Original Article
The evolutionary fitness of personality traits in a small-scale subsistence society

https://doi.org/10.1016/j.evolhumbehav.2013.09.002Get rights and content

Abstract

The maintenance of personality variation remains an unexplained puzzle in evolutionary biology. Despite evidence among non-humans that personality variation affects fitness, few data exist to assess the personality–fitness relationship in humans. Among Tsimane forager–horticulturalists (n = 632), we test whether personality traits (assessed using a 43-item Big Five Inventory administered orally in native language) predict fertility, offspring survivorship, age of first reproduction, and other fitness correlates (extramarital affairs, conflicts, social visitation, food production, and several health measures). Among men, several personality factors associate with higher fertility, more time spent producing food and social visitation. Among women, the relationship between personality and fitness varies across regions of Tsimane territory. The only case of an intermediate personality level associated with highest fitness was found for Industriousness in men. We find that personality factors positively associated with fitness do not associate with greater health costs, although greater Extraversion and Openness may lead to more conflicts among men. Factor heritability ranges from 60% for Prosociality and Extraversion to 8% for Neuroticism. We interpret our results in light of evolutionary models that explain maintenance of personality variation, including incomplete directional selection, mutation–selection balance, condition-dependent reaction norms and fluctuating selection based on sex or spatial variability in selection pressures.

Introduction

Personality or “behavioral syndromes” are relatively stable dispositional traits and behaviors that have now been identified in a myriad of social species (Gosling, 2001, Sih et al., 2004), and with clear consequences on fitness (Smith & Blumstein, 2008). The canalization of personality during development and relative stability thereafter, despite varying circumstances over the life course that might otherwise favor greater plasticity, is an important problem attracting much theoretical and empirical attention (Dall et al., 2004, Dingemanse et al., 2010). Further, personality is highly heritable, yet how heritable genetic variation in personality traits is maintained over generations remains another conundrum (Buss & Hawley, 2011). If selection effects on personality vary over space or time, or by organismal state or condition, then variation in personality could be adaptive. Frequency-dependence could also affect fitness if payoffs vary based on the frequency of personalities in the population. However, empirical evidence to support these adaptive explanations is sparse in humans. One approach to studying the adaptive value of personality variation considers costs and benefits of specific dispositions, and how these may maintain multiple phenotypic equilibria along personality dimensions. Extraverted individuals may be bold, sociable and may obtain greater mating access, but may also incur greater risks of injury, morbidity and mortality (Nettle, 2006). Conscientious individuals may be goal-oriented, hard working, and cautious about health, but may also miss out on short-term mating and resource opportunities (Schmitt, 2004). Neurotic individuals may be prone to greater depression, anxiety and chronic stress, but may also be more risk-averse and vigilant concerning environmental dangers (Nettle, 2006).

A theory of personality that specifies its ontogeny, the contributions of life history strategy and social norms, and the ways by which selection pressures impact personality variation over space and time, remains lacking. Furthermore, existing models of decision-making in the social sciences rarely consider dispositional traits as critical components of behavioral strategies. The standard framework underlying decision-making in optimization models assumes that situational costs and benefits impact all individuals equally, except when individuals vary by condition or state (Almlund, Duckworth, Heckman, & Kautz, 2011). Across many species and taxa, however, individuals often act consistently across contexts and over time when the standard approach predicts more flexible “optimal” responses (Bell, Hankison, & Laskowski, 2009). What we can learn about personality variation in small-scale societies will aid our understanding of the selection pressures responsible for shaping human personality traits. It is in small-scale societies that humans have lived for the majority of our existence. Variations within and across modern small-scale societies in access to contraception, health care, formal legal systems, or the market economy present unique opportunities to understand the origins of human personality variation and how personality responds to socioecological context.

Most human personality studies to date are descriptive, aimed at testing the existence of a specific personality structure (e.g. Big Five) based on inductive factor analysis of self- or third-party reports (e.g. McCrae and Terracciano, 2005, Schmitt et al., 2007). Approaches in evolutionary psychology have traditionally focused on human universals, and with a few exceptions (e.g. Buss, 1991, MacDonald, 1995, Wilson, 1994), have only recently attempted to explain individual variability (Buss & Hawley, 2011). Almost all studies have been restricted to low-fertility, heterogeneous, modern populations. In these societies, personality variation in the “Big Five” is associated with a variety of outcomes affecting health, mortality, education and income (Ozer & Benet-Martinez, 2006). It also correlates with reproductive behavior among Australian and U.S. adults (Eaves et al., 1990, Jokela et al., 2011). Only two studies have examined fitness correlates of personality variation in natural fertility populations. Among rural Senegalese farmers, extraverted men and women with intermediate levels of neuroticism have more children (Alvergne, Jokela, & Lummaa, 2010). Among Ache forager–farmers, extraverted men have more children (Bailey et al., 2013). Both studies, however, have small sample sizes, do not consider potential costs of specific dispositions, or whether dispositions co-vary with observed fitness-related behavior.

