EAS Activities at Past AAA Meetings

2007

EXPANDING FRONTIERS IN EVOLUTIONARY ANTHROPOLOGY: APPROACHES TO COMPLEX AND MODERNIZING SOCIETIES

Organized and chaired by Wesley Allen-Arave (University of New Mexico) and Mary K. Shenk (University of North Carolina at Chapel Hill)

November 29, 2007, 8:00 to 9:45 am

SESSION ABSTRACT

Evolutionary anthropology had its start in the mid-1970s with the application of behavioral ecology to the study of modern hunter-gatherers and foraging horticulturalists.  These early studies centered on territoriality, land use, foraging behavior, and resource transfers.  As the field advanced through the 1980s and early 1990s, reproduction, parental and kin investment, cooperation, and altruism became important topics of research.  Most evolutionary anthropologists, however, continued to concentrate on traditional and relatively egalitarian societies where modern contraceptive technology was not widely used and globalization so far had only marginally impacted subsistence patterns.  Modern evolutionary anthropology seeks to explain human behavioral diversity across time and space as adaptive solutions to current ecological and cultural conditions.  In seeking to build this natural science of sociocultural variation, evolutionary anthropologists now study diverse research topics in diverse cultures and many researchers have begun to collect quantitative behavioral data from large-scale and/or rapidly changing societies to augment the field’s strong foundation of work in traditional, small-scale cultures.

This session brings together seven anthropologists applying an evolutionary framework to our understanding of behavior in structurally complex, industrialized, or modernizing societies.  The papers in this session apply the perspectives of life history theory, embodied capital theory, cultural transmission theory, and costly signaling theory to explore behaviors and behavioral strategies related to fertility, mortality, altruism, and risk.  Some papers focus on how small-scale societies are encountering the new technologies and socioeconomic changes associated with economic development and globalization, while others look at problems and questions unique to large-scale social systems with complex systems of hierarchy and/or developed market economic systems.

The papers in this session highlight several themes facing researchers working in complex or modernizing social systems.  First, that understanding behavior in societies characterized by stratification, contraceptive technology, routine stranger interaction, and/or an industrialized economy often requires new models or modifications of existing evolutionary models of behavior.  One example is the classic evolutionary quandary of the demographic transition.  This topic spawned a series of theoretical papers in the 1990s but empirical work remained rare until recently.  Tests of evolutionary models of the demographic transition form the core of two papers in this session and inform two others.  Second, that knowledge of particular cultural contexts is often essential for understanding and interpreting the applications of evolutionary theoretical models to complex cultures.  This point is demonstrated in papers by Anderson, Boone and Allen-Arave, Kress, Mace, and Shenk which utilize data that can only be adequately analyzed and interpreted with reference to the cultural context in which the behaviors under study are situated.  Finally, that behavior which may seem odd or exclusive to one type of society may fall into larger patterns best observed in careful cross-cultural comparisons.  This point is demonstrated by Low’s paper, which looks for fertility and mortality patterns in a large cross-cultural sample and examines the implications for emerging life history traits in developed nations.

PAPERS

A comparison of four models of the demographic transition in Otavalo, Ecuador

Howard Kress (University of Connecticut)

Over the last 60 years, social scientists have debated the causes of the demographic transition.  Various theories have attempted to explain the transition from high-mortality and high-fertility to low-mortality and low-fertility.  Most of these theories have focused on changes in infant mortality and modernization as key factors; however, recent theory suggests that cultural or evolutionary forces may play a role in the transition.  This paper provides a direct comparison of classic and contemporary theory of the demographic transition.  The paper presents data on research that was designed to test these theories simultaneously.  Specifically, the paper tests theories of Human Capital (Kaplan and Lancaster 2001), Moral Economy of Childbearing (Handwerker 1989), Diffusion (Bongaarts and Cotts Watkins 1996), and Infant Mortality Rate Reductions (Palloni and Rafalimanana 1999).  Results are from a nine-month study in Otavalo, Ecuador and consist of data from 240 interviews.  Results indicate that the evolutionary model proposed by Kaplan and Lancaster captures the most variation in fertility.  These results, though, can only be understood in the specific cultural context of Otavalo.  The conclusion of the research is that if evolutionary models of human fertility are used, the results must be informed by cultural data.

