The Alliance Hypothesis for Human Friendship

Background

Exploration of the cognitive systems underlying human friendship will be advanced by identifying the evolved functions these systems perform. Here we propose that human friendship is caused, in part, by cognitive mechanisms designed to assemble support groups for potential conflicts. We use game theory to identify computations about friends that can increase performance in multi-agent conflicts. This analysis suggests that people would benefit from: 1) ranking friends, 2) hiding friend-ranking, and 3) ranking friends according to their own position in partners'' rankings. These possible tactics motivate the hypotheses that people possess egocentric and allocentric representations of the social world, that people are motivated to conceal this information, and that egocentric friend-ranking is determined by allocentric representations of partners'' friend-rankings (more than others'' traits).

Methodology/Principal Findings

We report results from three studies that confirm predictions derived from the alliance hypothesis. Our main empirical finding, replicated in three studies, was that people''s rankings of their ten closest friends were predicted by their own perceived rank among their partners'' other friends. This relationship remained strong after controlling for a variety of factors such as perceived similarity, familiarity, and benefits.

Conclusions/Significance

Our results suggest that the alliance hypothesis merits further attention as a candidate explanation for human friendship.     

Non‐verbal mate retention behaviour in women and its relation to couple's relationship adjustment and satisfaction

The function of mate retention strategies is to preserve the bond between romantic partners and to prevent desertion, which is also why such behaviours are often elicited by the presence of a potential rival. Most existing studies on mate retention are based on self‐reports, which are prone to various biases. In this study, we observed non‐verbal behaviour of 47 long‐term romantic heterosexual couples during their interview with a female experimenter, whom the women in the couples may have perceived as a potential rival. We measured non‐verbal displays of mate retention tactics (direct guarding, intimacy inducement, appearance enhancement, and love and care display), as well as relationship maintenance strategies of openness and positivity. We expected that relationship quality as reported by both partners in the couple would be positively associated with ‘benefit‐provisioning’ mate retention behaviours and negatively associated with a ‘cost‐inflicting’ mate retention tactic (direct guarding). Relationship quality was assessed by Spanier's Dyadic Adjustment Scale using both the total score of dyadic adjustment and a Dyadic Satisfaction subscale. We found a significant association between both partners' dyadic adjustment and satisfaction and woman's appearance enhancement behaviours (such as self‐touch, hair flip and primp) in the presence of her partner and a potential rival. We also found a negative association between partner's dyadic adjustment and woman's direct guarding of partner using short looks, and a positive association between woman's intimacy‐inducing behaviour using long gaze and her own dyadic satisfaction and her partner's dyadic adjustment. Our results highlight the specific role of female attractiveness in later stages of romantic relationships within the context of mate retention and draw attention to the significance of the use of observational techniques in addition to self‐reports.     

Friends without benefits: When we react negatively to helpful and generous friends

Being able to identify reliable friends and allies is key to surviving and thriving in the social world. Many cooperative accounts of friendship argue that people select friends based on how helpful and generous they are. While people certainly like helpful and generous others, here we explore a context in which people might respond negatively to a friend being prosocial: When one's friend is more helpful or generous toward another friend. We argue that such preferential prosociality prompts negative reactions, even when the alternative is a friend being less prosocial overall, because giving preferentially to another friend may be viewed as a threat of potential displacement of one's own friendship. In four studies (N?=?702), we predict and find that people respond negatively to a friend who was more helpful (Studies 1–2) and generous (Studies 3–4), preferring instead that a friend be less helpful and generous overall. Importantly, this preferential prosociality was viewed as particularly negative when the recipient was another friend and was seen as much less negative when the recipient was a relative (Study 1 and 4) or a romantic partner (Study 2). We discuss the implications of these results for cooperation and alliance-based accounts of friendship.     

What’s in a Friendship? Partner Visibility Supports Cognitive Collaboration between Friends

Not all cognitive collaborations are equally effective. We tested whether friendship and communication influenced collaborative efficiency by randomly assigning participants to complete a cognitive task with a friend or non-friend, while visible to their partner or separated by a partition. Collaborative efficiency was indexed by comparing each pair’s performance to an optimal individual performance model of the same two people. The outcome was a strong interaction between friendship and partner visibility. Friends collaborated more efficiently than non-friends when visible to one another, but a partition that prevented pair members from seeing one another reduced the collaborative efficiency of friends and non-friends to a similar lower level. Secondary measures suggested that verbal communication differences, but not psychophysiological arousal, contributed to these effects. Analysis of covariance indicated that females contributed more than males to overall levels of collaboration, but that the interaction of friendship and visibility was independent of that effect. These findings highlight the critical role of partner visibility in the collaborative success of friends.     

