People desire friends who have qualities such as similarity, the ability to provide support, opportunities for socialisation, assistance in achieving self-serving goals and access to potential mates. In the current study, we aimed to examine the degree of overlap between the friends people consider ideal and those they actually have. Using a sample of 6224 participants from 12 countries, we found that, across five dimensions of friendship, ideal and actual friendships considerably overlapped and ideal preferences were significant and strong predictors of actual friendships. Still, the overlap between ideal and actual friendships was not complete; rather, a small deficit emerged, with participants getting less of what they wanted. These results were consistent across the 12 countries in our sample. We also found significant effects of the Dark Triad personality traits on several dimensions of friendship.

Why is lower social class often associated with greater empathy? We test whether less wealthy people experience more challenges, thereby motivating them to help others as a means of securing future social support, and, in turn, cultivating empathy over recurrent experiences. Study 1 (N = 513) showed that wealth was negatively correlated with empathy through fewer challenges and less helping. Study 2 (N = 915) replicated this effect while revealing unexpected curvilinear associations. Among people helping less overall (measured monthly across 1 year), wealth led to lower empathy through fewer challenges and less helping. However, among people helping more overall, more helping led to decreasing empathy. Study 3 (N = 500) replicated the sequential indirect effect of wealth on empathic concern at low, but not high, helping in a two-wave preregistered study. Results suggest that repeated acts of helping can cultivate empathic concern-but, only for those helping a moderate amount.

Cooperation often relies on people's ability to discern in whom to invest their limited resources. One solution to this partner choice dilemma entails estimating one's fitness interdependence with others. Previous studies indicate that estimates of fitness interdependence (i.e., perceived shared fate) motivate cooperation, but the sources of information that contribute to perceived shared fate remain an elusive question. We examine links among ten sources of interdependence (i.e., attributes and experiences that yoke partners' fitness), perceived shared fate, and cooperation towards three relationship targets (an acquaintance, cousin, and sibling) in a smallscale horticultural society of Northern Nicaragua. While eight sources of interdependence showed positive bivariate associations with perceived shared fate, only relatedness, commensality, and shared subsistence activities were uniquely associated with higher perceived shared fate. Moreover, perceived shared fate (1) was associated with more cooperation across seven fitness-relevant domains, (2) statistically mediated associations between relatedness and cooperation (Study 1: N = 146, Obs. = 437), and (3) had strong effects on cooperation (i.e., forgoing money to buy rice for a partner; Study 2: N = 36, Obs. = 108). Results indicate that perceived shared fate arises from sources of fitness interdependence and cooperation is proximally guided by perceived shared fate. Estimates of fitness interdependence by way of perceptions of shared fate may therefore offer a simple solution to partner choice dilemmas: Help partners if one has a positive stake in their welfare.

Friendships help people achieve many fundamental adaptive functions, such as forming coalitions and successful alloparenting. As such, one might theorize that humans are designed to seek out and maintain as many friendships as possible. However, with time and energy as finite resources, humans face a unique challenge in determining the number of friendships they can sustain - do they prioritize maximizing friendship quantity (potentially at the expense of investment in each friend, thus compromising friendship quality), or do they prioritize friendship quality (potentially at the expense of friendship quantity)? Here, we introduce the novel concept of friendship making orientation - a stable individual difference in motivation towards maximizing friendship quantity - and develop a tool for measuring this trait. Across 5 studies (NTotal = 5707), we created and confirmed the Friendship Making Orientation Scale's factor structure, finding that the scale is composed of two factors: preference for a wide friendship network and desire for intimacy with many friends. We further demonstrate strong test-retest reliability, as well as construct and discriminant validity. Critically, we demonstrate that the scale achieves measurement invariance across three geographically diverse countries (i.e., Egypt, the Philippines, and Brazil), and is associated with friend-seeking motivation and behaviors over and above relevant controls. Friendship Making Orientation offers a novel method for assessing an unexplored aspect of friendships and provides a crucial tool for understanding this valued relationship.

Friendships have helped humans solve many different recurrent adaptive challenges across evolutionary time, ranging from physical defense to finding mates. While some of the challenges equally affected both men and women, others are linked to one's sex and have therefore led to some differences in the structures and functions of male and female friendships in order to effectively solve these challenges. For example, previous research has found robust evidence for enhanced intimacy and emotional sharing in women's (as compared to men's) friendships. This difference is reflected in the qualities men and women report seeking in their friends: Women, more than men, prefer friends who provide them with emotional support. We replicate this finding in five studies spanning four countries (N = 967) to provide preliminary evidence that this preference is found cross-societally. Consistent with a functional account of friendship, the extent to which emotional support from same-sex friends is prized differs by sex.

