gar-den work. When Ye’kwana mothershave very young dependent children,they spend less time in economic ac-tivities, but do not compromise thetime allocated to child care.7 Amongthe Hiwi and Ache, South Americanhorticulturalists and foragers, nursingmothers decrease their foraging effortrelative to nonnursing mothers.18,44

Nursing Hadza women, hunter-gath-erers living in the woodlands and sa-vannas of northern Tanzania, havelower foraging returns and returnrates than do women with older chil-dren.45 Nursing Maya mothers allo-cate significantly less time to field

work; nursing mothers with infantsless than a year old spend no time infield work.46 These studies suggestthat mothers with a nursing infantand older children balance the in-creased demands of a newborn by re-ducing the time spent in domestic ac-tivities, foraging activities, or fieldwork rather than that spent in childcare.

A sample of natural-fertility popula-tions for which time allocation dataare available shows that children allo-cate some time to child care (Fig. 4).But regardless of mode of subsistence,children cross-culturally allocate sub-stantially more time to economic ac-tivities. While allomaternal child careoffsets material time constraints tosome extent, it is economically thatchildren make their greatest contribu-tion. These patterns suggest that therate of human reproduction may bemore sensitive to help directed towardeconomic activities that support chil-dren during childhood and juvenilitythan to help directed toward childcare during infancy.

HUMAN SUBSISTENCE FROM ACHILD’S PERSPECTIVE

From the viewpoint of children, es-pecially young children trying tomake it on their own, the human feed-ing niche is complex. All human dietsincorporate a diverse suite of plant,animal, and often aquatic resources.In addition to food procurement,many of the foods incorporated in the

human diet require processing: butch-ering, cracking, hulling, winnowing,shelling, leaching, pounding, grindingor cooking. Young children are inca-pable of these processing tasks andjuveniles, depending on the particulartask, may or may not have thestrength or skill to accomplish them.47

Besides processing, in many environ-ments dependence on stored food forat least part of the year is critical tosolving overwintering or hypersea-sonal problems in resource availabil-ity and predictable annual resourceshortfalls. Storage may require bon-ing, drying, salting, and smoking aswell as the use of specialized tools,structures and containers. In most hu-man environments, besides food pro-curement and processing, survivalalso minimally entails proficiency attool manufacture and the technologyrequired to obtain shelter and cloth-ing, activities that may be beyond thegrasp of a child’s ability. Only underdire circumstances does a child liveon his or her own and concomitantlyfund all of the tasks necessary for sur-vival.

Low-Cost Juvenile Help

Although human juveniles are eco-nomically dependent, they also are apotential source of low-cost help forseveral reasons. Helping may have en-ergetic costs and trade-offs withhealth risks, but because juveniles arenot yet fully grown, sexually mature,or competing for mating opportuni-

TABLE 1. MEAN PROPORTION OF DIRECT CHILD CARE RECEIVED BY AN INFANT FROM VARIOUS CARETAKERS

Mothers Fathers Siblings GrandmothersOtherRelated/Unrelated

Yekwana 49% 2.7% � 16.7% 11.2% 20.6%� 1.9%

Agta 51.7% 4.4% � 10.2% 7.6% —� 1.1%

Maya 46.1% 1.6% � 31.6% 1.2% 11.2%b

� 4.6% 2.8%Alyawaraa 53% �1% 31% — 16%Toba 50% — � 33% 12.5% —

� 4%Efec 50% 6% � 13% 9% 9%

� 14%a Values reported for carrying children only. Values for male and female children reported as an aggregate.b Value for related individuals among whom aunts comprise 8.4%.c Focal follow data.

Sources: Yekwana114; Agta115; Maya87; Alyawara116; Toba117; Efe (Ivey Henry, unpublished data).

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ties, they do not compromise theirown reproductive success by helping.In addressing the evolutionary puzzleof why helpers help, emphasis hasbeen placed on the indirect fitnessbenefits to the helper. However, re-cent research suggests this emphasismay overshadow positive direct bene-fits. The cost of helping may be low,especially when the benefits to a help-er’s survival are high.22 This point hasclear implications with respect to hu-mans, since juveniles, who fall shortof where they need to be to make it ontheir own, have much to gain by main-taining their dependence. Eventhough juveniles are sexually imma-ture and closely related to their sib-lings, the cost for them to help mayalso be low for another reason.

The human feeding niche has theimportant feature of incorporating abroad diversity of foods that requirevariable procurement and processingcosts. While juveniles depend on oth-ers for some of what they need to sur-vive, they also may be successful atprocuring some of the resources theyneed and, indeed, some in excess oftheir own consumption needs.

Where children live in safe environ-ments with easy-to-obtain foods, theymay spend considerable time foragingfor fruit or shellfish, snack foods thatare an ideal package size for childrenand require little processing. Under

these circumstances, children maycontribute substantially to their ca-loric intake.20,28,48–50 For example,among the Hadza, children provide50% of their caloric intake by the age

of five.49 Among subsistence agricul-turalists, Maya boys by the age ofseven and girls by the age of six pro-duce 50% of what they consume.51

Within any one subsistence regime,children’s economic dependence alsoimportantly varies across resourcesand tasks. Among foragers in theWestern Desert of Australia, for in-stance, children can successfully cap-

ture small game, such as reptiles androdents, and collect grubs and fruit,but depend on others for seed cakesand larger game.52 In other food pro-duction activities children may pro-duce enough to meet their own needsas well as the needs of their siblings.In another forager example, a Pumeboy living on the llanos of Venezuelais successful at bringing home a catchof fish, enough fish to feed himself andhis siblings. But he also depends onshares of processed plant food andlarger game from others.53 Amongsubsistence agriculturalists, aneleven-year-old Maya boy spends 3%of daylight hours, or about a half anhour a day, harvesting enough maizeper day to meet his own consumptionneeds as well as those of his siblings.However, he does not directly con-sume the maize he harvests, but doesso after it has been shelled, leached,soaked, ground into meal, and pro-cessed as tortillas, tasks that his oldersisters and mother perform.63

How much time children spend at atask and how efficient they are at thattask has been theoretically linkedthrough opportunity costs, or theforegone benefit to an individual toinvest in one activity and not an-other.54,55 One interesting point thatcomes out of this is that even thoughchildren may be less efficient thanadults, the opportunity cost for them

While juveniles dependon others for some ofwhat they need tosurvive, they also maybe successful atprocuring some of theresources they needand, indeed, some inexcess of their ownconsumption needs.

