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Humans are characterized by an unusual level of prosociality. Despite this, considerable indirect evidence suggests that biological kinship plays an important role in altruistic behaviour. All previous reports of the influence of kin selection on human altruism have, however, used correlational (rather than experimental) designs, or imposed only a hypothetical or negligible time cost on participants. Since these research designs fail either to control for confounding variables or to meet the criteria required as a test of Hamilton's rule for kin selection (that the altruist pays a true cost), they fail to establish unequivocally whether kin selection plays a role. We show that individuals from two different cultures behave in accordance with Hamilton's rule by acting more altruistically (imposing a higher physical cost upon themselves) towards more closely related individuals. Three possible sources of confound were ruled out: generational effects, sexual attraction and reciprocity. Performance on the task however did not exhibit a perfect linear relationship with relatedness, which might reflect either the intrusion of other variables (e.g. cultural differences in the way kinship is costed) or that our behavioural measure is insufficiently sensitive to fine-tuned differences in the way individuals view their social world. These findings provide the first unequivocal experimental evidence that kinship plays a role in moderating altruistic behaviour. Kinship thus represents a baseline against which individuals pitch other criteria (including reciprocity, prosociality, obligation and a moral sense) when deciding how to behave towards others. [ABSTRACT FROM AUTHOR]

Competing Interests: Declaration of competing interest The author declares no conflicts of interest.

Healthy social life requires relationships in different levels of personal closeness. Based on ethological, sociological, and psychological evidence, social networks have been divided into five layers, gradually increasing in size and decreasing in personal closeness. Is this division also reflected in brain processing of social networks? During functional MRI, 21 participants compared their personal closeness to different individuals. We examined the brain volume showing differential activation for varying layers of closeness and found that a disproportionately large portion of this volume (80%) exhibited preference for individuals closest to participants, while separate brain regions showed preference for all other layers. Moreover, this bipartition reflected cortical preference for different sizes of physical spaces, as well as distinct subsystems of the default mode network. Our results support a division of the neurocognitive processing of social networks into two patterns depending on personal closeness, reflecting the unique role intimately close individuals play in our social lives., (© 2024. The Author(s).)

Human communities have self-organizing properties in which specific Dunbar Numbers may be invoked to explain group attachments. By analysing Wikipedia editing histories across a wide range of subject pages, we show that there is an emergent coherence in the size of transient groups formed to edit the content of subject texts, with two peaks averaging at around N = 8 for the size corresponding to maximal contention, and at around N = 4 as a regular team. These values are consistent with the observed sizes of conversational groups, as well as the hierarchical structuring of Dunbar graphs. We use a model of bipartite trust to derive a scaling law that fits the data and may apply to all group size distributions when these are based on attraction to a seeded group process. In addition to providing further evidence that even spontaneous communities of strangers are self-organizing, the results have important implications for the governance of the Wikipedia commons and for the security of all online social platforms and associations., Competing Interests: We declare we have no competing interests., (© 2024 The Author(s).)

While social networks are typically relatively stable in size over time, major changes in life circumstances can result in opportunities to acquire new friends. How young adults manage their relationships with their wider network of friends and family during such transitions is, however, not well understood. Using a prospective longitudinal design, we investigate changes in the size and composition of complete egocentric networks of two cohorts of young adults moving away from home to college. We show that, although networks grow rapidly due to an influx of new friends made at college, the social overload that would result is partially mitigated through the progressive loss of pre-transition friendships (but not family relationships). In addition, most of the new relationships are placed in the outermost, emotionally less close network layers that are less costly to maintain. In contrast, the more intimate inner layers of the network remain stable in size, with efforts being made to conserve these relationships. The overriding importance of face-to-face interaction in creating and maintaining ties (compared to digital media) results in the emotional quality of a tie being traded off against the constraints imposed by physical distance. The most reliable predictor of the proportion of original members with whom relationships were maintained post-transition was pre-transition network size, with weaker effects due to geographical proximity and personal popularity in the new social context. These findings have implications for managing transitions to a new environment at any life stage., Competing Interests: Declaration of Competing Interest none., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Group-living creates stresses that, all else equal, naturally lead to group fragmentation, and hence loss of the benefits that group-living provides. How species that live in large stable groups counteract these forces is not well understood. I use comparative data on grooming networks and cognitive abilities in primates to show that living in large, stable groups has involved a series of structural solutions designed to create chains of 'friendship' (friends-of-friends effects), increased investment in bonding behaviours (made possible by dietary adjustments) to ensure that coalitions work effectively, and neuronally expensive cognitive skills of the kind known to underpin social relationships in humans. The first ensures that individuals synchronise their activity cycles; the second allows the stresses created by group-living to be defused; and the third allows a large number of weak ties to be managed. Between them, these create a form of multilevel sociality based on strong versus weak ties similar to that found in human social networks. In primates, these strategies appear successively at quite specific group sizes, suggesting that they are solutions to 'glass ceilings' that would otherwise limit the range of group sizes that animals can live in (and hence the habitats they can occupy). This sequence maps closely onto the grades now known to underpin the Social Brain Hypothesis and the fractal pattern that is known to optimise information flow round networks.