Testing evolutionary models of the demographic transition in urban south India

Mary K. Shenk (UNC Chapel Hill)

This paper examines the demographic transition in the city of Bangalore, India during the course of the 20th century using a dataset that allows detailed comparison between 2 and 3 generations of fertility data for parents, children, and siblings.  I begin by showing the patterns of fertility change in Bangalore over the course of the 20th century, both in the entire sample and then in two social class groups that experience different social and economic environments.  I then test how well two evolutionary models of the demographic transition predict these trends both overall and within classes.  The models tested include the embodied capital model of the demographic transition proposed by Kaplan (1996) and Kaplan and Lancaster (2000) and the cultural transmission model proposed by Boyd & Richerson (1988, 2004).  I find that both models have a relatively good ability to predict observed fertility declines, but that the pattern and pace of fertility transition as well as the predictive ability of the two evolutionary models vary systematically between social classes.

HIV/AIDS risk behaviors in South Africa: Testing predictions from life history theory

Kermyt G. Anderson (University of Oklahoma)

Life history theory predicts that when mortality is high, sexual maturity will occur earlier, and fertility rates will increase.  This prediction has implications for engagement in HIV/AIDS risk behaviors.  HIV/AIDS prevalence is highest in developing countries with high mortality rates, even excluding mortality from HIV/AIDS.  Yet variation in local mortality within countries may contribute to regional variation in HIV/AIDS prevalence.  Two specific hypotheses are tested using data from South Africa, a country with high HIV/AIDS prevalence.  The first hypothesis is that high mortality is positively associated with greater engagement in HIV/AIDS risk behaviors, measured by such proxies as being sexual active, using a condom, cumulative and desired fertility, and age at menarche and first sex.  The second hypothesis is that life expectancy at birth is negatively associated with lower engagement in HIV/AIDS risk behaviors.  Data on sexual behavior for 11,735 women come from the 1998 Demographic and Health Survey.  Mortality rates and life expectancy at birth are measured at the level of the magisterial district, roughly equivalent to US counties, using data from the 1995, 1997 and 1998 October Household Surveys.  Results are consistent with the predictions, suggesting mortality is associated with HIV/AIDS risk behaviors.  However, multivariate analysis reveals the importance of intermediate variables, especially education level and race/ethnicity.

Modern Women's Lives in Ecological and Anthropological Perspective

Bobbi Low (University of Michigan)

Across species, there exist some strong patterns in life history.  In other species, ecological drivers are often clear; yet we imagine these same patterns to be largely or purely cultural in humans.  For example:  the nastier, more brutish and short life is, the earlier it pays, in biological terms, to reproduce.  I examine women's life patterns in 172 nations, exploring where the patterns 'fit' the general patterns, where they diverge, and the extent to which the developed/least developed nation life history patterns can be generalized.   Extremeness, range of variation, and predictability of resources, and even survivorship appear to affect human life histories, as well as those of other species.  The answers we find may have policy implications for women's lives and fertility.

Surviving Titanic: The View from Behavioral Ecology

James L. Boone (University of New Mexico) and Wesley Allen-Arave (University of New Mexico)

Aside from the well-known and somewhat over-rated spoken rule favoring women and children, there were a number of social and life history characteristics that affected passengers' chances of surviving the Titanic disaster of April 14-15, 1912.  We examine the effects of age in adult (aged 15 and older) men and women.  In general, we find that in third class (the largest ticket class, and which had the lowest survival rates), women aged 15 to 24 had a higher than expected survival rate, while men aged 25-34 had a higher than expected survival rate.  We examine two hypotheses that might explain this pattern: first, that men weigh reproductive value against reputation in making the choice of whether to behave altruistically or not; and two, that the higher than expected survival of women results from sexually selected altruistic displays on the part of men.  Titanic age and survival data are compared with similar data from other disasters, specifically the Donner Party and the Mormon Hand Cart Migration.