Figure 4. Proportion of time that juveniles allocate cross-culturally to child care (gray portion of bar) and economic work (black portion ofbar). Values averaged for males and females. Sources: Maya, 7–15-year-olds51; Canchicos, 6–10-year-olds131; Kikuyu, 6–10-year-olds133;Logoli, 6–10-year-olds133; Zimbabwe, 10–18-year-olds33; Botswana, 7–15-year-olds134; Java, 6–14-year-olds84; Nepal, 6–14-year-olds84;Machiquenga, 6–13-year-olds128; Ifaluk, 0–15-year-olds.35



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to participate in certain tasks is lowsince they can do little else with theirtime at a higher rate of return. Forexample, the Maya boy in the earlierexample can harvest maize at a returnrate of 30 kilos per hour, or 0.61% thatof the mean adult male return rate.Nonetheless, he allocates as muchtime to the task as an adult since fewother activities offer a higher rate ofreturn.56

In sum, the opportunity cost to ju-veniles for helping is attenuated sincethey are not competing for mating op-portunities and there are few compet-ing ways to spend their time.Although some procurement, manu-facture, and processing activities maybe beyond their scope, from the juve-nile’s point of view human subsistencehas the important characteristic of in-corporating a broad diversity of re-sources and tasks to procure and pro-cess those resources. They can

accomplish some activities at the levelof their own consumption and pro-duce a surplus for others. Conse-quently, in traditional subsistenceeconomies helping is often embeddedin the same general suite of activitiesas self-provisioning. If a Pume girl, forexample, spends several hours pro-cessing bitter manioc, some portion ofthat work contributes to her own well-being and some portion goes towardsubsidizing her siblings. This under-scores one of the central methodolog-ical concerns in situating children inthe context of cooperative breeding,how to define help and helpers.

DO CHILDREN HELP?

One question that is central to as-sessing the adaptive significance ofcooperative breeding is whether help-ers really help? The weight of evidencefrom animal behavior studies shows

that helpers have a positive effect onparental reproductive success.25,57–60

However, this remains unclear withrespect to humans for several reasons.Help from children is difficult to cali-brate and measure and, together withecological variation in the mode ofsubsistence, has contributed to pro-longed debate over what otherwisemight be simple empirical questions:Are children helping? And is the effectof their help on parental time and re-productive budgets positive? Identify-ing what conditions variation in chil-dren’s economic contributions and itseffect on fertility has been furtherhampered because of differences inhow children’s economic value is eval-uated (Box 1).

How we think about help and help-ers depends in part on whether thequestion is approached from the eth-nographic present or from evolution-ary past. From the point of view of the

Box 1. Do Children Help?

Interest in children’s economiclives has been launched from twomain theoretic domains, behavioralecology and economics. Both ap-proaches to the quality-quantitytradeoff emphasize the cost of raisingchildren (parental investment offsetby children’s contributions).77,78–80

The proliferation of empirical studiesthat followed the theoretic link madebetween children’s labor and fertil-ity81–83 illustrated that the relationshipwas far from straightforward, in part

because of methodological differ-ences in how children’s economicvalue was defined. For example,studies in both the economic and an-thropology literature have found thatchildren spend considerable time inproductive field and domestic activi-ties and are an economic benefit tothe family. Other studies have foundthat when consumption is balancedagainst production (net production)children are a net cost until and evenafter sexual maturity. Still others have

found that wealth flows (cumulativelifetime consumption up to each ageminus cumulative lifetime productionup to that age) are such that the nettransfer of resources and labor isdownward from parent to child wellinto adulthood. What conditions vari-ation in the relationship between helpand reproductive rate remains un-clear since each measure answersdifferent questions about children’svalue as helpers.

Analytic Perspective Questions Addressed Studies

Time (Production) How much time do children spend working? What is theage and sex patterning of children’s activities?

82–85

Net production (Productionminus consumption)

At what age do juveniles reach independence? What isthe age-specific cost (production-consumption) ofchildren?

29,86–89

Wealth flows (Cumulative netproduction minus cumulativenet consumption)

What is the net cost of childbearing? At what age dochildren pay back their lifetime cumulative cost? Is thenet flow of wealth upward from children to parents ordownward to children? Are children a net economicasset?

35,86,90–94

Timing across the family life cycle(Age-specific production andconsumption across thedemographic family life cycle)

What effect does children’s production have on parentaltime? How does the level of consumer demands andparental and child production change with family size?How much do children as a group offset their cost andthe cost of siblings? Who subsidizes dependent youngand how does that change as the family matures?

62,88,95

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modern mother, since juveniles aresubsidized any contribution a childmakes offsets his or her cost and theamount of subsidy that a mother orsomeone else has to make. From thepoint of view of a mother in the evo-lutionary past, since juveniles in alllikelihood were autonomous feeders,help would have been assistance be-yond self-provisioning. In otherwords, is a helper an economically in-dependent individual? If so, in themodern human case does that excludeyoung children and juveniles as help-ers? Or, given that the human subsis-tence niche is such that juveniles andeven young children may not be inde-pendent but are nonetheless signifi-cant producers at some tasks, such asharvesting, fishing, collecting shell-fish, foraging for fruit, and haulingwater, should helpers be more broadlydefined?

Time allocation data collected dur-ing a year-long study with a group ofMaya subsistence agriculturalists(Box 2) is used as an example to eval-uate both of these vantage points interms of alleviating parental time con-straints. Rather than delimiting help-ers based on age or developmental sta-tus, helpers can define themselvesusing the age- and sex-specific rela-tionship between an individual’s levelof production and consumption.

As children grow and develop, theyconsume more, but also producemore (Fig. 5). Infants are fully depen-dent on others, but children increas-ingly contribute to their own con-sumption. From the age of seven untilfifteen years, Maya boys producemore than half of what they consume.The transition from net consumer tonet producer occurs during their six-teenth year, marking the close of de-pendency. Maya females, not shownin Figure 5, have a similar transition,but become net producers a year ear-lier, by the age of fifteen years.

Figure 6 shows variation in the ageof economic independence for the fewnatural-fertility populations for whichboth production and consumptiondata are reported in the same cur-rency. While Piro, Machiguenga, andAche children provide 20% to 25% oftheir own calorie needs by the age ofeighteen years, Maya eighteen-year-olds have been surplus producers for

several years. Reflecting this differ-ence, variation in the timing of thetransition to net production influ-ences parental time budgets so thatPiro and Machiguenga older maleadults work much harder than domale Maya adults.61

Positive net production empiricallymarks the duration of offspring inde-pendence and is a clear indicationthat these teenagers, who live in theirnatal home several years more, notonly are paying for themselves but aregenerating a production surplus. Ju-veniles, though not net producers,produce much of what they consume.Importantly, if total work is disaggre-gated, juveniles are net producerswhen they perform easier tasks suchas harvesting and water collection,but are not when they engage in othertasks such as planting maize and foodpreparation.56 Thus, a Maya juvenile

may harvest enough maize to feedhimself and his family, but consumesshares of his father’s hunting returns,emphasizing that juveniles, while notindependent, may offset their owncost and also provide help beyondself-provisioning through some tasks.