Sex differences in friendship preferences

Friendships can help us solve a number of challenges, increasing our welfare and fitness. Across evolutionary time, some of the many challenges that friendships helped to solve may have differed between men and women. By considering the specific and potentially distinct recurrent problems men's and women's friendships helped them solve, we can derive predictions about the qualities that would have made men's and women's same-sex friends ideal partners. This logic leads to several predictions about the specific friend preferences that may be differentially prized by men and women. Across three studies (N = 745) with U.S. participants—assessing ideal hypothetical friends, actual friends, and using a paradigm adapted from behavioral economics—we find that men, compared to women, more highly value same-sex friends who are physically formidable, possess high status, possess wealth, and afford access to potential mates. In contrast, women, compared to men, more highly value friends who provide emotional support, intimacy, and useful social information. Findings suggest that the specific friendship qualities men and women preferred differed by sex in ways consistent with a functional account of friendship.     

Reciprocity and communication of partner quality

In a cooperative exchange, the size of a partner''s contribution is likely to depend both on the partner''s ability to supply help and on the partner''s need for help in return. Referring to such needs and abilities as aspects of partner quality, it follows that variation in the amount of help offered in a relationship could transmit information about partner quality. A plausible behaviour might then be to vary the investment in a partner according to available information about partner quality and to invest little in a partner who offers little in return. Thus, regulation of a relationship through communication of partner quality would tend to follow the principle of reciprocity. In an analysis of an iterated game where players have private information about their needs and abilities, I verify this possibility by describing an evolutionarily stable state space strategy, referred to as ''state-dependent reciprocity'', entailing communication of partner quality. Although the evolution of cooperation has been studied in great detail, there has been no previous analysis of communication of needs and abilities in a relationship. It may well be that such communication is of major importance for the evolution of cooperative behaviour in nature.     

Male sexual attractiveness predicts differential ovulatory shifts in female extra-pair attraction and male mate retention

Because ancestral women faced trade-offs in choosing mates, they may have evolved to pursue a dual-mating strategy in which they secured investment through one partner and obtained good genes through others. The dual-mating theory predicts that women will display greater interest in extra-pair sex near ovulation, especially if they are mated to a primary male partner who is low in sexual attractiveness. Forty-three normally ovulating women rated their partner's sexual attractiveness and separately reported their own desires and their partner's mate retention behaviors at high and low fertility (confirmed using luteinizing hormone tests). In the high-fertility session relative to the low, women who assessed their partners as being lower in sexual attractiveness reported greater extra-pair desires and more expressed love and attention from their male partners. Women's desire for their own partners did not differ significantly between high and low-fertility sessions.     

Sex (similarities and) differences in friendship jealousy

Friendships provide material benefits, bolster health, and may help solve adaptive challenges. However, a recurrent obstacle to sustaining those friendships—and thus enjoying many friendship-mediated fitness benefits—is interference from other people. Friendship jealousy may be well-designed for helping both men and women meet the recurrent, adaptive challenge of retaining friends in the face of such third-party interference. Although we thus expect several sex similarities in the general cognitive architecture of friendship jealousy (e.g., it is attuned to friend value), there are also sex differences in friendship structures and historical functions, which might influence the inputs of friendship jealousy (e.g., the value of any one friendship). If so, we should also expect some sex differences in friendship jealousy. Findings from a reanalysis of previously-published data and a new experiment, including both U.S. student and adult community participants (N = 993), provide initial support for three predicted sex differences: women (versus men) report greater friendship jealousy at the prospective loss of best friends to others, men (versus women) report greater friendship jealousy at the prospective loss of acquaintances to others, and men's (but not women's) friendship jealousy is enhanced in the context of intergroup contests.     