Does fermentation enhance the safety of beverages? This study investigates kombucha's resilience to microbial invasion from human hands. Kombucha, an ancient fermented tea, relies on a biofilm known as a symbiotic community of bacteria and yeast (SCOBY) and a starter culture to ferment sweet tea, producing various metabolites including gluconic acid, acetic acid, ethanol, and carbon dioxide. While fermented foods have been historically associated with potential health benefits, direct evidence comparing the prevalence of human pathogens in fermented versus non-fermented beverages has been lacking. To address this, we conducted a citizen science experiment at Arizona State University, introducing swabs from people's palms into both kombucha and a sweet tea control. Over 30 days, we monitored the bacterial and fungal composition using 16S and ITS rRNA sequencing. Our findings reveal that kombucha maintained a relatively stable microbial profile and physical appearance, with a typical SCOBY biofilm developing. In stark contrast, the sweet tea experienced a dramatic change in microbial composition and was visibly compromised by rapidly growing microorganisms. Importantly, this suggests that the complex microbial ecosystem of kombucha can limit the growth of foreign microbes introduced from human hands. Given that human pathogens were absent from the kombucha while present in the tea samples, our results indicate that the antimicrobial properties of fermentation byproducts and the physical barrier of the SCOBY may contribute to this resilience. Further research is warranted to fully elucidate the mechanisms underlying kombucha's resistance to microbial invasion.

Friendships vary in closeness and endurance: Some friendships are closer while others are more casual; some are more enduring while others last only as long as they are convenient. The present study examined friendship preferences in these dimensions. Using data from a cross-cultural sample ( N = 6224) taken from 12 countries, we found that the vast majority of participants preferred a combination of close and casual friendships, with a preference for having more close friendships. Similarly, most participants preferred a combination of enduring and convenience friendships, with a preference for having more enduring friendships. Additionally, participants’ preferences closely matched their actual friendships, with these findings being generally consistent across cultures. We found also that participants who preferred friendships that enabled them to receive support and social input were more likely to prefer close and enduring friendships over casual and convenient ones. The opposite was true for participants who preferred friendships that allowed them to achieve self-serving goals and gain access to mates. Furthermore, we found significant but small direct and indirect effects of the Dark Triad personality traits, especially Machiavellianism, on preferences for closeness and endurance in friendship.

We test the hypothesis that wealth is associated with lower empathic concern because wealthier people experience fewer challenges and give help less often to people in need. As predicted, Study 1 (N = 513; cross-sectional) showed wealth was negatively associated with trait empathic concern through experiencing fewer challenges and providing less help. Study 2 (N = 915; longitudinal) replicated this while also revealing unexpected curvilinear associations. At low levels of helping (measured monthly across one year), wealth led to lower empathy through experiencing fewer challenges and less helping. However, more helping led to decreasing empathy at higher levels of helping. Moreover, person-centered analyses showed that people experienced higher state-level empathic concern in months in which they provided moderate amounts of help relative to when they helped less than their usual amount of helping. Results suggest people can cultivate empathy through helping, but only for those who help a moderate amount.

Cooperation is ubiquitous across biological and social systems and is critical for the healthy functioning of those systems. Across these many systems, cooperation is underlain by a set of shared principles. Here we give an overview of those principles and describe diverse systems in which cooperation is manifested, from genomes to human groups to human-machine interactions. We also offer a universal language that can be used to talk about cooperation across systems and, thus, facilitate interdisciplinary research and collaborations. By creating a shared understanding and common language for cooperation science, we will be better able to identify and address key issues in the study of cooperation in each of the realms in which it occurs.

Friendship is a unique and underexplored area of human sociality. Research suggests that humans have preferences for characteristics in their friends that maximize the benefits of these relationships. Yet, whereas more friends might increase friendship benefits, humans also have limited time, resources, and energy to invest in finding high-quality friends, making it likely that the nature of these preferences differs depending on the resources an individual has available to invest in this goal. Across two studies (total N = 693), we investigated how this trade-off may function by investigating the nature of friendship preferences. In Study 1, we utilized the budget paradigm method from behavioral economics to investigate the necessities and luxuries in friendship preferences. In Study 2, we replicated these preferences with a novel method and extended our investigation into understanding the hierarchical nature of these preferences. Taken together, our results provide a promising starting point for research investigating trade-offs between necessities and luxuries in friendship preferences.