DOES CHILDREN’S HELP AFFECTPARENTAL TIME OR

REPRODUCTIVE RATE?

Parents are in the position of bal-ancing all of their dependents’ con-sumption demands and the compet-ing demands on their time to supportboth younger and older children. Toassess whether constraints on parents’time are offset by their children’s con-tributions, children’s helping behavioris viewed from the perspective of

shifts in labor supply across the lifecycle of the family.

Many demographic changes occuracross the life cycle of the family.Marriage unions form and sometimelater a child is born. First-born chil-dren become productive workerswhile their mothers are in their primechildbearing years. Children bornlater are still young when their oldersiblings marry, leave their natalhouseholds, and begin families oftheir own. Parents age and becomegrandparents while they have severaldependent children living at home. Aschildren enter and leave the familythrough births and marriages, notonly does the number of children fluc-tuate, but so does the ratio of youngchildren, who consume little and pro-duce even less, to older children, whoconsume more but also producemore.

Maya reproductive histories can beused to construct a heuristic frame-work to track demographic pressureacross the family life cycle as a familymatures and consumer demands andlabor supply shift. In the first step,demographic changes across the fam-ily life cycle are constructed using theaverage age at marriage, which is 21.6years for Maya males and 18.7 yearsfor females; the average birth intervalat each parity, which is about twoyears; and the average completed fam-ily size, which is 7.2 people. A lifetable appropriate to a village’s level ofmortality is then superimposed to cal-culate the age distribution of surviv-ing household members. To modelchildren’s economic value from theviewpoint of a parent, average age-and sex-specific levels of productionand consumption derived from thetime-allocation data are folded intothese demographic changes so thatconsumption demand relative to laborsupply can be observed across the lifecycle of the family. These methods arefully explained in Lee and Kramer62

and Kramer.63

Figure 7 shows a mother’s and fa-ther’s production (time spent in do-mestic work, field work, and wage la-bor for those males who participate;see Box 2) relative to their family’stotal consumption from the onset ofmarriage to the birth of the first child,the birth of the last child, and the de-

How we think abouthelp and helpersdepends in part onwhether the question isapproached from theethnographic present orfrom evolutionary past.



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parture of the first and last child. Thedeficit in parental production relativeto family consumption increases dur-ing early parities, but is greatest afterthe last child is born, when most chil-dren are living at home.

The relative economic contribu-tions (time spent in domestic work,field work, and wage labor) of moth-ers, fathers, and children of differentdevelopmental status across the fam-ily life cycle are shown in Figure 8.Parents are responsible for virtually

all of their family’s production duringthe first five years of marriage. Chil-dren’s contributions play a substantialrole during the peak demographicsqueeze during the mid-portion of thefamily life cycle when parents haveseven to eight children living at homeand are least able to support their de-pendent young.

The inverse of parental share ofhousehold production indicates bywhat factor parents would have to in-crease their production if children

made no economic contribution. Ifchildren produced nothing, Maya par-ents would have to work 2.5 times ashard as they do to maintain their chil-dren’s consumption between the 20thand 33rd years of the family life cycle.Were it not for the economic contri-butions of children, parents in theirfourth and fifth decades would have toincrease their work effort by 150%,each parent working more than 16.5hours a day (real-time hours). Parentscould not possibly sustain this level of

Box 2. Maya Study Population and Data

The Xculoc Maya are subsistencemaize farmers who live in a village witha population of 316 in the interior of theYucatan Peninsula, Mexico. The major-ity of calories they consume come frommaize, though the Maya also cultivatebeans, squash, sweet potatoes, pea-nuts, and other fruits and vegetables.Honey is collected for sale and smallquantities of maize may be exchangedin the village store for limited commod-ities such as vegetable oil, eggs, andcandles. Otherwise, no cash crops aregrown. At the time of this study therewas no running water or electricity inthe village. Villagers participated mini-mally in wage labor and the cash econ-omy. Although a rustic primary schoolhad been built in the village, classeswere held infrequently and schoolinghad little impact on children’s labor.Maya children live and work in theirparents’ household until they are intheir late teens or early twenties, whenthey marry and begin families of theirown. Exogamy is uncommon. Aftermarriage, most couples stay in the vil-lage and establish fields of their own.Mothers and fathers spend equalamounts of time supporting their fam-ily, with both allocating 6.5 hours dailyto productive activities other than childcare during their prime reproductiveyears.

Time allocation data were collectedduring a year-long study using scansampling and focal follow tech-niques.96–98 Each participant wasscan-sampled every other weekthroughout the field season for an ob-servation period of three to four hours.

During an observation period, an indi-vidual’s activity was recorded every 15minutes. The following analyses in-clude 112 individuals ages 0 to 65years and draw on over 17,000 scanobservations. These are used to com-pile a detailed profile of the proportionof time an individual spends in agricul-tural activities (ground preparation,planting, weeding, harvesting, andtransporting field goods); domestictasks (washing, cleaning, food pro-cessing, food preparation, cooking,chopping firewood, hauling water, andtending animals), and wage labor.

For work effort to be comparableacross age and sex classes, two ad-justments are made to convert thesimple proportion of time an individ-ual spends working into an estimateof individual production. For example,if a mother and her twelve-year-olddaughter chop wood for an hour theyare likely to return to the village withvery different quantities of wood. Fortheir work effort to be comparable,the first adjustment accounts for therelative efficiency (return rates) ofchildren compared to adults and ofmales compared to females. The sec-ond adjustment accounts for differ-ences in the value of time.77 Becausesome tasks require greater caloric ex-penditure, more strenuous tasks areweighted more heavily than are lessenergetically costly tasks.99,100

An individual’s consumption in-cludes his or her performing the samesuite of activities mentioned above.Consumption is expressed in hoursand is proportional to an individual’s

daily caloric requirements and house-hold size. Maya children consumeabout 3.3 million calories from birth toage 20. Although food is one of themost important resources, measuringproduction or consumption based oncalories alone underestimates bothchildren’s dependence on and contri-bution to a wide variety of domesticand field activities that do not have acaloric (or monetary) output. For ex-ample, some portion of the time that ajuvenile spends each day hauling wa-ter provides water that is consumedby her siblings. Some portion of thetime a mother spends each day pro-cessing food provides food con-sumed by the juvenile. Among theMaya, 80% of time spent working isrelated to food production: process-ing, preparation, hauling water, col-lecting firewood—activities that arenecessary for survival and thatwomen and children primarily fund.Expressing production and con-sumption as time has the advantageof expanding their definition beyondfood and including this range of othertasks. In the graphs that follow, pro-duction and consumption are ex-pressed in weighted hours as ex-plained earlier. Weighted hours areon an order of magnitude greater thanreal-time hours as follows: for individ-ual males age 0–6 years, 1.00–1.5;ages 7–15 years, 1.3–1.8; �15 years,1.5–2.3; for individual females ages0–6 years, 1.0–1.5; ages 7–15 years,1.2–1.8; and �15 years, 1.5–2.1. Foran overview of the study site andmethods, see Kramer.51,87,95

8 Kramer ARTICLES


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production. Children as a group offseta substantial portion of their own con-sumption, producing 80% of whatthey consume in the 20th year of thefamily life cycle and virtually all ofwhat they consume by the 28th year,when Maya parents still have severalchildren living at home.62,63 Across

the demographic life cycle of the fam-ily, Maya children play an importanteconomic and reproductive role byenabling parents to continue child-bearing during life-cycle stages whenparents themselves may not have suf-ficient time and resources to supporttheir family.

