Adaptive personality calibration in a human society: effects of embodied capital on prosocial traits

Behavioral Ecology The official journal of the ISBE International Society for Behavioral Ecology Behavioral Ecology (2015), 26(4), 1071–1082. doi:10.1093/beheco/arv051 Original Article Adaptive personality calibration in a human society: effects of embodied capital on prosocial traits Christopher R. von Rueden,a Aaron W. Lukaszewski,b and Michael Gurvenc aJepson School of Leadership Studies, University of Richmond, Jepson Hall, 28 Westhampton Way, Richmond, VA 23173, USAbDepartment of Psychology, Oklahoma State University, Stillwater OK 74074, USAcDepartment of Anthropology, University of California, Santa Barbara, CA 93106, USA Received 24 October 2014; revised 15 March 2015; accepted 29 March 2015; Advance Access publication 24 April 2015. Evolutionary theories of personality origins have stimulated much empirical research in recent years, but pertinent data from smallscale human societies have been in short supply. We investigate adaptively patterned personality variation among Tsimane’ foragerhorticulturalists. Based on a consideration of cost-benefit tradeoffs that likely maintain variation in human prosociality, we hypothesize that individual differences in prosocial personality traits are facultatively calibrated to variation in “embodied capital”—that is, knowledge, skills, or somatic traits that increase expected future fitness. In support of this hypothesis, 2 components of embodied capital—physical strength and formal education—associated positively with Tsimane’ prosocial leadership orientation (PLO), a broad personality dimension representing gregarious cooperation, interpersonal warmth, and pursuit of leadership. Moreover, using pedigrees to compute heritability estimates, strength and education had additive effects on the heritable variance in PLO, which suggests that prosocial traits may be “reactively heritable” by virtue of their calibration to condition-dependent components of embodied capital. Although alternative explanations must be falsified in future research, our findings 1) provide one of the first demonstrations of adaptively patterned personality variation in a small-scale society and 2) illustrate the potential power of an adaptationist approach to elucidate the causal underpinnings of heritable personality variation. Key words: cooperation; embodied capital; leadership; personality; prosociality; reactive heritability. INTRODUCTION Personality variation refers to individual differences in behavioral phenotypes that are relatively stable over time and across situations (Sih et al. 2004; Penke et al. 2007; John et al. 2008). Such individual differences—also referred to as behavioral syndromes (Sih et al. 2004)—are ubiquitous within social species and predictive of fitness-related outcomes (Nettle 2005; Roberts et al. 2007; Gurven et al. 2014), including reproductive success (Smith and Blumstein 2008; Alvergne et al. 2010; Jokela 2012; Bailey et al. 2013; Berg et al. 2014; Gurven et al. 2014). Recent years have seen a surge of interest among evolutionary behavioral scientists in explaining the ultimate and proximate origins of personality variation in multiple species (Sih et al. 2004; Wolf et al. 2007; Smith and Blumstein 2008; Dingemanse et al. 2010), including humans (Nettle 2006; Alvergne et al. 2010; Buss and Hawley 2011; C. R. von Rueden and A. W. Lukaszewski equally contributed to this work. Address correspondence to C. R. von Rueden. E-mail: . © The Author 2015. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For Downloaded from https://academic.oup.com/beheco/article-abstract/26/4/1071/210310 permissions, please e-mail: by guest on 12 June 2018 Del Giudice et al. 2011; Lukaszewski and Roney 2011; Jokela 2012; Verweij et al. 2012; Bailey et al. 2013; Lukaszewski 2013; Berg et al. 2014; Gurven et al. 2014). Evolutionary theories of adaptive personality variation, although heterogeneous in emphasis, tend to argue that natural selection will maintain personality differences within or between populations when a behavioral phenotype is subject to fluctuating or frequencydependent selection regimes—wherein the costs and benefits of different trait levels vary depending on individual circumstance (Sih et al. 2004; Nettle 2006; Penke et al. 2007; Dingemanse et al. 2010). For example, different levels along a personality continuum may be optimal for individuals in different socioecologies (Sih et al. 2004; Dingemanse et al. 2010; Penke 2011) or who differ in somatic condition or “state” (Luttbeg and Sih 2010; Lukaszewski 2013). A firstorder prediction from this theoretical perspective is that individuals’ personality traits will often be coupled to the circumstances under which their personality trait levels were ancestrally most adaptive. To date, most empirical research that has applied this perspective to elucidate adaptively patterned personality variation in humans has done so using subjects from Western, postindustrial societies 1072 with low fertility and formal legal institutions (e.g., Simpson et al. 1999; Pound et al. 2007; Sell et al. 2009; Jonason et al. 2010; Campiero Ciani 2011; Holtzman et al. 2011; Lukaszewski and Roney 2011; Lukaszewski 2013). Although such research can inform evolutionary theories of personality, it is limited in that modern societies differ in important respects from the small-scale, natural fertility societies in which humans have spent most of our existence (Henrich et al. 2010; Gurven et al. 2013a). In moving forward, therefore, it is important to test functional theories of personality origins using human subjects whose behaviors are evoked within the socioecological contexts of small-scale societies (Alvergne et al. 2010; Bailey et al. 2013; Gurven et al. 2014). The current research investigates adaptively patterned personality variation among the Tsimane’, a group of forager-horticulturalists indigenous to the Bolivian Amazon. Based on a consideration of cost-benefit tradeoffs that have likely maintained variation in aspects of prosociality over human evolution, we advance the hypothesis that individual differences in prosocial traits are facultatively calibrated (i.e., conditionally adjusted over ontogeny) to variation in “embodied capital”—that is, skills, abilities, or somatic traits that enhance an individual’s expected future fitness (Kaplan 1996). This hypothesis is similar to state-dependent personality models, which consider how body size (McElreath and Strimling 2006), reproductive value (Wolf et al. 2007), productivity (Biro and Stamps 2008) and other aspects of phenotype can regulate personality. To evaluate our hypothesis empirically, we test whether prosocial traits (related to leadership and gregarious cooperation) are positively associated with 2 important components of embodied capital among the Tsimane’: physical strength and locally valued knowledge. Facultative calibration of prosocial traits to (...truncated)