Objective: In a UK national census sample, women from the upper and lower socioeconomic (SES) classes achieve parity in completed family size, despite marked differences in both birth rates and offspring survival rates. We test the hypothesis that women adopt reproductive strategies that manipulate age at first reproduction to achieve this., Methods: We use a Monte-Carlo modeling approach parameterized with current UK lifehistory data to simulate the reproductive lifehistories of 64,000 individuals from different SES classes, with parameter values at each successive time step drawn from a statistical distribution defined by the census data., Results: We show that, if they are to achieve parity with women in the higher socioeconomic classes, women in lower socioeconomic classes must begin reproducing 5.65 years earlier on average than women in the higher SES classes in order to offset the higher class-specific mortality and infertility rates that they experience. The model predicts very closely the observed differences in age at first reproduction in the census data., Conclusions: Opting to delay reproduction in order to purse an education-based professional career may be a high risk strategy that many lower SES women are unwilling and unable to pursue. As a result, reproducing as early as possible may be the best strategy available to them., Competing Interests: Conflict of InterestThe authors declare that they have no conflict of interest., (© The Author(s) 2024.)

Music is widely recognised as a human universal, yet there is no agreed explanation for its function, or why and when it evolved. I summarise experimental evidence that the primary function of musicking lies in social bonding, both at the dyadic and community levels, via the effect that performing any form of music has on the brain's endorphin system (the principal neurohormonal basis for social bonding in primates). The many other functions associated with music-making (mate choice, pleasure, coalition signalling, etc) are all better understood as derivative of this, either as secondary selection pressures or as windows of evolutionary opportunity (exaptations). If music's function is primarily as an adjunct of the social bonding mechanism (a feature it shares with laughter, feasting, storytelling and the rituals of religion), then reverse engineering the problem suggests that the capacity for music-making most likely evolved with the appearance of archaic humans. This agrees well with anatomical evidence for the capacity to sing., Competing Interests: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Dunbar.)

The personal network of relationships is structured in circles of friendships, that go from the most intense relationships to the least intense ones. While this is a well established result, little is known about the stability of those circles and their evolution in time. To shed light on this issue, we study the temporal evolution of friendships among teenagers during two consecutive academic years by means of a survey administered on five occasions. We show that the first two circles, best friends and friends, can be clearly observed in the survey but also that being in one or the other leads to more or less stable relationships. We find that being in the same class is one of the key drivers of friendship evolution. We also observe an almost constant degree of reciprocity in the relationships, around 60%, a percentage influenced both by being in the same class and by gender homophily. Not only do our results confirm the mounting evidence supporting the circle structure of human social networks, but they also show that these structures persist in time despite the turnover of individual relationships-a fact that may prove particularly useful for understanding the social environment in middle schools., (© 2023. Springer Nature Limited.)

