Is friendship akin to kinship?

Although unrelated friends are genetically equivalent to strangers, several lines of reasoning suggest that close friendship may sometimes activate processes more relevant to kinship and that this may be especially true for women. We compared responses to strangers, friends, and kin in two studies designed to address distinct domains for which kinship is known to have functional significance: incest avoidance and nepotism. Study 1 examined emotional responses to imagined sexual contact with kin, friends, and strangers. Results revealed that women, compared to men, treated friends more like kin. Study 2 examined benevolent attributions to actual kin, friends, and strangers. Results revealed that women treated friends very much like kin, whereas men treated friends very much like strangers. The current findings support a domain-specific over a domain-general approach to understanding intimate relationships and raise a number of interesting questions about the modular structure of cognitive and affective processes involved in these relationships.     

Do Birds of a Feather Flock Together?

Cooperation requires that individuals are able to identify, and preferentially associate with, others who have compatible preferences and the shared background knowledge needed to solve interpersonal coordination problems. The present study investigates the nature of such similarity within social networks, asking: What do friends have in common? And what is the relationship between similarity and altruism? The results show that similarity declines with frequency of contact; similarity in general is a significant predictor of altruism and emotional closeness; and, specifically, sharing a sense of humor, hobbies and interests, moral beliefs, and being from the same area are the best predictors. These results shed light on the structure of relationships within networks and provide a possible checklist for predicting attitudes toward strangers, and in-group identification.     

Hamilton's rule and kin competition: the Kipsigis case

Evolutionary studies of human behavior have emphasized the importance of kin selection in explaining social institutions and fitness outcomes. Our relatives can nevertheless be competitors as well as sources of altruism. This is particularly likely when there is local competition over resources, where conflict can lead to strife among nondispersing relatives, reducing or even negating the effects of relatedness on promoting altruism. Here, I present demographic data on a land-limited human population, utilizing large within-population variation in land ownership to determine the interactions between local resource competition and the benefits of kin in enhancing child survival, a key component of fitness in this population. As predicted, wealth affects the extent of kin altruism, in that paternal relatives (specifically father's brothers) appear to buffer young children from mortality much more effectively in rich than in poor households. This interaction effect is interpreted as evidence that the extent of nepotism among humans depends critically on resource availability. Further unanticipated evidence that maternal kin play a role in buffering children from mortality in situations where paternal kin control few resources speaks to the important role that specific local circumstance plays in shaping kin contributions to child welfare.     

Social dynamics of mammals: Reproductive success, kinship and individual fitness

Apparent altruism, in which an individual seemingly decreases its evolutionary fitness by assisting others, can confer benefits if the individual assists kin. Thus, an animal can increase its total or inclusive fitness by producing offspring (direct fitness) and/or helping kin to reproduce (indirect fitness). Although kin selection has been suggested as the mechanism underlying the formation of mammalian societies, many species act as if they attempt to maximize the direct fitness component of their inclusive fitness.     

Altruism among kin vs. nonkin: effects of cost of help and reciprocal exchange

Evolutionary principles suggest that there will be differences in the nature of altruism directed toward kin vs. nonkin. The present study sought to explore these differences. Participants were 295 undergraduate students who each completed a questionnaire about help exchanged with siblings, cousins, acquaintances or friends. For siblings, cousins and acquaintances, greater relatedness was associated with higher levels of helping. Friends were an exception, however, receiving as much or more help than kin. Consistent with an evolutionary analysis, as the cost of helping increased, kin received a larger share of the help given, whereas nonkin received a smaller share. For low-cost help, people helped friends more than siblings; for medium-cost help, they helped siblings and friends equally; and for high-cost help, they expressed a greater willingness to help siblings than friends. As expected, the level of reciprocal exchange was higher among acquaintances than among friends; however, there was also an unexpectedly high level of reciprocal exchange among kin.     

Altruism and fairness: Unnatural selection?

Darwin admitted that the evolution of moral phenomena such as altruism and fairness, which are usually in opposition to the maximization of individual reproductive success, was not easily accounted for by natural selection. Later, authors have proposed additional mechanisms, including kin selection, inclusive fitness, and reciprocal altruism. In the present work, we explore the extent to which sexual selection has played a role in the appearance of human moral traits. It has been suggested that because certain moral virtues, including altruism and kindness, are sexually attractive, their evolution could have been shaped by the process of sexual selection. Our review suggests that although it is possible that sexual selection played such a role, it is difficult to determine the extent of its relevance, the specific form of this influence, and its interplay with other evolutionary mechanisms.     