What qualities are New Mexicans signaling with their charitable contributions?

Wesley Allen-Arave (University of New Mexico)

Charitable donations are one of the most important sources of funding for public assistance and service institutions in our society, yet we do not know how well leading theoretical explanations for why people contribute to public goods explain variation in who donates to charities and in what circumstances.  I present data on voluntary charitable donations made by 506 New Mexican households to evaluate the explanatory power of evolutionary-minded theories that seek to account for unreciprocated giving to non-kin through reputational effects.  The theoretical models evaluated in this talk contend that altruistic donors gain reputations for generosity and attract favorable attention by being viewed as 1) a beneficent partner for turn-taking exchanges of favors, 2) someone who commands access to resources and social prestige, or 3) a desirable mate who will provide for his spouse and children.  Several anthropologists have argued that the theoretical models considered here account for seemingly unreciprocated food provisioning in small-scale societies.  However, unlike previous studies conducted in small-scale societies where donors and recipients continue to interact and opportunities abound for recipients to directly repay donors, the case of charitable donors in our own society provides a better test case for these theories by offering a more unambiguous instance of generosity that cannot be accounted for by direct reciprocation.

GENDERED INEQUALITIES IN EVOLUTIONARY PERSPECTIVE

Organized by Rebecca Bliege Bird (Stanford University) and Monique Borgerhoff Mulder (University of California at Davis), chaired by Rebecca Bliege Bird.

November 30, 2007, 4:00 to 5:45 pm

SESSION ABSTRACT

Gendered inequities in access to rights and resources are affected by a diverse set of dynamic processes – ecological, historical, institutional, spatial, and developmental. Mainstream anthropological approaches to this issue have emphasized historical and institutional processes: the roles of gender ideology, colonialism and globalization, and economic development in creating and fostering gendered inequalities. Increasingly, anthropologists are beginning to pay attention to the role of individual decision-making and agency in structuring inequality, focusing on how and why the economic and ecological tradeoffs men and women face in particular social, cultural and ecological environments create different strategies of action. What is lacking so far is a way to integrate the two approaches—the historical and institutional with the ecological and developmental—using formal theory.  In this session we bring together a diverse set of investigators who are developing integrative explanatory models using evolutionary ecological theory to explain cultural variation in sex and gendered inequalities. Specific presentations will cover institutional factors such as divisions of labor that privilege certain gender and age groups and prejudice others; analyses of marriage as an arena of conflict; and developmental sex differences that influence gendered inequalities in health, exposure to disease and accidents, and wellbeing. The goal of this session is to demonstrate the integrative nature of evolutionary approaches to inequality by showcasing studies that investigate why gender inequalities form, and how they are shaped by extrinsic ecological and social formations.

PAPERS

Coming out of the Kitchen: How Tanzanian Pimbwe Women use Marriage to Outcompete Men

Monique Borgerhoff Mulder (UC Davis)

Parental investment theory provides a strong basis for making predictions about how men's and women's mating strategies might vary. More specifically it has generated a large number of successful predictions regarding gender differences in reproductive strategy. There are however many situations in which traditional sex roles are not observed, and studies in non-humans are beginning to determine how and why this might be. In this talk I examine data from the Pimbwe, a horticultural population in Rukwa (western Tanzania), where variation in women’s reproductive success equals, and in some contexts exceeds that of men. In particular I look at how women use monogamous marriage to outcompete men. The implications of these results for claims of universal sex differences are discussed.

