The social evolution of individual differences: Future directions for a comparative science of personality in social behavior

Personality is essential for understanding the evolution of cooperation and conflict in behavior. However, personality science remains disconnected from the field of social evolution, limiting our ability to explain how personality and plasticity shape phenotypic adaptation in social behavior. Researchers also lack an integrative framework for comparing personality in the contextualized and multifaceted behaviors central to social interactions among humans and other animals. Here we address these challenges by developing a social evolutionary approach to personality, synthesizing theory, methods, and organizing questions in the study of individuality and sociality in behavior. We critically review current measurement practices and introduce social reaction norm models for comparative research on the evolution of personality in social environments. These models demonstrate that social plasticity affects the heritable variance of personality, and that individual differences in social plasticity can further modify the rate and direction of adaptive social evolution. Future empirical studies of frequency- and density-dependent social selection on personality are crucial for further developing this framework and testing adaptive theory of social niche specialization.

Capital as an Integrative Conceptualisation of Human Characteristics, Behaviour, and Outcomes Predicting Reproductive Success and Evolutionary Fitness

According to evolutionary theory, human cognition and behaviour are based on adaptations selected for their contribution to reproduction in the past, which in the present may result in differential reproductive success and inclusive fitness. Because this depiction is broad and human behaviour often separated from this ultimate outcome (e.g., increasing childlessness), evolutionary theory can only incompletely account for human everyday behaviour. Moreover, effects of most studied traits and characteristics on mating and reproductive success turned out not to be robust. In this article, an abstract descriptive level for evaluating human characteristics, behaviour, and outcomes is proposed, as a predictor of long-term reproductive success and fitness. Characteristics, behaviour, and outcomes are assessed in terms of attained and maintained capital, defined by more concrete (e.g., mating success, personality traits) and abstract (e.g., influence, received attention) facets, thus extending constructs like embodied capital and social capital theory, which focuses on resources embedded in social relationships. Situations are framed as opportunities to gain capital, and situational factors function as elicitors for gaining and evaluating capital. Combined capital facets should more robustly predict reproductive success and (theoretically) fitness than individual fitness predictors. Different ways of defining and testing these associations are outlined, including a method for empirically examining the psychometric utility of introducing a capital concept. Further theorising and empirical research should more precisely define capital and its facets, and test associations with (correlates of) reproductive success and fitness.

Extracting Agency and Communion From the Big Five: A Four-Way Competition

Agency and communion are the two fundamental content dimensions in psychology. The two dimensions figure prominently in many psychological realms (personality, social, self, motivational, cross-cultural, etc.). In contemporary research, however, personality is most commonly measured within the Big Five framework. We developed novel agency and communion scales based on the items from the most popular nonpropriety measure of the Big Five-the Big Five Inventory. We compared four alternative scale-construction methods: expert rating, target scale, ant colony, and brute force. Across three samples (N_total = 942), all methods yielded reliable and valid agency and communion scales. Our research provides two main contributions. For psychometric theory, it extends knowledge on the four scale-construction methods and their relative convergence. For psychometric practice, it enables researchers to examine agency and communion hypotheses with extant Big Five Inventory data sets, including those collected in their own labs as well as openly accessible, large-scale data sets.

Physical Strength Partly Explains Sex Differences in Trait Anxiety in Young Americans

Among the most consistent sex differences to emerge from personality research is that women score higher than men on the Big Five personality trait Neuroticism. However, there are few functionally coherent explanations for this sex difference. The current studies tested whether this sex difference is due, in part, to variation in physical capital. Two preregistered studies (total N = 878 U.S. students) found that sex differences in the anxiety facet of Neuroticism were mediated by variation in physical strength and self-perceived formidability. Study 1 (N = 374) did not find a predicted mediation effect for overall Neuroticism but found a mediation effect for anxiety (the facet of Neuroticism most strongly associated with grip strength). Study 2 (N = 504) predicted and replicated this mediation effect. Further, sex differences in anxiety were serially mediated by grip strength and self-perceived formidability. These findings add to a nascent literature suggesting that differences in physical attributes may partially explain sex differences in personality.