Here we investigate the relationship between personality and fitness among Tsimane forager–horticulturalists of Bolivia with four goals in mind. We first explore the relationship between reproductive success (RS) and two sets of personality dimensions: the traditional Big Five and a population-specific Big Two derived from exploratory factor analysis (“pro-sociality” and “industriousness”) for both men and women. While among Tsimane the Big Five correlate with observed behavior and are replicable, their internal consistency is lower than commonly encountered; for this reason, we provide the Big Two as an alternative and more robust personality structure (Gurven, von Rueden, Massenkoff, Kaplan, & Lero Vie, 2013). Fitness is proxied as age-specific cumulative fertility, offspring survivorship, and age at first reproduction (AFR). In natural fertility populations without deliberate fertility control, these are reliable measures of future genetic representation. AFR is the most indirect fitness measure of the three, but has been linked to fitness among Tsimane and other subsistence populations (von Rueden, Gurven, & Kaplan, 2011).

Second, we consider potential costs of several personality traits by testing for longitudinal associations with health indicators and frequency of social conflicts among men in a society lacking modern healthcare and a formal legal system. Prospective study permits causal inference beyond simple correlation between personality and health. Third, we examine several behavioral correlates of personality that relate to fitness, such as time spent in productive tasks, direct care of offspring, social visitation and number of extramarital affairs. Fourth, we assess the heritability of personality by considering multigenerational pedigrees. Heritability describes the proportion of variance in an observable trait within a population that is due to inter-individual variance in genetic factors. Hence, the heritability of personality describes the extent to which selection can maintain variability in personality within a particular population (Falconer & Mackay, 1996). As our estimate is not based on an adoption or twin study, and so does not completely control for shared environment, we label our estimate “quasi”-heritability. We consider different modes of selection in the interpretation of personality–fitness associations: stabilizing, directional and fluctuating (in which the fittest phenotype within a population varies across time, space or by individual condition). Highest fitness at intermediate levels of personality traits is consistent with stabilizing selection on personality, while linear effects are consistent with directional selection. Variation in personality and the personality–fitness relationship by geographic region is consistent with spatially fluctuating selection. Fluctuating selection is also consistent with personality–fitness relationships differing among men and women. Men and women may be expected to differ in personality because their reproductive, social and parental investment strategies may diverge (Schmitt, Realo, Voracek, & Allik, 2008).

Section snippets

Study population

Tsimane are semi-sedentary forager–horticulturalists of Amazonian Bolivia, inhabiting over 90 villages ranging from 50 to 500 individuals. They cultivate plantains, rice, corn, and sweet manioc in small swiddens, and regularly fish and hunt for meat. These foods together provide over 90% of the calories in the diet, with the remainder coming mainly from store-bought items or trade with itinerant merchants. Tsimane live in extended family clusters, where the majority of food- and labor-sharing

Results

Descriptive statistics for personality measures and covariates by sex are provided in Table 1.

Discussion

In a natural fertility context with rapid population growth, we find that greater E, C, O and I, and lower N, are associated with higher fertility and reproductive success among men. Fitness differences due to personality among men are realized through increased fertility rather than offspring survival. These differences persist when other potential confounds like age, schooling and Spanish fluency are controlled. Furthermore, only men lower in I had an earlier age at first birth, so earlier

Conclusion

The evolutionary fitness consequences of human personality variation have been difficult to assess because of the main focus on modern societies with controlled fertility (Alvergne et al., 2010, Gurven et al., 2013). Yet even in modern societies, personality predicts income, occupation, reproduction, marital stability and other fitness-relevant outcomes in both sexes (Ozer & Benet-Martinez, 2006). Personality has equally strong effects as cognitive ability and socioeconomic status in predicting

Supplementary Materials

The following are the Supplementary data to this article.

Gurvenetal EHB Supplement 8.10.2013 CLEAN

Acknowledgment

We are grateful to all Tsimane for their participation, and to THLHP personnel. Marino Lero Vie assisted with data collection.

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    Funding was provided by NIH/NIA (R01AG024119 and R56AG024119).

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