DEMOGRAPHIC ADVANTAGEOF THE DISTRIBUTION OFOFFSPRING DEPENDENCY

One potential advantage of the dis-tribution of children’s dependencyand productivity over several develop-mental stages is its implications with

Figure 5. The age-specific relationship of production to consumption from birth to the age of twenty years for an average Maya male child(see Box 1). The dotted line shows total consumption at each age. Bars show the portion of consumption not met by production (gray) andthe portion offset by production (white). Black portion of bar denotes production in excess of consumption.

Figure 6. Age-specific production minus consumption for Maya females (dotted line) and males (solid line). Markers denote the age atwhich production equals consumption for the Maya51 and three South American populations.86



regard to the demographic and geo-graphic proliferation of humans.While hominid evolution is tradition-ally explained as long-term directionalselection to constant environmentaltrends, Potts64 pointed out that theidea of an environment of evolution-

ary adaptiveness is inconsistent withwhat is now known about local andglobal dynamics in the environmentalpast. Rather than digging in adap-tively to fill a particular niche, an al-ternate means of coping with variableenvironmental conditions is to in-

crease versatility. Concomitant withlocomotor, cognitive, and social traitsthat broaden adaptability is the evolu-tion of a human life-history patternthat is favorable to reproductive suc-cess and survival in a range of envi-ronmental conditions.

Figure 7. Maya mothers’ and fathers’ production relative to family consumption across the family life cycle. Family consumption is the sumof consumption across family members. Parental deficit is the family consumption minus the mother’s and father’s production.

Figure 8. Time that Maya mothers, fathers, young children (0–7 years), juveniles (8–16 years) and older coresidential children (17–21 years)allocate to production across the family life cycle expressed as proportion of total household production.

10 Kramer ARTICLES


Population growth in long-lived,large mammals is generally limited bygenerational time. The great ape lifehistory of long birth intervals, ex-tended infancy, and relatively high ju-venile mortality places further limitson lifetime reproductive success. Thepopulation growth potential of thegreat apes is near demographic equi-librium.65–67 Humans, in contrast, arecapable of staggering populationgrowth. Because the duration of keydevelopmental stages varies acrossprimate life histories, constraints onreproductive rate also vary. Great apemothers manage the demands ofslow-growing young through pro-tracted infancy. For human mothers,slow growth and dependency is dis-tributed across infancy, childhood,and juvenility, producing a very differ-ent effect on population growth po-tential.

Although young children are nolonger dependent on mother’s milk,childhood in many ways is an exten-sion of infancy in that young children,who are unable to procure and pro-cess the specialized foods they need,depend on provisioning by others fortheir survival.68,69 If the periods of in-fancy and childhood are consideredtogether from the point of view of theyoung’s dependence, their duration iscomparable to the period of infancy inthe great apes (Fig. 2). However, fromthe point of view of the human motherthere is a significant distinction. Oncea child is weaned, a mother can phys-iologically prepare to bear anotherchild and reinvest energy from nurs-ing to pregnancy.70,71 The insertion,then expansion, of childhood as a de-velopmental stage and the shift in de-pendency from breastfeeding to foodprovisioning allow mothers to pro-duce children in relatively rapid suc-cession.

Yet the potential gain in lifetime fer-tility from an increase in the pace ofreproduction could also be counteredby elevated mortality risk because ma-ternal time and resources are spreadtoo thin. The traditional view of pasthuman population growth is thatgains in fertility were kept in check byhigh mortality, with the net effect oflow population growth. Nonetheless,population growth was sufficient toproduce an estimated world popula-

tion of one billion before the onset ofthe modern demographic transition.This magnitude of growth, though di-minutive compared to what it wouldbe during the modern transition, isdemographically remarkable com-pared to the great ape populationgrowth dilemma. Differences in pri-mate infant and juvenile mortalityrates72 suggest that human mothersmaintain relatively short birth inter-vals without increasing child mortal-ity. Redistributing the cost of repro-duction appears to be a key to holdingin balance the human life-historycombination of a relatively rapid rateof reproduction and a high probabil-ity of child survival.

Because human offspring are subsi-dized during childhood and juvenility,mothers commit to raising multipleyoung.73 Several lines of evidence sug-gest that some form of allomaternalassistance had to be in place beforemothers consigned to producing long-term dependent offspring, and thatthe level of dependence and commit-ment to cooperative breeding in-creased over time.10,74 In all likeli-hood, the direction of selection wasfrom self-provisioning juveniles to ex-tended dependence. Hrdy75 points outthat some degree of allomaternal as-sistance from immature individuals,postreproductive females, and possi-bly fathers was already available, andthat prolonged offspring dependencecame as a corollary of such coopera-tive breeding.

Provisioning juveniles allowed hu-mans to move into habitats theycould not have lived in if juveniles

were self-feeders.17 It is often tacitlyassumed that as provisioning ex-tended further into juvenility, self-provisioning decreased in step. Inthose ecological situations where itdid not—where juveniles were ableto provision themselves to some ex-tent or had foraging, processing, orprocurement returns for some re-sources in excess of their consump-tion—the demands of extended de-pendency may have been furtheroffset through their transfers toother dependents. Dental patternssuggest that the protraction of child-hood and the feasibility of the shortbirth intervals characteristic of mod-ern natural fertility populations arelikely to have occurred fairly re-cently following the emergence andgeographic dispersal of Homo erec-tus.76 The push then toward increas-ing commitment to allomaternal as-sistance may have been punctuatedboth before extended dependenceand again with geographic expan-sion and a further shortening ofbirth intervals.

Several points are made in sum-mary. First, the most difficult timefor mothers to meet their children’sconsumption needs is when childrenare older, not younger, when theyrequire food and other investmentsrather than child care. Followingthis, while juveniles assist in childcare, they spend far more time ineconomic activities. Second, thetime children allocate to economictasks is variable, but the differencescross-cut mode of subsistence. Thissuggests that children’s helping be-havior varies with costs and benefitsspecific to subsistence ecology andsocial organization, not withwhether a child is a hunter-gatherer,agriculturalist, or pastoralist per se(Box 3). Third, from a child’s pointof view, the human feeding nichehas the important characteristic ofincorporating a broad diversity ofresources that require variable cap-ture and processing costs. In manyecological situations, while childrenare subsidized, they are able to pro-duce some of what they need at thelevel of their own consumption andsome of what they need in excess oftheir own consumption.

