David M. G. Lewis

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.

In their commentary on our paper, Brandner et al. commit an elementary statistical mistake that leads to entirely erroneous conclusions. When this statistical error is corrected, the effects described in our original paper appear exactly as reported. In principle, we could end our reply there. However, ending our reply there would be a lost opportunity for promoting best practices in Error Management Theory (EMT) research. The commentators make several other missteps that present the opportunity to draw attention to important principles in EMT research and offer clarifications that we hope assist in the operationalization, design, and interpretation of EMT-inspired studies in the future. We discuss these points and provide several EMT research scenarios to help to illustrate a key principle. We hope this reply highlights some of the key elements of best practices in EMT research and sheds light on pitfalls that researchers must make sure to avoid.

Semchenko and colleagues (in press) recently disentangled two evolutionary hypotheses and demonstrated that heterosexual men have mate preferences for both the morphological cue of women's lumbar curvature and the behavioral cue of back arching: Men are attracted to an intermediate degree of lumbar curvature in both short-term and long-term mating contexts, and, independent of this preference, are attracted to lordosis behavior in short-term, but not long-term, mating contexts. No research to date has investigated whether women are aware of these preferences. There are a priori reasons to expect this to be the case: An awareness of these preferences could functionally guide both appearance-enhancement and intrasexual competition strategies. Here, we tested whether women have accurate perceptions of men's preferences in the lumbar region. Across two studies (Ns = 177, 293), we found that women's perceptions align precisely with men's preferences: Women perceive men to be attracted to cues to lordosis behavior in short-term but not long-term contexts, and to be attracted to an intermediate angle of lumbar curvature independent of mating context. We hope these findings, which document previously unknown features of women's mating psychology, inspire investigations into how women might adaptively use their accurate perceptions of men's mate preferences.
•We tested whether women accurately perceive men's mate preferences.•Women accurately perceived men's preference for lumbar curvature.•Women accurately perceived men's context-dependent attraction to lordosis behavior.

In recent years, researchers have discovered much about how disgust works, its neural basis, its relationship with immune function, its connection with mating, and some of its antecedents and consequents. Despite these advances in our understanding, an under-explored area is how disgust may be used to serve a communicative function, including how individuals might strategically downplay or exaggerate the disgust display in front of different audiences. Here, we generated two hypotheses about potential communicative functions of disgust, and tested these hypotheses in four countries (Turkey, Croatia, Germany, and Norway). We found no evidence in support of either hypothesis in any country. Discussion focuses on the likely falsity of the two central hypotheses, alternative interpretations of our findings, and directions for future research.