Elsevier

Evolution and Human Behavior

Volume 35, Issue 5, September 2014, Pages 384-388
Evolution and Human Behavior

Original Article
Amazonian horticulturalists live in larger, more related groups than hunter–gatherers

https://doi.org/10.1016/j.evolhumbehav.2014.05.003Get rights and content

Abstract

The relatedness of human groups has important ramifications for kin (group) selection to favor more collective action and invites the potential for more exploitation by political leaders. Endogamous marriages among kin create intensive kinship systems with high group relatedness, while exogamous marriages among nonrelatives create extensive kinship with low group relatedness. Here, a sample of 58 societies (7,565 adults living in 353 residential groups) shows that average group relatedness is higher in lowland horticulturalists than in hunter–gatherers. Higher relatedness in horticulturalists is remarkable given that village sizes are larger, harboring over twice the average number of adults than in hunter–gatherer camps. The relatedness differential between subsistence regimes increases for larger group sizes. Large and dense networks of kin may have favored an increased propensity for some forms of in-group cooperation and political inequality that emerged relatively recently in human history, after the advent of farming.

Introduction

Although still hotly debated in some circles, a key paradigm of evolutionary biology remains Hamilton's (1964) idea that consanguineal relatedness among individuals drives the evolution of cooperative behavior (Abbot et al., 2011, Forster et al., 2006; cf. Nowak et al., 2010, Wilson, 2005). Cooperation (behaviors that benefit another individual and are selected for because of beneficial effects on recipients, West, Griffin, & Gardner, 2007a) is generally thought to form around kinship, from bacteria (Gardner, West, & Griffin, 2007) to primates (Silk, 2002), although cooperation often extends to non-kin (Clutton-Brock, 2009). Humans are an unusual species in that we live in large and cooperative social groups with embedded families formed around stable pair-bonds that create sets of full siblings with life-long bonds (Alexander, 1979, Chapais, 2008, Rodseth et al., 1991). Hunter–Gatherers, the best contemporary representatives of earlier periods of human evolution, often have fluid fission/fusion social dynamics that break up the co-residence of sibling sets and lead to rather low levels of group relatedness (r  0.05, Hill et al., 2011). Models of contingent cooperation show that reciprocating strategies can increase when rare only at the high range of empirical estimates of human group relatedness (r > 0.05, Boyd, Schonmann, & Vicente, 2014).

Bugos (1985) distinguishes intensive versus extensive human kinship systems. An extensive kinship system is characterized by marriages among genealogically and geographically distant individuals to create a diffuse kinship network (Fix, 1999, MacDonald and Hewlett, 1999). Extensive kinship networks may be more adaptive for nomadic peoples in unpredictable environments by providing more residential options and insurance in times of crises (Fix, 1999, Yellen and Harpending, 1972). In contrast, intensive kinship systems often include marriage alliances between co-resident lineages which generate cousin marriages and converging networks of kin (Lévi-Strauss, 1949). In addition to endogamy and limited dispersal, social norms that favor kin marriages (Flinn and Low, 1986, Walker and Bailey, 2014) and population dynamics like lineal fissions (Neel and Salzano, 1966, Walker and Hill, 2014) increase the average relatedness of human groups under intensive kinship regimes. Here, groups are defined as people living close together in local residential communities which may be temporary (e.g., hunter–gatherer camps) or more permanent (e.g., horticultural villages). Group relatedness is important because it governs how strongly kin (or group) selection favors cooperation (Hamilton, 1964, Queller, 1992, West et al., 2007a, West et al., 2007b). Some pertinent examples of cooperation and collective action problems in humans include building structures, clearing fields, sharing feasts, raiding enemies, keeping the peace, and forming institutions; all of which appear to have increased in importance and scale across human history from hunting-and-gathering ancestors to more recent and intensive modes of agricultural-based subsistence (Johnson & Earle, 2000).

Bowles (2006) has argued that hunter–gatherer groups may have had sufficiently high relatedness to explain human cooperation in a between-group competition model. However, others have discounted between-group genetic differentiation as a primary driver of human cooperation (Langergraber et al., 2011). These two previous studies used empirical estimates of group relatedness based on genetic differentiation among human groups yet have yielded opposite conclusions. The current study uses comparative genealogical datasets to estimate human group relatedness across hunter–gatherer and horticultural subsistence regimes. An advantage of genealogies and census data is that analyses include all adults living together in a relatedness matrix without having to rely on genetic sampling issues or assumptions of population structure such as demes within ethnolinguistic boundaries. Particular attention is given here to horticultural populations where the potential for dense networks of kin, due at least in part to limited dispersal of sibling sets and more inbreeding, leads to higher group relatedness.

Section snippets

Methods

Most genealogies and censuses used in this study are available online at KinSources (http://kinsources.net). Additional societies have been added from recent studies of co-residence in hunter–gatherers (Hill et al., 2011) and lowland horticulturalists (Walker et al., 2013) (Table 1). Both of these publications only analyzed primary kin (parents, offspring, and siblings) living together in the same group. A difference in relatedness is anticipated since an average hunter–gatherer lives with 1.8

Results

The average group size for horticulturalists in the sample is 35 adults (range 3–192) which is more than twice that for hunter–gatherers of 16 adults (range 3–118). Despite larger groups, the average coefficient of relatedness for horticultural villages is 0.11 (bootstrapped 95% confidence interval 0.090–0.126), higher than that for hunter–gatherer camps at 0.08 (0.076–0.090). The difference cannot be explained by differential data quality because both samples have the same average genealogical

Discussion

Results show that horticulturalists live in larger and more related groups, at least in part because of increased inbreeding. Inbreeding can arise from higher reproductive skew (successful males, especially headmen, married polygynously) producing more sets of at least half siblings whose children may intermarry, and from social norms of marriage patterns like endogamy, prescribed kin marriages, and sister exchange systems, all common strategies to form alliances between lineages (Chapais, 2008

Acknowledgments

This paper benefited from the genealogies and documentation made available at KinSources (http://kinsources.net) and from data, help, and advice from Drew Bailey, Woodrow Denham, Kim Hill, Ed Hagen, Karen Kramer, and Jeremy Koster. Several anonymous reviewers provided helpful corrections. Financial support was provided by a National Geographic Society Research and Exploration grant (#9165-12).

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