Culturing Mammalian Tissues for Cytogenetic Investigations

Biologists often tend to assume that natural selection acts fundamentally on individuals or on species. It is now appreciated that this is an oversimplification. Present understanding of the unit of natural selection, with its pivotal importance in evolution, largely derives from a consideration of the problem of why altruism exists. This paper reviews evidence for and against the theories that evolution acts essentially on genes, on individuals, on kin, or on larger groups.     

Kin recognition: Competition and cooperation in Impatiens (Balsaminaceae)

The ability to recognize kin is an important element in social behavior and can lead to the evolution of altruism. Recently, it has been shown that plants are capable of kin recognition through root interactions. Here we tested for kin recognition in a North American species of Impatiens that has a high opportunity of growing with kin and responds strongly to aboveground competition. We measured how the plants responded to the aboveground light quality cues of competition and to the presence of root neighbors and determined whether the responses depended on whether the neighbors were siblings or strangers. The study families were identified by DNA sequencing as members of the same species, provisionally identified as Impatiens pallida (hereafter I. cf. pallida). We found that I. cf. pallida plants were capable of kin recognition, but only in the presence of another plant's roots. Several traits responded to relatedness in shared pots, including increased leaf to root allocation with strangers and increased stem elongation and branchiness in response to kin, potentially indicating both increased competition toward strangers and reduced interference (cooperation) toward kin. Impatiens cf. pallida responded to both competition cues simultaneously, with the responses to the aboveground competition cue dependent on the presence of the belowground competition cue.     

Altruism, cheating, and anticheater adaptations in cellular slime molds

Cellular slime molds (CSMs) possess a remarkable life cycle that encompasses an extreme act of altruism. CSM cells live as individual amoebae until starved, then aggregate and ultimately transform themselves into a multicellular fruiting body. This fruiting body consists of stalk cells (altruists that eventually die) and spores (the beneficiaries of this sacrifice). Altruistic systems such as this are vulnerable to cheaters, which are individuals unrelated to the altruists that obtain the benefits provided by them without reciprocating. Here, we investigate two forces that can maintain CSM altruism despite cheating: kin selection and anticheater adaptations. First, we present new kinship-based models based on CSM developmental biology to evaluate the efficacy of kin selection. These models show that stalk-making genotypes can still be maintained when aggregations are initiated by multiple "founder" spores, provided that spores of stalkless fruiting bodies have low rates of dispersal and dispersal success is a concave function of stalk height. Second, we review proposals that several features of CSM development, such as the chemical suppression of the redifferentiation of prestalk cells into prespores, act as anticheater adaptations.     

Cooperative nest defence in red-winged blackbirds: reciprocal altruism,kinship or by-product mutualism?

Male red-winged blackbirds (Agelaius phoeniceus) often cooperate with their neighbours in defending nests against predators. Some studies have suggested that this is an example of by-product mutualism, whereas others have suggested the possibility of reciprocal altruism. No study has addressed the possibility of kin-selected cooperation in nest defence in this species. Reciprocal altruism, kin selection and by-product mutualism are not mutually exclusive alternatives, but few studies of territorial neighbours have tested for multiple mechanisms simultaneously. We test for these three possibilities in a population of red-winged blackbirds. We used simulated defections to test for reciprocal altruism. We used analysis of microsatellite loci to test for kin selection between adult male neighbours. We also used microsatellite loci to test for by-product mutualism resulting from nest defence of offspring sired on neighbouring territories. We found that male red-winged blackbirds cooperate in nest defence primarily as a form of reciprocal altruism. Experimental males reduced their level of nest defence relative to controls following simulated defection by a neighbour. In contrast to some earlier studies, we found no evidence for by-product mutualism: males did not defend nests where they had sired extra-pair offspring. We also found no evidence for kin selection: males were no more cooperative with more closely related neighbours. Considered alongside the results from other studies, our study suggests that mechanisms stabilizing cooperation in red-winged blackbirds may vary among populations.     

Evolution of eusociality by kin selection: the effect of inbreeding between siblings

The effect of sib-sib inbreeding on the evolution of eusocial altruism in Hymenoptera by kin selection is examined by computer simulations. Inbreeding has minor effects on the ratio of relatedness to siblings: relatedness to offspring, but this ratio remains approximately one no matter what the degree of inbreeding. This implies that although inbreeding increases relatedness to siblings, relatedness to offspring increases to the same degree. Hence, inbreeding does not make the evolution of altruism more likely. If all the brothers of (non-mating) altruists outbreed, thereby increasing the frequency of altruism alleles in the outbred fraction of the population especially at low gene frequency, then altruism can be promoted by inbreeding. However, this is an indirect advantage, not attributable to inbreeding per se.     