People make friends for a variety of reasons. The current study aimed to explore these reasons and the role of the Dark Triad in predicting them, using self-report questionnaires in a sample drawn from 12 countries. We found that the most important reasons for making friends were having people around with desirable traits such as compatibility, who could provide support and be relied upon in times of need, and with whom one could frequently socialize. Less important reasons included achieving self-serving goals such as career advancement and securing an intimate partner. We found small effects of age, sex, and relationship status. Furthermore, higher scores in Machiavellianism and Narcissism were associated with a greater emphasis on all reasons, particularly on achieving self-serving goals. Conversely, higher Psychopathy scores were associated with a lower importance placed on all reasons except mating, where they were associated with higher importance.

Did the COVID-19 pandemic bring people together or push them apart? While infectious diseases tend to push people apart, crises can also bring people together through positive interdependence. We studied this question by asking an international sample (N = 1,006) about their inclinations to cooperate, perceptions of interdependence (i.e., shared fate), and perceived risk as well as local prevalence of COVID-19 infection across 14 time points from March to August, 2020. While perceived interdependence with others tended to increase during this time period, inclinations to cooperate decreased over time. At the within-person level, higher local prevalence of COVID-19 attenuated increases in perceived interdependence with others, and was associated with lower inclinations to cooperate. At the between-person level, people with high perceived interdependence with others reported more stable, or increasing, inclinations to cooperate over time than people with low perceived interdependence. Establishing a high sense of perceived interdependence with others may thus allow people to maintain cooperation during crises, even in the face of challenging circumstances such as those posed by a highly transmissible virus.

A common trope in popular media is that men who are having romantic affairs tend to spend more lavishly on their extrapair partner than their inpair partner. However, empirical literature supporting this gift-giving phenomenon is limited. From an evolutionary perspective, willingness and ability to provide resources are highly desirable traits in male partners. In addition, the reputational and physical risks of engaging in extrapair relationships are greater for women than men. This suggests that female extrapair partners may place an even greater value on partners who provide robust resources, and men's behavior may be attuned to this reality. Across three studies (N = 523), the current research explored men's and women's hypothetical, actual, and stereotyped gift-giving preferences for extrapair and inpair partners. Contrary to predictions-and participants' own reported stereotypes-results revealed no significant differences between men's and women's hypothetical gift expenditures, as well as significantly lower hypothetical spending on extrapair as compared to inpair relationships overall. The present research suggests that gift giving may help signal a long-term commitment to existing inpair partners more so than serving as an enticement for prospective, or existing, extrapair partners.

Understanding how genetics influences human psychology is something that the evolutionary sciences emphasize. However, the functions of complex genetic influences on behavior have been overlooked in favor of perspectives that posit unitary influences of genes on behavior. One such example is the belief that human growth, development, and behavior are influenced uniformly by their genes even though previous research has highlighted the genetic conflict endemic in these domains. Although much psychological research has robustly documented areas in which we see the footprints of genetic conflict in human behavior, these areas are referred to by different names that prevent researchers from making connections under a unifying framework. In this article, I outline what genetic conflict is and how genetic conflict can provide a unifying framework for psychological investigations of social relationships. I also discuss avenues for future research on genetic conflict in humans and the importance of considering cultural, ecological, and other developmental factors when researching the genetic influences on human behavior.

Assessing interdependence between self and specific others has a rich history in psychological science. Here, we report a novel scale to measure Shared Fate. Studies 1 and 2 (N1 = 198, N2 = 216) show that the Shared Fate scale has two factors assessing perceived shared fate and emotional shared fate with a target, has good reliability (Cronbach’s alpha = 0.81 to 0.91; McDonald’s omega = 0.89 to 0.96), and predicts participants’ willingness to help interdependent others. Studies 3 and 4 (N3 = 695, N4 = 629) indicate that the Shared Fate scale has good discriminant, convergent, concurrent, and predictive validity as well as test-retest reliability across a 14-day period. Taken together, our results suggest that Shared Fate scale is a useful instrument for work on cooperation, interdependence, and social behavior. However, there are still many open questions about the cognitive architecture underlying perceptions of shared fate and how shared fate interacts with genetic relatedness and other sources of fitness interdependence.