An Adaptationist Framework for Personality Science

The field of personality psychology aspires to construct an overarching theory of human nature and individual differences: one that specifies the psychological mechanisms that underpin both universal and variable aspects of thought, emotion, and behaviour. Here, we argue that the adaptationist toolkit of evolutionary psychology provides a powerful meta–theory for characterizing the psychological mechanisms that give rise to within–person, between–person, and cross–cultural variations. We first outline a mechanism–centred adaptationist framework for personality science, which makes a clear ontological distinction between (i) psychological mechanisms designed to generate behavioural decisions and (ii) heuristic trait concepts that function to perceive, describe, and influence others behaviour and reputation in everyday life. We illustrate the utility of the adaptationist framework by reporting three empirical studies. Each study supports the hypothesis that the anger programme—a putative emotional adaptation—is a behaviour–regulating mechanism whose outputs are described in the parlance of the person description factor called ‘Agreeableness’. We conclude that the most productive way forward is to build theory–based models of specific psychological mechanisms, including their culturally evolved design features, until they constitute a comprehensive depiction of human nature and its multifaceted variations. © 2020 European Association of Personality Psychology

Embodied Capital and Risk-Related Traits, Attitudes, Behaviors, and Outcomes: An Exploratory Examination of Attractiveness, Cognitive Ability, and Physical Ability

The relative state model posits two nonindependent pathways to risk. The need-based pathway suggests people take risks when nonrisky options are unlikely to meet their needs. The ability-based pathway suggests people take risks when they possess resources or abilities making them more capable of successfully “pulling off” risk-taking. Growing laboratory and field evidence supports need-based risk-taking. However, little is known about ability-based risk-taking. We examined whether three indicators of embodied capital (attractiveness, cognitive ability, and physical dexterity) were associated with risk-related personality traits, risk-attitudes, behavioral risk-taking, and outcomes associated with risk-taking. Among 328 community members recruited to maximize variance on risk-propensity, we demonstrate that embodied capital indices predict various instantiations of risk-propensity consistent with the relative state model.

Toward Integration of the Niche Diversity Hypothesis With Other Explanations for Personality Covariation: Reply to Međedović's (2019) Commentary on Lukaszewski et al. (2017)

The hypothesis of a human-universal personality structure is undermined by cross-cultural studies in small-scale societies. To explain cross-population differences in patterns of behavioral covariance, we proposed the niche diversity hypothesis, which holds that the degree of behavioral covariation within a population is inversely related to the number and diversity of niches within its socioecology. This hypothesis is formalized as a computational model, and its predictions have been supported empirically. Herein, we respond to several important issues regarding this line of research that were raised in a recent commentary: (1) the study of specific behavioral syndromes should be integrated into the niche diversity model; (2) environmental harshness might alternatively explain our cross-cultural findings regarding effects of niche diversity; and (3) better definitions of behavioral traits are needed for future research. We conclude that the niche diversity hypothesis can be integrated with other explanations for cross-cultural differences in personality covariation.

Niche diversity can explain cross-cultural differences in personality structure

The covariance structure of personality traits derived from statistical models (for example, Big Five) is often assumed to be a human universal. Cross-cultural studies have challenged this view, finding that less-complex societies exhibit stronger covariation among behavioural characteristics, resulting in fewer derived personality factors. To explain these results, we propose the niche diversity hypothesis, in which a greater diversity of social and ecological niches elicits a broader range of multivariate behavioural profiles and, hence, lower trait covariance in a population. We formalize this as a computational model, which reproduces empirical results from recent cross-cultural studies and also yields an additional prediction for which we find empirical support. This work provides a general explanation for population differences in personality structure in both humans and other animals and suggests a substantial reimagining of personality research: instead of reifying statistical descriptions of manifest personality structures, research should focus more on modelling their underlying causes.

The dynamics of men's cooperation and social status in a small-scale society

We propose that networks of cooperation and allocation of social status co-emerge in human groups. We substantiate this hypothesis with one of the first longitudinal studies of cooperation in a preindustrial society, spanning 8 years. Using longitudinal social network analysis of cooperation among men, we find large effects of kinship, reciprocity and transitivity in the nomination of cooperation partners over time. Independent of these effects, we show that (i) higher-status individuals gain more cooperation partners, and (ii) individuals gain status by cooperating with individuals of higher status than themselves. We posit that human hierarchies are more egalitarian relative to other primates species, owing in part to greater interdependence between cooperation and status hierarchy.