The period of dependency for hu-

Redistributing the cost ofreproduction appears tobe a key to holding inbalance the human life-history combination of arelatively rapid rate ofreproduction and a highprobability of childsurvival.



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mans, while prolonged compared tothat of the great apes, is spread overthe developmental stages of infancy,childhood, and juvenility. Because itis not confined to infancy, physiologi-cal constraints on reproductive rateare attenuated. Short birth intervalspresent other trade-offs and an alloca-tion problem for mothers. Not only isdependency distributed across severallife-history stages, but so is the cost ofdependency. The Maya data suggestthat parents only have enough time tosupport about four children. In situa-tions where the reproductive rate ex-ceeds this level of fertility, as it does inmost natural-fertility populations re-gardless of mode of production, chil-dren’s contributions are an alternateor additional means of maintainingthe relatively short birth intervalscharacteristic of modern human re-production. The relatively fast rate ofhuman reproduction appears to besensitive not only to children’s self-sustaining activities but to direct helpto mothers. In addition to their chil-dren, mothers receive help from theirspouses, their own mothers, other fe-male kin, and unrelated individuals.Flexibility in allocare strategies to al-leviate constraints on maternal timeand resource budgets under variableecological and demographic condi-tions gives rise to a number of path-ways to cooperative breeding.

ACKNOWLEDGMENTS

Abiding appreciation to the Mayaand Pume for giving me the ethno-graphic opportunity to observe theirchildren at work and play; for gra-ciously providing food, shelter, andcompany; and for allowing me to col-lect demographic and time-allocationdata to my heart’s content. Much ofthe methodological and theoretic de-velopment presented here benefitedfrom collaboration with and generousinput from Ronald Lee and HillardKaplan. I am grateful for the previouscontributions of Sarah Hrdy, whosework has done much to bring humansas cooperative breeders to the foreand has given much insight into hu-mans as cooperative breeders. I thankJane Lancaster, Sarah Hrdy, BruceWinterhalder, Kristen Hawkes, JohnFleagle, and the reviewers for theirinvaluable comments on previousdrafts.

REFERENCES

1 Brown JL. 1987. Helping and communalbreeding in birds. Princeton: Princeton Univer-sity Press.2 Altmann J. 1980. Baboon mothers and infants.Cambridge: Harvard University Press.3 Nicolson NA. 1987. Infants, mothers, and otherfemales. In: Smuts BB, Cheney DL, Seyfarth RM,Wrangham RW, Struhsaker TT, editors. Primatesocieties. Chicago: University of Chicago Press. p330–342.4 Lancaster J, Lancaster C. 1987. The watershed:

change in parental-investment and family-forma-tion strategies in the course of human evolution.In: Lancaster J, Altmann J, Rossi A, Sherrod L,editors. Parenting across the life span. New York:Aldine De Gruyter. p 187–205.

5 Kaplan H. 1996. A theory of fertility and paren-tal investment in traditional and modern humansocieties. Yearbook Phys Anthropol 39:91–135.

6 Bogin B. 1999. Patterns of human growth.Cambridge: Cambridge University Press.

7 Hames R. 1988. The allocation of parental careamong the Ye’Kawana. In: Betzig L, BorgerhoffMulder M, Turke P, editors. Human reproductivebehavior. Cambridge: Cambridge UniversityPress. p 237–251.

8 Hewlett B. 1991. Demography and childcare inpreindustrial societies. J Anthropol Res 47:1–37.

9 Hill K, Hurtado M. 1996. Ache life history. NewYork: Aldine de Gruyter.

10 Hrdy SB. 1999. Mother nature. New York:Pantheon Books.

11 Lancaster J. 1991. A feminist and evolution-ary biologist looks at women. Yearbook Phys An-thropol 34:1–11.

12 Lee RB. 1979. The !Kung San: men, womenand work in a foraging society. Cambridge: Cam-bridge University Press.

13 Panter-Brick C. 1989. Motherhood and sub-sistence work: the Tamang of rural Nepal. J Bio-soc Sci 23:137–154.

14 Alvarez HP. 2000. Grandmother hypothesisand primate life histories. Am J Phys Anthropol113:435–450.

15 Hawkes K, O’Connell J, Blurton Jones N.1989. Hardworking Hadza grandmothers. In:Standen V, Foley RA, editors. Comparative socio-ecology: the behavioral ecology of humans andother mammals. London: Basil Blackwell Press.p 341–366.

16 Hawkes K, O’Connell J, Blurton Jones N.1997. Hadza women’s time allocation, offspringprovisioning and the evolution of long postmeno-pausal life spans. Curr Anthropol 38:551–577.

17 O’Connell JF, Hawkes K, Blurton Jones NG.1999. Grandmothering and the evolution ofHomo erectus. J Hum Evol 36:461–485.

Box 3. Sources of Cross-Cultural Variation in Children’s Helping Behavior

The role that ecology and subsis-tence play in conditioning the age pat-terning of children’s work has a longtradition of study.108,109 How peoplemake a living has an obvious link tovariation in the economic contributionof children since subsistence, as itssimplest, delimits the kinds of tasks

children can perform. However, empir-ical studies show that children’s eco-nomic participation expresses a rangeof variation within the general subsis-tence regime and that mode of produc-tion alone—knowing whether a child isa hunter-gatherer or an agriculturist—isnot highly informative in predicting the

level of children’s economic contribu-tion. Rather, the extent to which chil-dren work is conditioned not only bythe kinds of economic tasks availableto them but by the costs and benefitsassociated with participating in thosetasks and the opportunity costs ofspending their time in other ways.

