David M. G. Lewis

Over the past half century, the field of personality psychology has developed several descriptive taxonomies, but progress toward a genuinely explanatory framework for between- and within-person variation in psychology and behavior has been elusive. We argue that this is largely due to (1) neglecting a key distinction between personality traits and the underlying psychological mechanisms that generate the behaviors captured by those trait constructs, and (2) insufficient focus on attempting to explain variation in the operation of those mechanisms. Here, we illustrate the utility of a mechanism-centered approach for personality psychology by empirically demonstrating that variation in the activation of the evolved psychological mechanism of jealousy is systematically linked to HEXACO Emotionality and NEO Neuroticism. People in monogamous long-term relationships (N = 204) read vignettes depicting varying levels of infidelity threat and wrote what they would think, feel, say, and do in response to the scenarios described. Two different groups of independent raters read the participants' written responses: one group assessed the activation of participants' jealousy by rating their expressed jealousy levels, and the other completed observer reports of the participants' HEXACO Emotionality. As predicted, (1) as infidelity threat increased across scenarios, jealousy levels increased, and (2) increased jealousy predicted higher observer-reported Emotionality. Moreover, (3) participants who exhibited higher jealousy levels reported higher levels of NEO Neuroticism, (4) these self-reports of Neuroticism covaried with the observer reports of Emotionality, and (5) a common-cause model revealed that jealousy accounted for a significant proportion of this covariance between self-reported Neuroticism and observer-reported Emotionality. These findings support the hypothesis that jealousy is a central mechanism underlying variation in Emotionality (and the overlapping Big Five/FFM dimension Neuroticism). More broadly, the present findings suggest that an understanding of specific evolved psychological mechanisms, including both their species-typical design features and individual differences in their operation, can help explain variation on major personality dimensions and offers personality science a genuine integration of universal human nature and individual differences.

Balancing the benefits of social contact against infection risks is a fundamental challenge faced throughout human evolution. Prior work proposes that this trade-off is embedded in mental processes that track the interpersonal value of social partners. If this account is correct, the link between interpersonal value and infection-risky contact should be universal. We find that it is: Across 61 societies, people were consistently more willing to engage in infection-risky contact with high-value social partners. Diverse societal-level factors also related to infection-risky contact, but specifically for high-value targets rather than low-value ones. Findings support the universality of a social psychology of pathogen avoidance that considers the interpersonal value of targets while also being calibrated by societal factors. Interpersonal contact is essential for cooperation and care, but it also provides a route for pathogen transmission between individuals. Hence, a key challenge for social organisms involves maintaining beneficial social contact while also limiting exposure to infection risks. Existing proposals suggest that this trade-off is guided by the interpersonal value of specific social targets. In addition, ecological and cultural factors, including historical pathogen prevalence, individualism-collectivism, tightness–looseness, and relational mobility, might also shape how people avoid pathogens. To test whether and how infection-risky social contact varies with individual-level interpersonal value and these societal-level factors, we collected data from 47,756 participants across 61 societies. Participants were randomly assigned to picture a social target - either a friend, acquaintance, stranger, or enemy. They then rated their comfort with infection-risky contact with that target (e.g., drinking from the same water bottle) and whether they would touch the target when greeting them. In all societies, participants were more comfortable with infection-risky contact and more likely to engage in physical contact during greetings with targets of higher interpersonal value. Such associations varied only modestly across societies. Notably, the associations between infection-risky contact and societal-level factors depended on target interpersonal value and were concentrated among high-value targets—contact was lower where historical pathogen prevalence and tightness were greater and higher where individualism and relational mobility were greater. These findings suggest that infection-risky contact is primarily guided by the value of social targets, with ecological and cultural factors playing a lesser role.

This paper introduces a framework for understanding the functions of emotion and emotion expression at multiple levels of analysis. The framework clarifies several key but sometimes overlooked conceptual distinctions in emotion science, including the distinctions between emotions and their expressions, the individual and social functions of emotions, and functions and benefits. This framework also integrates ultimate and proximate levels of analysis by showing how functional hypotheses lead to concrete, testable proximate predictions about how emotions work. By organizing these distinctions and levels of analysis, this framework provides a basis for asking precise questions and testing new hypotheses about the structure and function of emotions and their expressions. This article is part of the theme issue ‘Mechanisms, development, phylogeny and functions of emotional expressions’.