Foraging Performance, Prosociality, and Kin Presence Do Not Predict Lifetime Reproductive Success in Batek Hunter-Gatherers

Identifying the determinants of reproductive success in small-scale societies is critical for understanding how natural selection has shaped human evolution and behavior. The available evidence suggests that status-accruing behaviors such as hunting and prosociality are pathways to reproductive success, but social egalitarianism may diminish this pathway. Here we introduce a mixed longitudinal/cross-sectional dataset based on 45 years of research with the Batek, a population of egalitarian rain forest hunter-gatherers in Peninsular Malaysia, and use it to test the effects of four predictors of lifetime reproductive success: (i) foraging return rate, (ii) sharing proclivity, (iii) cooperative foraging tendency, and (iv) kin presence. We found that none of these factors can explain variation in lifetime reproduction among males or females. We suggest that social egalitarianism, combined with strikingly low infant and juvenile mortality rates, can mediate the pathway between foraging, status-accruing behavior, and reproductive success. Our approach advocates for greater theoretical and empirical attention to quantitative social network measures, female foraging, and fitness outcomes.

State-dependent risk-taking

Who takes risks, and when? The relative state model proposes two non-independent selection pressures governing risk-taking: need-based and ability-based. The need-based account suggests that actors take risks when they cannot reach target states with low-risk options (consistent with risk-sensitivity theory). The ability-based account suggests that actors engage in risk-taking when they possess traits or abilities that increase the expected value of risk-taking (by increasing the probability of success, enhancing payoffs for success or buffering against failure). Adaptive risk-taking involves integrating both considerations. Risk-takers compute the expected value of risk-taking based on their state-the interaction of embodied capital relative to one's situation, to the same individual in other circumstances or to other individuals. We provide mathematical support for this dual pathway model, and show that it can predict who will take the most risks and when (e.g. when risk-taking will be performed by those in good, poor, intermediate or extreme state only). Results confirm and elaborate on the initial verbal model of state-dependent risk-taking: selection favours agents who calibrate risk-taking based on implicit computations of condition and/or competitive (dis)advantage, which in turn drives patterned individual differences in risk-taking behaviour.

Broadening horizons: Sample diversity and socioecological theory are essential to the future of psychological science

The present lack of sample diversity and ecological theory in psychological science fundamentally limits generalizability and obstructs scientific progress. A focus on the role of socioecology in shaping the evolution of morphology, physiology, and behavior has not yet been widely applied toward psychology. To date, evolutionary approaches to psychology have focused more on finding universals than explaining variability. However, contrasts between small-scale, kin-based rural subsistence societies and large-scale urban, market-based populations, have not been well appreciated. Nor has the variability within high-income countries, or the socioeconomic and cultural transformations affecting even the most remote tribal populations today. Elucidating the causes and effects of such broad changes on psychology and behavior is a fundamental concern of the social sciences; expanding study participants beyond students and other convenience samples is necessary to improve understanding of flexible psychological reaction norms among and within populations. Here I highlight two examples demonstrating how socioecological variability can help explain psychological trait expression: (i) the role of environmental harshness and unpredictability on shaping time preference and related traits, such as impulsivity, vigilance, and self-efficacy; and (ii) the effects of industrialization, market integration, and niche complexity on personality structure. These cases illustrate how appropriate theory can be a powerful tool to help determine choices of diverse study populations and improve the social sciences.

Sex differences in political leadership in an egalitarian society

We test the contribution of sex differences in physical formidability, education, and cooperation to the acquisition of political leadership in a small-scale society. Among forager-farmers from the Bolivian Amazon, we find that men are more likely to exercise different forms of political leadership, including verbal influence during community meetings, coordination of community projects, and dispute resolution. We show that these differences in leadership are not due to gender per se but are associated with men's greater number of cooperation partners, greater access to schooling, and greater body size and physical strength. Men's advantage in cooperation partner number is tied to their participation in larger groups and to the opportunity costs of women's intrahousehold labor. We argue these results highlight the mutual influence of sexual selection and the sexual division of labor in shaping how women and men acquire leadership.