Comparative analyses are the backbone of evolutionary analysis. However, their record in producing a consensus has not always been good. This is especially true of attempts to understand the factors responsible for the evolution of large brains, which have been embroiled in an increasingly polarised debate over the past three decades. We argue that most of these disputes arise from a number of conceptual errors and associated logical fallacies that are the result of a failure to adopt a biological systems-based approach to hypothesis-testing. We identify four principal classes of error: a failure to heed Tinbergen's Four Questions when testing biological hypotheses, misapplying Dobzhansky's Dictum when testing hypotheses of evolutionary adaptation, poorly chosen behavioural proxies for underlying hypotheses, and the use of inappropriate statistical methods. In the interests of progress, we urge a more careful and considered approach to comparative analyses, and the adoption of a broader, rather than a narrower, taxonomic perspective., (© 2023 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.)

Doctrinal religions that involve recognised gods, more formal theologies, moral codes, dedicated religious spaces and professional priesthoods emerged in two phases during the Neolithic. Almost all of these appeared in a narrow latitudinal band (the northern Subtropical Zone). I suggest that these developments were the result of a need to facilitate community bonding in response to scalar stresses that developed as community sizes increased dramatically beyond those typical of hunter-gatherer societies. Conditions for population growth (as indexed by rainfall patterns and the difference between pathogen load and the length of the growing season) were uniquely optimised in this zone, creating an environment of ecological release in which populations could grow unusually rapidly. The relationship between latitude, religion and language in contemporary societies suggests that the peculiar characteristics of the northern (but not the southern) Subtropical Zone were especially favourable for the evolution of large scale religions as a way of enforcing community cohesion., Competing Interests: The author declares no conflicts of interest., (© The Author(s) 2023.)

In contrast to long-term relationships, far less is known about the temporal evolution of transient relationships, although these constitute a substantial fraction of people's communication networks. Previous literature suggests that ratings of relationship emotional intensity decay gradually until the relationship ends. Using mobile phone data from three countries (US, UK, and Italy), we demonstrate that the volume of communication between ego and its transient alters does not display such a systematic decay, instead showing a lack of any dominant trends. This means that the communication volume of egos to groups of similar transient alters is stable. We show that alters with longer lifetimes in ego's network receive more calls, with the lifetime of the relationship being predictable from call volume within the first few weeks of first contact. This is observed across all three countries, which include samples of egos at different life stages. The relation between early call volume and lifetime is consistent with the suggestion that individuals initially engage with a new alter so as to evaluate their potential as a tie in terms of homophily., (© 2023. The Author(s).)

Seminal studies suggest that being mimicked increases experienced social closeness and prosocial behavior to a mimicking confederate (i.e., interaction partner). Here we reexamine these results by considering the role of empathy-related traits, an indirect proxy for endorphin uptake, and their combined effects as an explanation for these results. 180 female participants were mimicked or anti-mimicked in an interaction with a confederate. The effects of being mimicked versus anti-mimicked in relation to empathy-related traits and endorphin release (assessed indirectly via pain tolerance) on experienced closeness and prosocial behavior were assessed using Bayesian analyses. Our results suggest that high individual empathy-related traits increase social closeness to the anti-mimicking and mimicking confederate and to one's romantic partner, as compared to mimicry alone. Results furthermore strongly suggest that high individual empathy-related traits increase prosocial behavior (donations and willingness to help) as compared to mimicry alone. These findings extend previous work by highlighting that empathy-related traits are more influential in creating positive effects on social closeness and prosocial behavior than a one-shot mimicking encounter., (© 2023. The Author(s).)

The relationship between social behaviour and the microbiome is known to be reciprocal. Research in wild animal populations, particularly in primate social groups, has revealed the role that social interactions play in microbial transmission, whilst studies in laboratory animals have demonstrated that the gut microbiome can affect multiple aspects of behaviour, including social behaviour. Here we explore behavioural variation in a non-captive animal population with respect to the abundance of specific bacterial genera. Social behaviour based on grooming interactions is assessed in a population of rhesus macaques ( Macaca mulatta ), and combined with gut microbiome data. We focus our analyses on microbiome genera previously linked to sociability and autistic behaviours in rodents and humans. We show in this macaque population that some of these genera are also related to an individual's propensity to engage in social interactions. Interestingly, we find that several of the genera positively related to sociability, such as Faecalibacterium , are well known for their beneficial effects on health and their anti-inflammatory properties. In contrast, the genus Streptococcus , which includes pathogenic species, is more abundant in less sociable macaques. Our results indicate that microorganisms whose abundance varies with individual social behaviour also have functional links to host immune status. Overall, these findings highlight the connections between social behaviour, microbiome composition, and health in an animal population., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Johnson, Watson, Dunbar and Burnet.)