The precise functions of friendship and the information-processing architecture that evolved to navigate friendships remain unresolved. Research assessing which types of benefits friends provide or which benefits people value most is unlikely to enable researchers to discriminate between alternative evolutionary accounts of friendship, such as reciprocal altruism theory and alliance theory, because these theories generate similar predictions about these features of friendship. We suggest that greater progress toward adjudicating between alternative evolutionary accounts of friendship can be achieved through an increased focus on the underlying information-processing architecture that evolved to secure the functions that friendship evolved to serve. An alliance theory perspective suggests an architecture that hierarchically integrates cues: the mind should track resource sharing and other cues to alliance when information about a friend's alliance cannot be observed directly, but should discount those cues once alliance can be directly observed. By contrast, a reciprocal altruism perspective suggests an additive information-processing architecture: the mind should track resource sharing and other types of benefits a friend might deliver both when a friend's alliance cannot be observed directly and—contrasting alliance theory—when it can be. We outline diverse candidate inputs to alliance-detecting systems that have yet to be systematically empirically investigated, as well as candidate outputs that can reveal the underlying algorithms by which different friend behaviors are processed. We hope that, equipped with these tools, researchers can further illuminate the cognitive architecture of friendship and shed new light on the functions that friendship evolved to solve.

Cross-sex friendships are often characterized by psychological and behavioral patterns consistent with underlying mating motivations, and many romantic relationships begin as friendships. Despite the prevalence of mating outcomes in cross-sex friendships, little is known about the courtship behaviors in cross-sex friendships that translate into these outcomes. Guided by parental investment theory, sexual economics theory, and the principle that courtship behaviors are calibrated to the mate preferences of the courted sex, we elaborated and tested multiple novel predictions about a candidate courtship behavior in cross-sex friendships: financial provisioning. Findings provided robust support for half a dozen sex-differentiated predictions. First, men (N = 147) reported that they pay for more of bills shared with cross-sex friends, a perspective corroborated by reports from women (N = 441). Second, men's, but not women's, interest in mating with their cross-sex friends predicted their financial provisioning behavior. Third, women, but not men, perceived cross-sex friends who financial provision them to be more interested in mating with them. Notably, all these findings were explained as between-person differences rather than within-person effects. That is, men did not provision some female friends more than others depending on their attraction to those specific friends. Rather, some men consistently provisioned across their cross-sex friendships, whereas others did not. This pattern suggests that some men are inclined to view cross-sex friendships as mating opportunities whereas others do not, which may help explain some of the variation in platonic versus mating outcomes of cross-sex friendships. Collectively, these findings offer multiple new insights into the psychology underpinning cross-sex friendships.

Self-esteem is hypothesized to be an evolved psychological system that monitors and responds to cues to one's own relational value. Here, we employ a novel, cue-based approach to generate and test the hypothesis that self-esteem tracks specific, fitness-relevant morphological features. Specifically, we investigated whether females' self-esteem systematically varies as a function of their waist-to-hip ratio (WHR) and lumbar curvature (LC). Participants (N = 177) indicated on two body morph arrays—one varying in WHR and one varying in LC—the morphs that corresponded to (i) their current levels of these features, (ii) the levels they believed potential mates find most attractive, and (iii) the levels they viewed as ideal for themselves. They also completed the Body Esteem Scale and Rosenberg's Self-Esteem Scale. Results demonstrated that (1) participants internalized, as their own “ideal”, the levels of WHR and LC they perceived potential mates to desire, and (2) their self-esteem levels tracked the discrepancy between their idealized levels of WHR and LC and their own current levels of these features. These effects of WHR and LC on self-esteem were independent and fully mediated by their effects on body esteem. Discussion centers on how efforts to mitigate the negative effects of media exposure on women's self-esteem could potentially be improved by more precisely targeting the specific morphological cues to which self-esteem is sensitive.