Approaches to the Costs and Benefits of HelpingLearning and maternal experience 10,103,104

Indirect fitness benefits 35,94

Rotten-kid theorem 47,77

Task skill and strength requirements; tasks available to children; resource opportunities 47,48,105–108

Opportunity cost to train or educate children 47,79,109–111

How children learn 87

Health risks 86,106,112

Marginal value of alternate sources of help 113

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18 Hurtado M, Hawkes K, Hill K, Kaplan H.1992. Trade-offs between female food acquisitionand child care among Hiwi and Ache foragers.Hum Nat 3:1–28.19 Irons W. 1983. Human female reproductivestrategies. In: Wasser S, editor. Social behaviorof female vertebrates. New York: AcademicPress. p 169–213.20 Kaplan H, Lancaster J. 2000. The evolution-ary economics and psychology of the demo-graphic transition to low fertility. In: Cronk L,Chagnon N, Irons W, editors. Adaptation andhuman behavior: an anthropological perspective.New York: Aldine de Gruyter. p 283–322.21 Lancaster J, Kaplan H, Hill K, Hurtado AM.2000. The evolution of life history, intelligenceand diet among chimpanzees and human forag-ers. In: Tonneau F, Thompson NS, editors. Per-spectives in ethology, evolution, culture and be-havior. New York: Kluwer Academic. p 47–72.22 Clutton-Brock TH. 2002. Breeding together:kin selection and mutualism in cooperative ver-tebrates. Science 296:69–72.23 Clutton-Brock TH, Brotherton PNM, O’RiainMJ, Griffin AS, Gaynor D, Kansky R, Sharpe L,McIlrath GM. 2001. Contributions to cooperativerearing in meerkats. Anim Behav 61:705–710.24 Cockburn A. 1998. Evolution of helping be-havior in cooperatively breeding birds. Ann RevEcol Syst 29:141–177.25 Emlen ST. 1991. Evolution of cooperativebreeding in birds and mammals. In: Krebs JR,Davies NB, editors. Behavioral ecology: an evo-lutionary approach, 2nd ed. London: BlackwellScientific Publications. p 301–337.26 Skutch AF. 1987. Helpers at birds’ nests: aworldwide survey of cooperative breeding andrelated behavior. Iowa City: University of IowaPress.27 Solomon NG, French JA, editors. 1997. Coop-erative breeding in mammals. Cambridge: Cam-bridge University Press.28 Blurton Jones N, Hawkes K, O’Connell J.1997. Why do Hadza children forage? In: SegalN, Weisfeld GE, Weisfeld CC, editors. Unitingpsychology and biology: integrative perspectiveson human development. American PsychologicalAssociation. p 164–183.29 Cain M. 1977. The economic activities of chil-dren in a village in Bangladesh. Popul Dev Rev3:201–227.30 Flinn MV. 1988. Parent-offspring interactionsin a Caribbean village: daughter guarding. In:Betzig L, Borgerhoff Mulder M, Turke P, editors.Human reproductive behavior: a Darwinian per-spective. Cambridge: Cambridge UniversityPress. p 189–200.31 Ivey PK. 2000. Cooperative reproduction inIturi Forest hunter-gatherers: who cares for Efeinfants. Curr Anthropol 41:856–866.32 Lee RD, Bulatao RA. 1983. The demand forchildren: a critical essay. In: Bulatao RA, Lee RD,editors. Determinants of fertility in developingcountries. New York: Academic Press. p 233–287.33 Reynolds P. 1991. Dance civet cat: child la-bour in the Zambezi Valley. Athens: Ohio Univer-sity Press.34 Turke P. 1989. Evolution and demand forchildren. Popul Dev Rev 15:61–90.35 Turke P. 1988. Helpers at the nest: childcarenetworks on Ifaluk. In: Betzig L, Borgerhoff Mul-der M, Turke P, editors. Human reproductivebehavior. Cambridge: Cambridge UniversityPress. p 173–188.36 Bentley G, Jasienska G, Goldberg T. 1993. Isthe fertility of agriculturalists higher than that ofnonagriculturalists? Curr Anthropol 34:778–785.37 French JA. 1997. Proximate regulation of sin-

gular breeding in Callitrichid primates. In: So-lomon NG, French JA, editors. Cooperativebreeding in mammals. Cambridge: CambridgeUniversity Press. p 34–75.

38 Goldizen AW. 1987. Tamarins and marmo-sets: communal care of offspring. In: Smuts B,Cheney D, Seyfarth R, Wrangham R, StruhsakerT, editors. Primate societies. Chicago: Universityof Chicago Press. p 34–43.

39 Hrdy SB. 1977. The langurs of Abu. Cam-bridge: Harvard University Press.

40 Koenig A. 1995. Group size, composition, andreproductive success in wild common marmo-sets (Callithrix jacchus). Am J Primatol 35.

41 Tardif SD. 1997. The bioenergetics of parentalcare in marmosets and tamarins. In: SolomonNG, French JA, editors. Cooperative breeding inmammals. Cambridge: Cambridge UniversityPress. p 11–33.

42 Bove RB, Valeggia CR, Ellison PT. 2002. Girlhelpers and time allocation of nursing womenamong the Toba of Argentina. Hum Nat 13:457–472.

43 Hames R, Draper P. 2004. Women’s work,child care and helpers-at-the-nest in a hunter-gatherer society. Hum Nat 15:319–341.

44 Hurtado M, Hawkes K, Hill K, Kaplan H.1985. Female subsistence strategies among Achehunter-gatherers of eastern Paraguay. Hum Ecol13:1–28.

45 Marlowe F. 2003. A critical period for provi-sioning by Hadza men: implications for pairbonding. Evol Hum Behav 24:217–229.

46 Kramer KL. 2004. Reconsidering the cost ofchildbearing: the timing of children’s helping be-havior across the life cycle of Maya families. In:Alvard M, editor. Socioeconomic aspects of hu-man behavioral ecology. Amsterdam: Elsevier. p335–353.

47 Bock J. 2002. Learning, life history and pro-ductivity: children’s lives in the Okavango Delta,Botswana. Hum Nat 13:161–197.

48 Bliege Bird R, Bird D. 2002. Constraints ofknowing or constraints of growing? Fishing andcollecting by the children of Mer. Hum Nat 13:239–267.

49 Blurton Jones N, Hawkes K, O’Connell J.1989. Measuring and modeling costs of childrenin two foraging societies: implications for sched-ule of reproduction. In: Standen V, Foley R, edi-tors. Comparative socioecology: the behavioralecology of humans and other mammals. Oxford:Blackwell Scientific Publications. p 367–390.

50 Draper P, Cashdan E. 1988. Technologicalchange and child behavior among the !Kung.Ethnology 27:339–365.

51 Kramer KL. 2002. Variation in juvenile de-pendence: helping behavior among Maya chil-dren. Hum Nat 13:299–325.

52 Gould RA. 1980. Living archaeology. Cam-bridge: Cambridge University Press.

53 Greaves R. n.d. Fishing strategies and tech-nology. Ethnoarchaeological research strategiesfor Pume foragers in Venezuela.

54 Hames R. 1992. Time allocation. In: SmithEA, Winterhalder B, editors. Evolutionary ecol-ogy and human behavior. New York: Aldine DeGruyter. p 203–235.

55 Winterhalder B. 1983. Opportunity-cost for-aging models for stationary and mobile preda-tors. Am Nat 122:73–84.

56 Kramer KL. 1998. Variation in children’swork among modern Maya subsistence agricul-turalists. Ph.D. dissertation. University of NewMexico, Albuquerque.