The Consequences of Sex-Ratio Imbalances on Dyadic Power within Colombian Households

James Jones (Stanford University)

The relative abundance of the sexes has a profound impact on marriage markets, which in turn has consequences for a variety of human behavior. Two domains of particular interest include: (1) the balance of power within households, and (2) patterns of parental investment. In this paper, we build on previous work showing that parts of Colombia have experienced -- and continue to experience -- severe marriage squeezes as a consequence of the chronic excess male mortality due to violence. We present a theory of gendered power as a function of local marriage market conditions and test the predictions using measurements of the local intensity of marriage squeezes with an index of female empowerment.

Marriage and the sexual division of labor among hunter-gatherers: a bargaining approach

Michael Gurven (UC Santa Barbara)

Children may be viewed as public goods wherein both parents equally benefit genetically yet one parent often invests more heavily than the other and therefore pays a greater cost. Investment decisions by marriage partners will be affected by their relative bargaining power, which itself is impacted by the feasibility of outside options and the supply and demand of unique sets of skills, abilities and traits that partners possess. I apply these concepts to lay out a bargaining framework for understanding investment decisions in order to address questions concerning potential conflicts of interest over types and amount of work effort (i.e. the sexual division of labor) among men and women in small-scale subsistence societies. I show that the gains and costs of marriage are unlikely to be spread equally among marriage partners, yet marriage can still be a favorable outcome under a wide range of conditions. This framework subsumes both cooperative and conflictive views on the sexual division of labor. Finally, I apply these concepts to revisit questions of men’s labor motivations among Tsimane forager-horticulturalists of Bolivia.

Where Have All the Boomers Gone? Social and Ecological Influences on the Gendered Division of Labor among Western Desert Aborigines.

Rebecca Bird (Stanford University)

“Pre-contact” descriptions of subsistence in Australia’s arid zone portray the sexual division of foraging labor as one where women supplied more than 90% of the diet, plants provided the mainstay, there was considerable overlap and flexibility in gendered foraging decisions, and animals pursued by men were scarce and never more than a small percentage of the total harvest. However, data from the post-mission period provides a rather stark contrast to this generalization. In 1975, and 1984, women foraging near Utopia station brought in only about 10% of all foraged foods, while men focused almost exclusively on kangaroo hunting and women on tubers and fruit. These trends in the reduction in women's productivity and a strengthening of difference have also been noted in other remote aboriginal communities. Many have variously argued that these secular changes in the division of labor are the result of introduced technology, colonialism and the imposition of a welfare economy, deliberate policies of assimilation or "capture by flour", or changes in the local environment as a result of pastoral incursion, invasive species, and policies that disenfranchised aboriginal land-owners. We present data on contemporary foraging patterns among Martu men and women to address these issues. Martu women remain active and productive foragers, today supplying 52% of all bush foods acquired, much of that in the form of meat. We suggest that interactions between technological change, the process of colonialism, and the local ecological environment structure the degree to which women today remain important contributors to the subsistence economy.

Age and sex differences in risk and time preferences in southwestern Madagascar: A test of Daly and Wilson’s “risky/impulsive male” hypothesis

Bram Tucker (University of Georgia)

Evolutionary psychologists Daly and Wilson have used sexual selection theory and evidence from homicide rates to argue that young males are more prone to risk and less willing to wait for delayed rewards than people in other age-sex categories. This hypothesis is tested with choice experiment data from a large sample of Masikoro agropastoralists, Vezo fishers, and Mikea forager-farmers of southwestern Madagascar, using new data from a four season longitudinal project aimed at understanding how social learning and individual needs influence economic decision-making. Preliminary experiments using small, real rewards have found no significant differences by age or sex. It is possible that socially-learned gender roles and expectations have a greater influence on human decision-making than biological age and sex, at least for some types of decisions. That socially learned norms and behavior and cultural categories may trump biology and genetic tendencies is equally consistent with (cultural) evolutionary theory and findings from cultural anthropology.