Human Beings as Evolved Nepotists

Inclusive fitness theory provides a compelling explanation for the evolution of altruism among kin. However, a completely satisfactory account of non-kin altruism is still lacking. The present study compared the level of altruism found among siblings with that found among friends and mates and sought to reconcile the findings with an evolutionary explanation for human altruism. Participants (163 males and 156 females) completed a questionnaire about help given to a sibling, friend, or mate. Overall, participants gave friends and mates as much or more help than they gave siblings. However, as the cost of help increased, siblings received a progressively larger share of the help, whereas friends and mates received a progressively smaller share, despite the fact that participants were closer emotionally to friends and mates than they were to siblings. These findings help to explain the relative standing of friends and mates as recipients of altruistic aid.

Reciprocal altruism in bats and other mammals

In this paper five conditions are specified which must be met before reciprocal altruism, rather than kin selection, should be invoked. Four purported mammalian examples— social grooming in coati, cluster position in roosting pallid bats, information exchange among greater spear-nosed bats, and blood regurgitation among vampire bats—are examined to determine if reciprocal altruism is necessary to plausibly explain each situation. Results from a computer simulation which apportions the relative selective advantage of vampire bat food sharing to kin selection and reciprocal altruism are then presented. The results demonstrate that the increase in individual survivorship due to reciprocal food sharing events in this species provides a greater increase in inclusive fitness than can be attributed to aiding relatives. This analysis suggests that reciprocal altruism can be selectively more important than kin selection when altruistic behaviors in a relatively large social group occur frequently and provide a major fitness benefit to the recipient even when that recipient is related to the donor.     

Evolution of indirect reciprocity by social information: the role of trust and reputation in evolution of altruism

The complexity of human's cooperative behavior cannot be fully explained by theories of kin selection and group selection. If reciprocal altruism is to provide an explanation for altruistic behavior, it would have to depart from direct reciprocity, which requires dyads of individuals to interact repeatedly. For indirect reciprocity to rationalize cooperation among genetically unrelated or even culturally dissimilar individuals, information about the reputation of individuals must be assessed and propagated in a population. Here, we propose a new framework for the evolution of indirect reciprocity by social information: information selectively retrieved from and propagated through dynamically evolving networks of friends and acquaintances. We show that for indirect reciprocity to be evolutionarily stable, the differential probability of trusting and helping a reputable individual over a disreputable individual, at a point in time, must exceed the cost-to-benefit ratio of the altruistic act. In other words, the benefit received by the trustworthy must out-weigh the cost of helping the untrustworthy.     

Altruism and reciprocity among friends and kin in a Tibetan village

To explain the high level of altruism among unrelated friends, some people propose that friends may apply the feeling and emotions that underlie kin-selected nepotism to each other. Alternatively, altruism among friends may be sustained by an evolved psychology that is sensitive to the dynamic of contingent reciprocity. A recent study (Stewart-Williams, S., 2007. Altruism among kin vs. nonkin: effects of cost of help and reciprocal exchange. Evolution and Human Behavior, 28, 193–198) with North American students showed that the amount of help given and received was less likely to be balanced among friends (as among relatives) than among acquaintances, suggesting that exchange among friends and relatives may rely on similar psychological mechanisms. To assess whether the similar pattern in exchange reflects the similar psychology in exchange, I tested how different types of relationship regulated the amount of help given and received, and people’s emotional reaction to non-reciprocation. I asked 45 participants from a Tibetan village in China to imagine how unhappy they would feel if the target person (sibling, cousin, friend or acquaintance) failed to reciprocate their help in various contexts. I found that overall, friends and relatives were more tolerant to non-reciprocation than acquaintances were. For emotional help, friends were more likely to feel unhappy about non-reciprocation than relatives were, but were similar to relatives in responses to other types of help (aid during crisis, labor, and financial help). The study suggests that people may evaluate the importance of reciprocity differently in various situations, and exhibit different levels of sensitivity to the dynamic of reciprocity. Thus it calls for careful distinctions between friends and kin in the everyday lives of individuals.     