The COVID-19 pandemic drastically changed the ways in which social interactions happened. This is particularly true for valued social relationships such as friendships. Friendships provide many physical and mental health benefits, including buffering against loneliness during the pandemic, so contact and interactions with friends may have become more important during the pandemic. Additionally, an additional benefit of friendships is that they may have provided individuals with the ability to reduce their uncertainty and lower their risks during the pandemic. But, if individuals or their friends are not being careful about their risky behavior during this time, this could lead to conflict within these valued relationships. In this study, we investigated how COVID-19, demographics, and preferences for risk management influenced participants’ perceptions of their friendship during the pandemic. We found that participants who were more concerned about contracting COVID-19 and more likely to use risk retention reported having more conflict with their friends during the pandemic. These results suggest that there may be individual differences in risk tolerance of contracting COVID-19 and these differences fundamentally influence social relationships, loneliness, or social connectedness during the pandemic.

Until recently, few researchers have closely investigated women’s intrasexual competition. A meta-analysis of 14 published studies with 61 effect sizes (N = 2,100) assessed the size of the effect of women’s competition. I hypothesized that 1) exposure to competitive situations would cause women to behave more competitively than women not exposed to these situations and 2) age, domain of competition, and confidence in effect size coding would not moderate this effect. Results from the random effects meta-analysis indicated that women responded more competitively when shown high competition scenarios, r = 0.23, p < 0.001, 95% CI [.16, .31]. Additionally, age, domain of competition, and confidence in effect size coding did not moderate the effects of competition. Results suggest that there are small to medium-sized increases in women’s competition when primed with competition, but more studies are needed to fully understand women’s competition across domains.

After decades of research on the topic of reciprocity, there is still no consensus about the meaning of the term. Instead, there has been a proliferation of reciprocity terms with varied definitions, some of which overlap in ways that lead to confusion for scholars studying cooperation. In this paper, we provide a summary of 34 reciprocity terms and their definitions from across a variety of disciplines. We then report the results of a survey of cooperation experts spanning biology, anthropology, economics, sociology, and psychology (N = 85) about the extent to which they consider 30 of these definitions of reciprocity to be truly reciprocity. Experts also rated the extent to which they considered seventeen hypothetical scenarios to be examples of reciprocity. We used exploratory factor analysis and found that responses clustered around four categories of transfers: Balanced (e.g., Balanced reciprocity), Reputation-based (e.g., Generalized reciprocity), Debt-based (e.g., Calculated reciprocity), and Unconditional (e.g., Negative reciprocity). Although researchers agreed that the term reciprocity was useful and necessary, there was low agreement among scholars about what should be considered reciprocity. However, there was high agreement that unconditional transfers, which are characterized by a lack of expectations of repayment, should not be considered reciprocity. We propose that scholars of cooperation consider using these four categories when referring to cooperative transfers rather than using reciprocity terms in order to facilitate communication across disciplines, resolve issues related to ambiguous definitions of reciprocity, and provide a solution to the lack of consensus about what constitutes reciprocity.

Human sociality is governed by two types of social norms: injunctive norms, which prescribe what people ought to do, and descriptive norms, which reflect what people actually do. The process by which these norms emerge and their causal influences on cooperative behavior over time are not well understood. Here, we study these questions through social norms influencing mask wearing during the COVID-19 pandemic. Leveraging 2 years of data from the United States (18 time points; n = 915), we tracked mask wearing and perceived injunctive and descriptive mask wearing norms as the pandemic unfolded. Longitudinal trends suggested that norms and behavior were tightly coupled, changing quickly in response to public health recommendations. In addition, longitudinal modeling revealed that descriptive norms caused future increases in mask wearing across multiple waves of data collection. These cross-lagged causal effects of descriptive norms were large, even after controlling for non-social beliefs and demographic variables. Injunctive norms, by contrast, had less frequent and generally weaker causal effects on future mask wearing. During uncertain times, cooperative behavior is more strongly driven by what others are actually doing, rather than what others think ought to be done.

Given the importance of friendships during challenging times and the mixed associations between personality traits and disease-related behaviors, we investigated the correlations between personality traits and perceptions of friendships during the COVID-19 pandemic. Data were collected as part of a longitudinal investigation of the correlations between the pandemic and various cooperative relationships. In this investigation, we found that agreeableness and neuroticism predicted participants being more concerned about COVID-19 and bothered by friends' risky behavior, and extraversion predicted enjoying helping friends during the pandemic. Our results suggest that personality differences are associated with how individuals cope with friends' risky behaviors during the COVID-19 pandemic.