57 Emlen ST, Wrege PH. 1991. Breeding biologyof white-fronted bee-eaters at Nakuru: the influ-

ence of helpers on breeder fitness. J Anim Ecol60:309–326.58 Fairbanks L. 1990. Reciprocal benefits of al-lomothering for female vervet monkeys. AnimBehav 40:553–562.59 Moehlman PD. 1986. Ecology of cooperationin canids. In: Rubenstein DI, Wrangham RW,editors. Ecological aspects of social evolution:birds and mammals. Princeton: Princeton Uni-versity Press. p 64–86.60 Wiley RH, Rabenold KN. 1984. The evolutionof cooperative breeding by delayed reciprocityand queuing for favorable social positions. Evo-lution 38:609–621.61 Lee RD, Kaplan H, Kramer KL. 2002. Chil-dren and the elderly in the economic life cycle ofthe household: a comparative study of threegroups of horticulturalists and hunter-gatherers.Presented at the Annual Meeting of the Popula-tion Association of America, Atlanta, Georgia.62 Lee RD, Kramer KL. 2002. Children’s eco-nomic roles in the Maya family life cycle: Cain,Caldwell and Chayanov revisited. Popul Dev Rev28:475–499.63 Kramer KL. 2005. Maya children: helpers atthe farm. Cambridge: Harvard University Press.64 Potts R. 1998. Variability selection in hominidevolution. Evol Anthropol 7:81–96.65 Bogin B. 1998. Evolutionary and biologicalaspects of childhood. In: Panter-Brick C, editor.Biosocial perspectives on children. Cambridge:Cambridge University Press. p 10–44.66 Galdikas BM, Wood JW. 1990. Birth spacingpatterns in humans and apes. Am J Phys An-thropol 83:185–191.67 Nishida T, Takasaki H, Takahata Y. 1990. De-mography and reproductive profiles. In: NishidaT, editor. The chimpanzees of the Mahale Moun-tains: sexual and life history strategies. Tokyo:University of Tokyo Press. p 63–97.68 Ivey PK, Morrelli GA, Tronick EZ. 1994. Chil-dren’s resource acquisition strategies: food-get-ting among young Efe forager children of theIturi Forest, Zaire. Paper presented at the Societyfor Cross-Cultural Research, Santa Fe, NM.69 Morelli GA. 1997. Growing up female in aforaging and farming community. In: ZihlmanM, editor. Female biology, life history, and evo-lution. Princeton: Princeton University Press.70 Low BS. 2000. Why sex matters: a Darwinianlook at human behavior. Princeton: PrincetonUniversity Press.71 Zeller AC. 1987. A role for women in hominidevolution. Man 22:528–557.72 Kaplan H, Hill K, Lancaster J, Hurtado AM.2000. A theory of human life history evolution:diet, intelligence, and longevity. Evol Anthropol9:156–185.73 Lancaster J. 1997. An evolutionary history ofhuman reproductive strategies and the status ofwomen in relation to population growth and so-cial stratification. In: Gowaty PA, editor. Evolu-tionary feminism. New York: Chapman and Hall.p 466–488.74 Hrdy SB. n.d. Evolutionary context of humandevelopment: the cooperative breeding model.In: Carter DS, Ahnert L, editors. Attachment andbonding: a new synthesis. Cambridge: M.I.T.Press. In press.75 Hrdy SB. n.d. Comes the child before theman: how cooperative breeding and prolongedpostweaning dependence shaped human poten-tials. In: Hewlett B, Lamb M, editors. Huntergatherer childhood. Piscataway: Transactions. Inpress.76 Smith HB, Tompkins RL. 1995. Toward a lifehistory of the Hominidae. Ann Rev Anthropol24:257–279.



AQ: 1

AQ: 2

77 Becker GS. 1981. A treatise on the family.Cambridge: Harvard University Press.

78 Caldwell JC. 1983. Direct economic costs andbenefits of children. In: Bulatoa RA, Lee RD,editors. Determinants of fertility in developingcountries. New York: Academic Press. p 458–493.

79 Handwerker WP. 1986. The modern demo-graphic transition: an analysis of subsistencechoices and reproductive consequences. Am An-thropol 88:400–417.

80 Becker GS, Lewis HG. 1973. On the interac-tion between the quantity and quality of children.J Political Econ 81:s279–s288.

81 Caldwell JC. 1977. The economic rationalityof high fertility: an investigation illustrated withNigerian survey data. Popul Stud 31:5–27.

82 Kasarda JD. 1971. Economic structure andfertility: a comparative analysis. Demography 8:307–318.

83 Munroe RH, Munroe RL, Shimmin HS. 1984.Children’s work in four cultures: determinantsand consequences. Am Anthropol 86:339–379.

84 Nag M, White B, Peet R. 1978. An anthropo-logical approach to the study of the economicvalue of children in Java and Nepal. Curr An-thropol 19:293–306.

85 White B. 1975. The economic importance ofchildren in a Javanese village. In: Nag M, editor.Population and social organization. The Hague:Mouton. p 127–146.

86 Kaplan H. 1994. Evolutionary and wealthflows theories of fertility: empirical tests and newmodels. Popul Dev Rev 20:753–791.

87 Kramer KL. 2005. Maya children: helpers atthe farm. Cambridge: Harvard University Press.

88 Lindert P. 1983. The changing economic costsand benefits of having children. In: Bulatao RA,Lee RD, editors. Determinants of fertility in de-veloping countries. New York: Academic Press. p494–516.

89 Vlassoff M. 1979. Labour demand and eco-nomic utility of children: a case study in ruralIndia. Popul Stud 33:415–428.

90 Caldwell JC. 1976. Toward a restatement ofdemographic transition theory. Popul Dev Rev2:321–366.

91 Lee RD. 2000. A cross-cultural perspective onintergenerational transfers and the economic lifecycle. In: Mason A, Tapinos G, editors. Sharingthe wealth: demographic change and economictransfers between generations. Oxford: OxfordUniversity Press. p 17–56.

92 Mueller E. 1976. The economic value of chil-dren in peasant agriculture. In: Ridker R, editor.Population and development: the search for se-lective interventions. Baltimore: John HopkinsPress. p 98–153.

93 Stecklov G. 1999. Evaluating the economicreturns to childbearing in Cote d’Ivoire. PopulStud 53:1–17.

94 Turke P. 1989. Evolution and demand forchildren. Popul Dev Rev 15:61–90.

95 Kramer KL. 2004. Reconsidering the cost ofchildbearing: the timing of children’s helping be-havior across the life cycle of Maya families. In:Alvard M, editor. Socioeconomic aspects of hu-man behavioral ecology. Amsterdam: Elsevier. p335–353.