What drives sex differences in parasitism? A perspective from non-human primates

Charles Nunn (Max Planck Institute, UC Berkeley)

Infectious diseases and parasites can create gender inequalities when infections are sex-biased and negatively impact the ability of individuals to generate wealth or status. Sex differences in infection can occur through two main mechanisms: differential exposure to infectious agents, and differential susceptibility to infection. With increasing knowledge of infectious diseases in wild primates, it becomes possible to investigate these mechanisms, and to compare results to humans. In this presentation, I review data and theoretical models relevant to understanding sex differences in disease prevalence in non-human primates. First, I consider reproductive skew and exposure to pathogens by focusing on sexually transmitted diseases (STDs). I briefly describe a model for the spread of STDs in polygynous systems and present a comparative test of a key prediction from this model. This test reveals that STD prevalence is higher in female primates than in males, consistent with a sex difference in exposure. Second, I review evidence for differential susceptibility to disease and factors that might drive this difference. I review results across primates and other mammals showing higher prevalence of parasitic infections in males than females, and I investigate sex differences in physiological parameters related to disease susceptibility. The latter results reveal that healthy female primates have significantly higher total white blood cell counts, with eosinophils and lymphocytes generating this overall difference. Collectively, these results illustrate how consideration of susceptibility and exposure “filters” can provide insights to the drivers of sex differences in disease prevalence. Similar principles and approaches also apply to humans.

Discussant

Carol Ember (Human Relations Area Files and Yale University)

New research in the evolutionary ecology of food transfers

Organized and chaired by Brian Wood (Harvard University)

December 2, 2007 (Sunday) 10:15 am - 12 Noon

SESSION ABSTRACT

The crucial role of food sharing in human societies is a drastic adaptive shift from that observed in other primates. Theories of human social evolution have posited how selection acting upon individuals could have led to the common pattern of widespread food sharing within groups. In the last 20 years, behavioral ecological theory has expanded in order to investigate how a full suite of social, ecological, and life history conditions pattern food transfers within populations. This session presents new case studies that illustrate how the giving and receiving of food operates within the larger adaptive strategies of chimpanzees, hunter-gatherers, and horticulturalists. The puzzle of why humans and our closest primate relatives transfer food remains a fertile area of research, providing insight into the evolution of human social behavior. This session will highlight the critical importance of food sharing in human evolution, and test new hypotheses seeking to explain patterns of food transfers in contemporary populations.

PAPERS

Chimpanzees do not trade meat for sex

Ian Gilby (Harvard University)

Chimpanzee behavior is of particular interest to those seeking an understanding of the biological roots of human cooperation. The notion that male chimpanzees swap meat for mating with sexually receptive females is frequently cited in the anthropological literature. However, there is essentially no evidence supporting the ‘meat-for-sex’ hypothesis. Data from the Kasekela (Gombe National Park, Tanzania), Ngogo and Kanyawara (Kibale National Park, Uganda) chimpanzee communities demonstrate that 1) the presence of sexually receptive females does not increase hunting probability; 2) males do not share preferentially with sexually receptive females; and 3) sharing meat with sexually receptive females does not increase mating success. These results emphasize the need for re-evaluation of the role of exchange in early human evolution.

Why Do Chimpanzees Hunt and Share Meat, and What Does This Say About Food Transfers in Humans?

David Watts (Yale University)

Because chimpanzees hunt more often than other nonhuman primates, are highly effective predators (particularly of red colobus monkeys), and routinely share meat after successful hunts, they provide an important point of comparison for investigations of the evolutionary ecology of food transfers in humans. Multiple factors influence chimpanzee decisions about whether to hunt on any given prey encounter and about the distribution of meat. Some of these are also relevant to human, including the potential impact of male social relationships on meat sharing.  Chimpanzee meat sharing also differs from food transfers in humans in important respects. Notably, male chimpanzees do not provision mates or offspring; chimpanzees consume nearly all meat at hunt sites and do not have any sort of home bases, so individuals not present at hunts do not receive shares of carcasses; and variation in hunting ability, success at procuring meat from others, and presence at hunts leads to wide variation in meat intake. I present data on hunting and meat sharing by chimpanzees at Ngogo, in Kibale National Park, Uganda. The Ngogo chimpanzee community is unusually large and the males there are unusually successful hunters. Energy availability and male party size both influence hunting frequency at Ngogo, and males use meat as a political tool in their negotiation of social relationships. I use Ngogo data and comparative data from other chimpanzee research sites to address questions about ecological and social influences on hunting frequency and meat sharing, and I discuss the implications of the chimpanzee data for understanding the evolution of food sharing in humans.