Relatives growing together: pair density and kinship

Kin selection is often used to explain the evolution altruism towards relatives through favouring the evolution of kin recognition. However, it remains unclear whether kin recognition is affected by plant pair density and different degrees of relatedness. A two-factor experimental design of kinship (three kinship degrees including siblings, closely related strangers and distantly related strangers) and pair density (including relative small, medium and large pair densities) was conducted in this study. Plant competitive traits including rosette size, specific leaf area (SLA), stem elongation, root and leaf allocation, seed biomass and vegetative biomass were measured to reflect interactions among plants living with different relatives of Arabidopsis thaliana accessions [Columbia (Col-0), Landsberg erecta (Ler) and Wassileskijia (Ws)] in three different pair densities. The SLA only showed kinship effect, and siblings showed higher SLA than non-siblings in each pair density. The plant total biomass was only affected by pair density, which increased with decreases of pair density. The rosette size, stem elongation, root allocation and leaf allocation showed interactive effects of kinship and pair density. In the large pair density, the rosette size of siblings was lower than distantly related strangers, compared to closely related strangers; the stem elongation and root allocation were lower, while the seed biomass of siblings was higher than the closely or distantly related strangers. In the medium pair density, plants living with siblings or with closely related neighbours showed higher root allocation than with distantly related neighbours. In the small pair density, the plant rosette acted similarly to of which in the large pair density, and siblings showed higher root allocation than the two strangers. The other traits in each pair density showed no significant differences among kinship treatments. A relatively large pair density achieves kin recognition by deducing root competition ability and mutual shading, with increased efficient light capture and fitness. Similar such root and efficient light capture strategies were selected in medium pair density. Except the efficient light capture strategies, small pair density also displays allocation trade-offs between roots (decreased) and leaves (increased) for siblings. Moreover, kin responses on those attributes are also adjusted by kinship degree. Thus, kinships and pair density are important variables to mediate kin interactions.     

Kin selection is the key to altruism

Kin selection theory, also known as inclusive fitness theory, has been the subject of much debate and misunderstanding. Nevertheless, the idea that relatedness among individuals can drive the evolution of altruism has emerged as a central paradigm in evolutionary biology. Or has it? In two recent articles, E.O. Wilson argues that kin selection should no longer be considered the main explanation for the evolution of altruism in insect societies. Here, we discuss what these articles say about kin selection and how it relates to the theory. We conclude that kin selection remains the key explanation for the evolution of altruism in eusocial insects.     

Selective pressures for accurate altruism targeting: evidence from digital evolution for difficult-to-test aspects of inclusive fitness theory

Inclusive fitness theory predicts that natural selection will favour altruist genes that are more accurate in targeting altruism only to copies of themselves. In this paper, we provide evidence from digital evolution in support of this prediction by competing multiple altruist-targeting mechanisms that vary in their accuracy in determining whether a potential target for altruism carries a copy of the altruist gene. We compete altruism-targeting mechanisms based on (i) kinship (kin targeting), (ii) genetic similarity at a level greater than that expected of kin (similarity targeting), and (iii) perfect knowledge of the presence of an altruist gene (green beard targeting). Natural selection always favoured the most accurate targeting mechanism available. Our investigations also revealed that evolution did not increase the altruism level when all green beard altruists used the same phenotypic marker. The green beard altruism levels stably increased only when mutations that changed the altruism level also changed the marker (e.g. beard colour), such that beard colour reliably indicated the altruism level. For kin- and similarity-targeting mechanisms, we found that evolution was able to stably adjust altruism levels. Our results confirm that natural selection favours altruist genes that are increasingly accurate in targeting altruism to only their copies. Our work also emphasizes that the concept of targeting accuracy must include both the presence of an altruist gene and the level of altruism it produces.     

Kinship,Family, and Gender Effects in the Ultimatum Game

Kinship and reciprocity are two main predictors of altruism. The ultimatum game has been used to study altruism in many small-scale societies. We used the ultimatum game to examine effects of individuals’ family and kin relations on altruistic behavior in a kin-based horticultural community in rural Dominica. Results show sex-specific effects of kin on ultimatum game play. Average coefficient of relatedness to the village was negatively associated with women’s ultimatum game proposals and had little effect on men’s proposals. Number of brothers in the village was positively associated with men’s ultimatum game proposals and negatively associated with women’s proposals. Similarly, presence of father in the village was associated with higher proposals by men and lower proposals by women. We interpret the effect of brothers on men’s proposals as a consequence of local competition among brothers. We speculate that daughter-biased parental care in this community creates a sense of entitlement among women with brothers, which may explain the inverse relation between number of brothers and women’s ultimatum game proposals. The pattern of results may be consistent with how matrifocality affects cultural models of fairness differently along gender and family lines.     

Altruism via kin-selection strategies that rely on arbitrary tags with which they coevolve

Hamilton's rule explains when natural selection will favor altruism between conspecifics, given their degree of relatedness. In practice, indicators of relatedness (such as scent) coevolve with strategies based on these indicators, a fact not included in previous theories of kin recognition. Using a combination of simulation modeling and mathematical extension of Hamilton's rule, we demonstrate how altruism can emerge and be sustained in a coevolutionary setting where relatedness depends on an individual's social environment and varies from one locus to another. The results support a very general expectation of widespread, and not necessarily weak, conditional altruism in nature.