96 Altmann J. 1974. Observational study of be-havior: sampling methods. Behaviour 49:227–267.

97 Borgerhoff Mulder M, Caro TM. 1985. Theuse of quantitative observational techniques inanthropology. Curr Anthropol 26:323–335.98 Hames R. 1992. Time allocation. In: SmithEA, Winterhalder B, editors. Evolutionary ecol-ogy and human behavior. New York: Aldine DeGruyter. p 203–235.99 Durnin JV, Passmore R. 1967. Energy, workand leisure. London: Heinemann EducationalBooks.100 Ulijaszek S. 1995. Human energetics in bio-logical anthropology. Cambridge: CambridgeUniversity Press.101 Boserup E. 1965. The conditions for agricul-tural growth: the economics of agrarian changeunder population pressure. Chicago: Aldine.102 Clark C, Haswell M. 1967. The economics ofsubsistence agriculture. London: MacmillanPress.103 Ivey PK. 2000. Cooperative reproduction inIturi Forest hunter-gatherers: who cares for Efeinfants. Curr Anthropol 41:856–866.104 Lancaster J. 1971. Play mothering: the rela-tions between juvenile females and young infantsamong free-ranging vervet monkeys (Cercopithi-cus aethiops). Folia Primatol 15:161–182.105 Blurton Jones N, Hawkes K, Draper P. 1994.Foraging returns of !Kung adults and children:why didn’t !Kung children forage? J AnthropolRes 50:217–248.106 Blurton Jones N, Hawkes K, Draper P. 1994.Differences between Hadza and !Kung children’swork: affluence or practical reason. In: Burch ESJr, Ellana LJ, editors. Key issues in hunter-gath-erer research. Berg: Oxford Press. p 189–215.107 Blurton Jones N, Marlowe FW. 2002. Selec-tion for delayed maturity: does it take 20 years tohunt and gather? Hum Nat 13:199–238.108 Hawkes K, O’Connell J, Blurton Jones N.1995. Hadza children’s foraging: juvenile depen-dency, social arrangements, and mobility amonghunter-gatherers. Curr Anthropol 36:688–700.109 Kaplan H. 1997. The evolution of the humanlife course. In: Wackter KW, Finch CE, editors.Between Zeus and the salmon: the biodemogra-phy of longevity. Washington D.C.: NationalAcademy of Sciences. p 175–211.110 Kaplan H, Lancaster JB, Bock JA, JohnsonSE. 1995. Does observed fertility maximize fit-ness among New Mexican men? A test of anoptimality model and a new theory of parentalinvestment in the embodied capital of offspring.Hum Nat 6:325–360.111 Parsons DO, Goldin C. 1989. Parental altru-ism and self interest: child labor among late-nineteenth century American families. Econ In-quiry 27:637–659.112 Blurton Jones N, Hawkes K, O’Connell J.1997. Why do Hadza children forage? In: SegalN, Weisfeld GE, Weisfeld CC, editors. Unitingpsychology and biology: integrative perspectiveson human development. American PsychologicalAssociation. p 164–183.113 Kramer KL, McMillan GP. 1998. How Mayawomen respond to changing technology: the ef-fect of helping behavior on initiating reproduc-tion. Hum Nat 9:205–223.114 Hames R. 1988. The allocation of parentalcare among the Ye’Kawana. In: Betzig L, Borg-erhoff Mulder M, Turke P, editors. Human repro-ductive behavior. Cambridge: Cambridge Univer-sity Press. p 237–251.115 Goodman M, Griffin P, Estioko-Griffin A,

Grove J. 1985. The comparability of hunting andmothering among the Agta humer-gatherers ofthe Philippines. Sex Roles 12:1199–1209.

116 Denham WW. 1974. Infant transport amongthe Alyawara tribe, Central Australia. Oceania64:253–277.

117 Valeggia CR. n.d. Changing times for theArgentine Toba: who cares for the baby now? In:Bentley G, Mace R, editors. Biosocial SocietySymposium Series. Cambridge: Cambridge Uni-versity Press.

118 Wich SA, Utami-Atmoko SS, Mitra Setia T,Rijksen HR, Schurmann C, van Hooff J, vanSchaik CP. 2004. Life history of wild Sumatranorangutans (Pongo abelii). J Hum Evol 47:385–398.

119 Fossey D. 1979. Development of the moun-tain gorilla (Gorilla gorilla beringei): the firstthirty-six months. In: Hamburg DA, McCownER, editors. The great apes. Menlo Park: Ben-jamin-Cummings. p 139–186.

120 Watts DP. 1991. Mountain gorilla reproduc-tion and sexual behavior. Am J Primatol 24:211–226.

121 Pusey AE. 1983. Mother-offspring relation-ship in chimpanzees after weaning. Anim Behav31:363–377.

122 Pusey AE. 1990. Behavioural changes in ad-olescence in chimpanzees. Behaviour 115:203–246.

123 Knott C. 2001. Female reproductive ecologyof the apes: implications for human evolution.In: Ellison P, editor. Reproductive ecology andhuman evolution. New York: Aldine de Gruyter.p 429–463.

124 Eveleth PB, Tanner JM. 1990. Worldwidevariation in human growth. Cambridge: Cam-bridge University Press.

125 Bentley G, Goldberg T, Jasienska G. 1993.The fertility of agricultural and non-agriculturalsocieties. Popul Stud 47:269–281.

126 Howell N. 1979. Demography of the !Kung.New York: Academic Press.

127 Tucker B, Young A, n.d. Growing up Mikea:children’s time allocation and tuber foraging insouthwest Madagascar. In: Hewlett B, Lamb, ed-itors. Hunter-gatherer children. p 147–171.

128 Johnson A. 1975. Time allocation in aMachiguenga community. Ethnology 14:301–310.

129 Minge-Kalman W. 1978. Household econ-omy during the peasant to worker transition inthe Swiss Alps. Ethnology 17:183–196.

130 Skoufias E. 1994. Market wages, family com-position and the time allocation of children inagricultural households. J Dev Stud 30:335–360.

131 Erasmus C. 1955. Work patterns in a Mayavillage. Am Anthropol 57:322–333.

132 Fratkin E. 1989. Household variation andgender inequality in Ariaal pastoral production:results of a stratified time-allocation survey. AmAnthropol 91:430–440.

133 Munroe R, Koel A, Munroe R, Bolton R,Michelson C, Bolton C. 1983. Time allocation infour societies. Ethnology 22:355–370.

134 Bock J. 1995. The determinants of variationin children’s activities in a southern African com-munity. Ph.D. dissertation, University of NewMexico, Albuquerque.

© 2005 Wiley-Liss, Inc.

14 Kramer ARTICLES


AQ1 Page #s needed.

AQ2 Ref. incomplete.

AQ3 “TFR” should be spelled out at first use.

AQ4 Can “consumption . . . of activities” be clarified?


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