What Explains Personality Covariation? A Test of the Socioecological Complexity Hypothesis

Correlations among distinct behaviors are foundational to personality science, but the field remains far from a consensus regarding the causes of such covariation. We advance a novel explanation for personality covariation, which views trait covariance as being shaped within a particular socioecology. We hypothesize that the degree of personality covariation observed within a society will be inversely related to the society’s socioecological complexity, that is, its diversity of social and occupational niches. Using personality survey data from participant samples in 55 nations ( N = 17,637), we demonstrate that the Big Five dimensions are more strongly intercorrelated in less complex societies, where the complexity is indexed by nation-level measures of economic development, urbanization, and sectoral diversity. This inverse relationship is robust to control variables accounting for a number of methodological and response biases. Our findings support the socioecological complexity hypothesis and more generally bolster functionalist accounts of trait covariation.

The Tsimane Health and Life History Project: Integrating anthropology and biomedicine

The Tsimane Health and Life History Project, an integrated bio-behavioral study of the human life course, is designed to test competing hypotheses of human life-history evolution. One aim is to understand the bidirectional connections between life history and social behavior in a high-fertility, kin-based context lacking amenities of modern urban life (e.g. sanitation, banks, electricity). Another aim is to understand how a high pathogen burden influences health and well-being during development and adulthood. A third aim addresses how modernization shapes human life histories and sociality. Here we outline the project's goals, history, and main findings since its inception in 2002. We reflect on the implications of current findings and highlight the need for more coordinated ethnographic and biomedical study of contemporary nonindustrial populations to address broad questions that can situate evolutionary anthropology in a key position within the social and life sciences.

Men's status and reproductive success in 33 nonindustrial societies: Effects of subsistence, marriage system, and reproductive strategy

Social status motivates much of human behavior. However, status may have been a relatively weak target of selection for much of human evolution if ancestral foragers tended to be more egalitarian. We test the "egalitarianism hypothesis" that status has a significantly smaller effect on reproductive success (RS) in foragers compared with nonforagers. We also test between alternative male reproductive strategies, in particular whether reproductive benefits of status are due to lower offspring mortality (parental investment) or increased fertility (mating effort). We performed a phylogenetic multilevel metaanalysis of 288 statistical associations between measures of male status (physical formidability, hunting ability, material wealth, political influence) and RS (mating success, wife quality, fertility, offspring mortality, and number of surviving offspring) from 46 studies in 33 nonindustrial societies. We found a significant overall effect of status on RS (r = 0.19), though this effect was significantly lower than for nonhuman primates (r = 0.80). There was substantial variation due to marriage system and measure of RS, in particular status associated with offspring mortality only in polygynous societies (r = -0.08), and with wife quality only in monogamous societies (r = 0.15). However, the effects of status on RS did not differ significantly by status measure or subsistence type: foraging, horticulture, pastoralism, and agriculture. These results suggest that traits that facilitate status acquisition were not subject to substantially greater selection with domestication of plants and animals, and are part of reproductive strategies that enhance fertility more than offspring well-being.

Solving the puzzle of collective action through inter-individual differences

Models of collective action infrequently account for differences across individuals beyond a limited set of strategies, ignoring variation in endowment (e.g. physical condition, wealth, knowledge, personality, support), individual costs of effort, or expected gains from cooperation. However, behavioural research indicates these inter-individual differences can have significant effects on the dynamics of collective action. The papers contributed to this theme issue evaluate how individual differences affect the propensity to cooperate, and how they can catalyse others' likelihood of cooperation (e.g. via leadership). Many of the papers emphasize the relationship between individual decisions and socio-ecological context, particularly the effect of group size. All together, the papers in this theme issue provide a more complete picture of collective action, by embracing the reality of inter-individual variation and its multiple roles in the success or failure of collective action.

The Relative State Model: Integrating Need-Based and Ability-Based Pathways to Risk-Taking

Who takes risks, and why? Does risk-taking in one context predict risk-taking in other contexts? We seek to address these questions by considering two non-independent pathways to risk: need-based and ability-based. The need-based pathway suggests that risk-taking is a product of competitive disadvantage consistent with risk-sensitivity theory. The ability-based pathway suggests that people engage in risk-taking when they possess abilities or traits that increase the probability of successful risk-taking, the expected value of the risky behavior itself, and/or have signaling value. We provide a conceptual model of decision-making under risk-the relative state model-that integrates both pathways and explicates how situational and embodied factors influence the estimated costs and benefits of risk-taking in different contexts. This model may help to reconcile long-standing disagreements and issues regarding the etiology of risk-taking, such as the domain-generality versus domain-specificity of risk or differential engagement in antisocial and non-antisocial risk-taking.