Household Provisioning and Food Transfers among Hadza Hunter-Gatherers

Brian Wood (Harvard University)

In hunter-gatherer societies, the foods men acquire are often transferred to other's households, and such wide-scale sharing can serve to bolster men's reputations and social standing. If men distribute their foods evenly among camp members, then their work effort is producing group benefits and may indicate that direct household provisioning is not the primary motivation of men's foraging. Data recently gathered among the Hadza of northern Tanzania indicates that men's foods are in fact not so evenly shared as previously thought. While men are motivated to act generously toward camp members, they nevertheless bias food distributions to the advantage of their own households. This paper presents these new findings and further investigates how food scarcity, resource package size, and the skill requirement of different foods pattern the sharing of men's foods.

Food Distribution among Hunter-Gatherers in Northern Siberia: Tests of Evolutionary Hypotheses

John Ziker (Boise State University)

Empirical data on food procurement and distribution combined with socio-demographic information on givers and recipients in a community of indigenous Siberians are used to test hypotheses of non-market food transfers derived from evolutionary theory, including: kinship, reciprocal altruism, tolerated scrounging, and costly signaling. The frequency and volume of food distributed are analyzed by considering independent variables including relatedness, age and sex differences, household proximity, hunter status, stated rationales for sharing, previous sharing and other aid, relative amounts of food on hand, hunting season, and storage capacity. The paper illustrates how the giving and receiving of food operates within the flexible strategies of hunter-gatherers in an uncertain environment: a remote Arctic community in postsocialist Russia. The development and continuation of non-market food transfers in Siberia provides additional perspective into discussions of the evolution of human social behavior. Non-market food transfers in this case study are embedded in kinship and other social institutions that aim to provide a social safety net for family and community members in ways analogous to those discovered among other hunter-gatherers.

Food transfers among indigenous Nicaraguan horticulturalists

Jeremy Koster (Penn State University)

In recent decades, a number of evolutionary models have been developed to explain food transfers and sharing between households. These models include reciprocal altruism, kin selection, tolerated scrounging, and costly signaling. Of these models, reciprocal altruism has generally received the most robust support in studies that simultaneously evaluate multiple models. However, tests of reciprocity are sometimes hindered by the inability to account for contingent repayments that differ in type from the original exchange. For example, the recipient of a food transfer might repay the donor by lending a tool or by helping with childcare or agricultural labor. I use data from scan sampling observations, genealogical research, and household food consumption to examine relationships between food transfer and other types of assistance in a Mayangna community (Nicaragua). The degree of relatedness between households, age effects in household composition, and the geographical proximity of households are also considered as explanatory factors.

Meat and manioc: Men’s and women’s food-sharing networks in the Ecuadorian Amazon

John Patton (CSU Fullerton)

This paper will present new data to examine the relationship between food-sharing networks of men and women in Conambo, a small-scale horticultural foraging society in the Ecuadorian Amazon. In Conambo, men and women acquire and control the distribution of distinct sets of resources. Primarily, men hunt and distribute meat, and women garden and control the distribution of garden resources. In this paper, we model the food-sharing networks of husbands and wives by analyzing household-by-household food transfers. Food-sharing networks are analyzed in terms of each household’s needs (household size and the ratio of producers-to-consumers) as well as their relationships of kinship, spatial proximity, reciprocal sharing, status, and political alliance. We conclude by comparing gender-based food-sharing strategies to consider whether men and women in this society share food differently and how these strategies may relate to differences in the nature of the food resources, the structure of men’s and women’s relationships in the community, and the life history of the household.

Discussion

Bruce Winterhalder (UC Davis)