Economic Profits Enhance Trust,Perceived Integrity and Memory of Fairness in Interpersonal Judgment

Does money lead to trust in personality and intention of others? Humans have a strong tendency to judge the intention of others from their sequent behaviors. In general, people trust others who behave fairly, but not always. Here we show that judgments of both intentional aspects and memory of intentional behavior are automatically influenced by unintentional benefits from the behaviors of others. We conducted a reward-manipulated and repeated trust game by using real participants interacting with moving image partners on a computer screen. The participants assessed likability, trustworthiness, and perceived integrity of the partners in pre- and post-game questionnaires. The results of judgments of all three dimensions and the memory of frequency of each partner''s fair behavior (sharing) were strongly influenced by profitability in the trust game, even though all partners shared 75% of the profit and participants were told that profitability was randomly assigned to each partner. Furthermore, these effects were moderated by the gender of the participants: males were more sensitive to monetary profits than were females. The results reveal that humans automatically trust, approve the integrity of, and recall well the fair behavior of others who provide affectively positive outcomes such as monetary profits. We call this phenomenon the “affect ripple effect”.     

Women’s sexual strategies in pregnancy

Humans exhibit an unusual pattern of sexual behavior compared to other mammalian females. Women's extended sexuality has been hypothesized to be related to a variety of possible benefits, especially non-genetic reproductive benefits, such as securing male investment via reinforced pairbonds or paternity confusion. But sexual behavior also comes at a cost, particularly for pregnant women, in terms of energetic costs, potential disease, and possible harm to the fetus. We hypothesize, therefore, that sexual behavior in pregnant women should reflect adaptive strategies and that pregnant women will be particularly strategic about their sexual behavior in order to maximize potential benefits while minimizing potential costs. One hundred twelve pregnant women completed a survey of their partners' qualities and their sexual desires toward their primary partners and men other than their primary partners. Results showed that women's perceptions of relationship threat positively predicted sexual desire for primary partners, while their perceptions of their partner's investing qualities negatively predicted sexual desire for extra-pair mates. These qualities, as well as cues to partner's genetic quality and gestation age, also interacted in ways that suggest that pregnant women's sexual desires are sensitive to cues of future investment and relationship stability.     

Sexual jealousy as a facultative trait: Evidence from the pattern of sex differences in adults from China and the United States

Across two studies, 716 and 308 undergraduate students from the United States and mainland China, respectively, were administered a series of measures on jealousy, emotional responses to partner infidelity, family background, and personality. Across both studies for the U.S. and Chinese samples, a higher proportion of males than females reported more distress to a partner's imagined sexual infidelity than to emotional infidelity, whereas a higher proportion of females than males reported more distress to a partner's emotional infidelity than to sexual infidelity, consistent with theoretical expectations and previous empirical research. However, a much higher proportion of U.S. males and females reported more distress to sexual infidelity than their same-sex Chinese peers, suggesting that the tendency toward sexual jealousy might be facultatively influenced by sexual permissiveness in the general culture. The overall pattern of results is considered in terms of individual and contextual differences in the expression of jealousy, as well as in terms of the emotional and behavioral responses associated with jealousy reactions.     

Distinguishing Family from Friends

Kinship and friendship are key human relationships. Increasingly, data suggest that people are not less altruistic toward friends than close kin. Some accounts suggest that psychologically we do not distinguish between them; countering this is evidence that kinship provides a unique explanatory factor. Using the Implicit Association Test, we examined how people implicitly think about close friends versus close kin in three contexts. In Experiment 1, we examined generic attitudinal dispositions toward friends and family. In Experiment 2, attitude similarity as a marker of family and friends was examined, and in Experiments 3 and 4, strength of in-group membership for family and friends was examined. Findings show that differences exist in implicit cognitive associations toward family and friends. There is some evidence that people hold more positive general dispositions toward friends, associate attitude similarity more with friends, consider family as more representative of the in-group than friends, but see friends as more in-group than distant kin.     