Disgust is a protective emotion that coordinates a suite of cognitive and behavioral changes to reduce people's likelihood of infection. Existing research demonstrates that people react with disgust to pathogenic stimuli compared to non-pathogenic stimuli, but there has been a paucity of research examining whether people are equipped with a more finely graded disgust response: do people discriminate between pathogen threats of different magnitudes and react with more disgust toward pathogen threats of a greater magnitude? We derived multiple novel predictions from the Threat-Dependent Disgust hypothesis and tested them across three experiments involving participants from the United States and India (total n = 1,333). In Study 1 (n = 428), we tested the prediction that people would be more disgusted by a pathogen threat touching their hand relative to their foot, given that touching a pathogen with the hand is more likely to result in the pathogen entering the body envelope (e.g., through the mouth). In Study 2 (n = 453), we tested the prediction that people would be more disgusted by a pathogen threat touching another person's hand relative to another person's foot; people touch others more with their hands than with their feet, so a conspecific with a contaminated hand poses a greater disease threat than one with a pathogen on their foot. In Study 3 (n = 452), we tested the prediction that people would be more disgusted by skin wounds caused by pathogenic infections than by surgical incisions; although both wounds pose the risk of exposure to another's bodily fluids, wounds caused by pathogenic infections pose a greater disease threat. Results across all three experiments suggest that disgust is sensitive to the magnitude of pathogen threat in a more finely tuned manner than previously demonstrated. Discussion focuses on interpretation of this gradient-like sensitivity of disgust, incorporating these findings into the broader disgust literature, and future directions.

Objectives
Sexual selection via contest competition has equipped countless organisms with weaponry in their appendages to overpower their opponents. Here, we tested (1) whether greater upper limb length—measured as span controlling for biacromial width—confers an advantage in contest competition among adult humans, (2) several possible means by which upper limb length might increase success in intrasexual contest competition, and (3) whether, consistent with male–male contest competition creating stronger selection pressures than female–female contest competition, male Homo sapiens have greater upper limb length.

Methods
We collected fight statistics and facial and body photographs from professional combatants (N = 715) in the Ultimate Fighting Championship (UFC; Study 1). Sexual dimorphism in upper limb length was then examined via diverse and demographically representative samples from four studies (total N = 6915), from Croatian adolescents and older Singaporean adults to United States Army personnel born across all major world regions (Studies 2a–2d).

Results
First, we found that greater upper limb length is associated with increased success in intrasexual contest competition, an effect driven by both the capacity to grapple opponents to submission and to knock opponents unconscious (Study 1). Second, we found unequivocal, cross-cultural evidence of unique sexual dimorphism in upper limb length after controlling for allometry: across four studies, men exhibited longer upper limbs than women (Studies 2a–2d).

Conclusion
Upper limb length may have been shaped by intrasexual selection, with implications across the biological, anthropological, and psychological sciences.

With dementia incidence projected to escalate significantly within the next 25 years, the United Nations declared 2021-2030 the Decade of Healthy Ageing, emphasising cognition as a crucial element. As a leading discipline in cognition and ageing research, psychology is well-equipped to offer insights for translational research, clinical practice, and policy-making. In this comprehensive review, we discuss the current state of knowledge on age-related changes in cognition and psychological health. We discuss cognitive changes during ageing, including (a) heterogeneity in the rate, trajectory, and characteristics of decline experienced by older adults, (b) the role of cognitive reserve in age-related cognitive decline, and (c) the potential for cognitive training to slow this decline. We also examine ageing and cognition through multiple theoretical perspectives. We highlight critical unresolved issues, such as the disparate implications of subjective versus objective measures of cognitive decline and the insufficient evaluation of cognitive training programs. We suggest future research directions, and emphasise interdisciplinary collaboration to create a more comprehensive understanding of the factors that modulate cognitive ageing.

From cockroaches and cuttlefish to crocodiles and chimpanzees, organisms across diverse taxa are equipped with physical and psychological systems for courting opposite-sex conspecifics. In this chapter, we focus on the colorful—literally and figuratively—collection of courtship ornaments, tactics, and strategies of one primate species: Homo sapiens. Humans use their vocal qualities—deep voices, soft voices, expressive voices—to show their dominance, kindness, and intelligence. They dance dynamically, kiss passionately, and offer caring (as well as deceptive) compliments. Humans’ courtship signals and the psychophysical systems that detect them span the senses: visual, auditory, tactile, olfactory, and gustatory. We review research across these perceptual modalities and offer suggestions for future work into the many uncharted areas of this fascinating domain.