Paying attention pays off: Kea improve in loose-string cooperation by attending to partner

Previous studies showed that kea are able to cooperate in experiments based on the loose-string paradigm, but success rates were low, except when tested in stable dyads. We trained kea with low success rates to attend to the handling of the string by a human partner. This vastly improved subsequent coordination during cooperation with kea partners. Furthermore, we tested the kea dyads with a delayed entry set-up (delays of 2, 4 or 6 s) and with two different lengths of string. Increasing length of delay and reducing string length had negative effects on the success rate. This suggests that a complete understanding of the actions was not present as the kea were not able to fully overcome factors that inhibit coordination. Lastly, we tested the three subjects showing the greatest ability to wait for the partner in a partner-choice paradigm: one subject in the central position could pull a loose string with one partner after another in the order it preferred. As a result, the second subjects had much longer waiting times than previously experienced. We could show that the central kea learned to visit both set-ups and that the second partner was able to wait for the partner to arrive before attempting to pull on the string, averaging twice as long as in the standard delay. Taken together, the results from this study highlight that cooperation in the loose-string paradigm is largely dependent on coordination of the subjects. While attention to the partner's actions greatly improved performance, the greater waiting times achieved in the final set-up also suggest that utilizing more ecologically relevant (subject is busy VS subject is held back) delays could further improve the performance of non-human subjects in the delayed loose-string paradigm.     

Human Love Styles and Attitudes toward Pets

Pets have a positive influence on humans’ physical and psychological wellbeing, as well as their close relationships. In general, people who hold a positive view of people in turn hold a favorable view of pets. Despite this, previous studies on the relationship between love toward people and love toward pets were inconclusive. In this study, we take a fresh look at the nexus of the domains of love and pets by exploring whether people’s views of love with a romantic partner are associated with attitudes toward their pets. To answer this question, we administered scales to assess human love styles and attitudes toward pets. Participants completed measures on their attitudes toward romantic love, romantic relationship satisfaction, amount of social support, self-esteem, and attitudes toward pets. Human love styles assessed include Eros (passionate), Ludus (game-playing), Storge (friendship), Pragma (practical), Mania (obsessive), and Agape (selfless). Factor analyses of the pet attitudes measure revealed two factors—positive and negative attitudes toward pets. An erotic love style was linked to the endorsement of favorable pet attitudes, while a ludic love style was associated with the endorsement of unfavorable pet attitudes. Subsequent gender-specific correlations indicated that favorable pet attitudes were associated with high levels of social support for men, but low levels of support for women. Future research might address whether pets are instrumental in the development of love styles, whether love styles change after adoption of a pet, and if attitudes toward a romantic partner are similar to attitudes toward a particularly close companion animal.     

Qualities and Inequalities in Online Social Networks through the Lens of the Generalized Friendship Paradox

The friendship paradox is the phenomenon that in social networks, people on average have fewer friends than their friends do. The generalized friendship paradox is an extension to attributes other than the number of friends. The friendship paradox and its generalized version have gathered recent attention due to the information they provide about network structure and local inequalities. In this paper, we propose several measures of nodal qualities which capture different aspects of their activities and influence in online social networks. Using these measures we analyse the prevalence of the generalized friendship paradox over Twitter and we report high levels of prevalence (up to over 90% of nodes). We contend that this prevalence of the friendship paradox and its generalized version arise because of the hierarchical nature of the connections in the network. This hierarchy is nested as opposed to being star-like. We conclude that these paradoxes are collective phenomena not created merely by a minority of well-connected or high-attribute nodes. Moreover, our results show that a large fraction of individuals can experience the generalized friendship paradox even in the absence of a significant correlation between degrees and attributes.     

First tests of Euclidean preference integration in friendship: Euclidean friend value and power of choice on the friend market

Close friendships are associated with greater happiness and improved health; historically, they would likely have provided beneficial fitness outcomes. Yet each friendship requires one's finite time and resources to develop and maintain. Because people can maintain only so many close relationships, including friendships, at any one time, choosing which prospective friends to pursue and invest in is likely to have been a recurrent adaptive problem. Moreover, not all friends are created equal; some might be kind but unintelligent, some intelligent but disloyal, and so on. How might people integrate their friend preferences to make friend choices? Work using a Euclidean model of mate preferences has had significant success in elucidating this integration challenge in the domain of mating. Here, we apply this model to the domain of friendship, specifically exploring same-sex best and close friendships. We test and find some support for several critical predictions derived from a Euclidean integration hypothesis: People with higher Euclidean friend value (a) have best friends who better fulfill their best friend preferences, (b) have higher friend-value ideal best friends, and (c) have higher friend-value actual best friends. We also (d) replicate existing similar findings with regard to mating and (e) additionally provide a first test of whether people's Euclidean friend value (versus mate value) is a better predictor of their friend outcomes, and vice versa, finding some, albeit mixed, support for the dissocialbility of these constructs.     

Birth order and relationships

Previous studies (Salmon 1999; Salmon and Daly 1998) have found that sex and birth order are strong predictors of familial sentiments. Middleborns tend to be less family-oriented than firstborns or lastborns, while sex differences seem to focus on the utility of kin in certain domains. If this is a reflection of middleborns receiving a lesser degree of support from kin (particularly in terms of parental investment), are middleborns turning to reciprocal alliances outside the family, becoming friendship specialists? Are there comparable birth order differences with respect to mating strategies? In this study, the impact of birth order on attitudes toward family, friends, and mating were examined. Two hundred and forty-five undergraduates completed a questionnaire relating to their attitudes toward friends and family as well as some aspects of mating behavior. Birth order did have a significant impact in several areas. Middleborns expressed more positive views toward friends and less positive opinions of family in general. They were less inclined to help family in need than firstborns or lastborns. Mating strategies also appeared to be influenced by birth order, most notably in the area of infidelity, with middleborns being the least likely birth order to cheat on a sexual partner.     

State‐dependent cooperation in burying beetles: parents adjust their contribution towards care based on both their own and their partner's size

Handicapping experiments on species with biparental care show that a focal parent increases its contribution when its partner is handicapped. Such results are interpreted as evidence for negotiation, whereby each parent adjusts its amount of care to that of its partner. However, it is currently unclear whether the focal parent responds to a change in its handicapped partner's behaviour or state. To address this gap, we conducted an experiment on the burying beetle Nicrophorus vespilloides where we first generated different‐sized males and females by varying the duration of larval development. We then used a 2 × 2 factorial design in which a small or large male was paired with a small or large female. Small females provided less direct care (food provisioning and interactions with larvae) than large females, and both males and females provided less direct care when paired with a small partner. Thus, the focal parent adjusted its contribution towards care based on both its own state and that of its partner. There was also evidence for negotiation between the two parents as the focal parent adjusted its contribution based on the amount of care by its partner. However, there was no evidence that negotiation accounted for how the focal parent responded to its partner's size. Our results have important implications for our understanding of biparental cooperation as they show that each parent adjusts its contribution not only based on the amount of care provided by its partner but also based on its own state and its partner's state.     

Keeping up with the Joneses: Interpersonal Prediction Errors and the Correlation of Behavior in a Tandem Sequential Choice Task

In many settings, copying, learning from or assigning value to group behavior is rational because such behavior can often act as a proxy for valuable returns. However, such herd behavior can also be pathologically misleading by coaxing individuals into behaviors that are otherwise irrational and it may be one source of the irrational behaviors underlying market bubbles and crashes. Using a two-person tandem investment game, we sought to examine the neural and behavioral responses of herd instincts in situations stripped of the incentive to be influenced by the choices of one''s partner. We show that the investments of the two subjects correlate over time if they are made aware of their partner''s choices even though these choices have no impact on either player''s earnings. We computed an “interpersonal prediction error”, the difference between the investment decisions of the two subjects after each choice. BOLD responses in the striatum, implicated in valuation and action selection, were highly correlated with this interpersonal prediction error. The revelation of the partner''s investment occurred after all useful information about the market had already been revealed. This effect was confirmed in two separate experiments where the impact of the time of revelation of the partner''s choice was tested at 2 seconds and 6 seconds after a subject''s choice; however, the effect was absent in a control condition with a computer partner. These findings strongly support the existence of mechanisms that drive correlated behavior even in contexts where there is no explicit advantage to do so.     

African elephants (Loxodonta africana) recognize visual attention from face and body orientation

How do animals determine when others are able and disposed to receive their communicative signals? In particular, it is futile to make a silent gesture when the intended audience cannot see it. Some non-human primates use the head and body orientation of their audience to infer visual attentiveness when signalling, but whether species relying less on visual information use such cues when producing visual signals is unknown. Here, we test whether African elephants (Loxodonta africana) are sensitive to the visual perspective of a human experimenter. We examined whether the frequency of gestures of head and trunk, produced to request food, was influenced by indications of an experimenter''s visual attention. Elephants signalled significantly more towards the experimenter when her face was oriented towards them, except when her body faced away from them. These results suggest that elephants understand the importance of visual attention for effective communication.