In anthropoid primates, social grooming is the principal mechanism (mediated by the central nervous system endorphin system) that underpins social bonding. However, the time available for social grooming is limited, and this imposes an upper limit on the size of group that can be bonded in this way. I suggest that, when hominins needed to increase the size of their groups beyond the limit that could be bonded by grooming, they co-opted laughter (a modified version of the play vocalization found widely among the catarrhine primates) as a form of chorusing to fill the gap. I show, first, that human laughter both upregulates the brain's endorphin system and increases the sense of bonding between those who laugh together. I then use a reverse engineering approach to model group sizes and grooming time requirements for fossil hominin species to search for pinch points where a phase shift in bonding mechanisms might have occurred. The results suggest that the most likely time for the origin of human-like laughter is the appearance of the genus Homo ca 2.5 Ma. This article is part of the theme issue 'Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience'.

Intense sociality has been a catalyst for human culture and civilization, and our social relationships at a personal level play a pivotal role in our health and well-being. These relationships are, however, sensitive to the time we invest in them. To understand how and why this should be, we first outline the evolutionary background in primate sociality from which our human social world has emerged. We then review defining features of that human sociality, putting forward a framework within which one can understand the consequences of mass social isolation during the COVID-19 pandemic, including mental health deterioration, stress, sleep disturbance and substance misuse. We outline recent research on the neural basis of prolonged social isolation, highlighting especially higher-order neural circuits such as the default mode network. Our survey of studies covers the negative effects of prolonged social deprivation and the multifaceted drivers of day-to-day pandemic experiences., (© 2022. Springer Nature Limited.)

Primates live in stable social groups in which they form differentiated relationships with group members and use a range of communication including facial expressions, vocalizations and gestures. However, how these different types of communication are used to regulate social interactions, and what cognitive skills underpin this communication, is still unclear. The aim of this special issue is to examine the types of cognitive skills underpinning the flexible and complex communication that is used to maintain the bonded social relationships found in primates and humans. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Characterizing non-human primate social complexity and its cognitive bases has proved challenging. Using principal component analyses, we show that primate social, ecological and reproductive behaviours condense into two components: socioecological complexity (including most social and ecological variables) and reproductive cooperation (comprising mainly a suite of behaviours associated with pairbonded monogamy). We contextualize these results using a meta-analysis of 44 published analyses of primate brain evolution. These studies yield two main consistent results: cognition, sociality and cooperative behaviours are associated with absolute brain volume, neocortex size and neocortex ratio, whereas diet composition and life history are consistently associated with relative brain size. We use a path analysis to evaluate the causal relationships among these variables, demonstrating that social group size is predicted by the neocortex, whereas ecological traits are predicted by the volume of brain structures other than the neocortex. That a range of social and technical behaviours covary, and are correlated with social group size and brain size, suggests that primate cognition has evolved along a continuum resulting in an increasingly flexible, domain-general capacity to solve a range of socioecological challenges culminating in a capacity for, and reliance on, innovation and social information use in the great apes and humans. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Primate social bonds are described as being especially complex in their nature, and primates have unusually large brains for their body size compared to other mammals. Communication in primates has attracted considerable attention because of the important role it plays in social bonding. It has been proposed that differentiated social relationships are cognitively complex because primates need to continuously update their knowledge about different types of social bonds. Therefore, primates infer whether an opportunity for social interaction is rewarding (valuable to individual goals) based on their knowledge of the social relationships of the interactants. However, exposure to distraction and stress has detrimental effects on the dopaminergic system, suggesting that understanding social relationships as rewarding is affected in these conditions. This paper proposes that complex communication evolved to augment the capacity to form social relationships during stress through flexibly modifying intentionality in communication (audience checking, response waiting and elaboration). Intentional communication may upregulate dopamine dynamics to allow recognition that an interaction is rewarding during stress. By examining these associations between complexity of communication and stress, we provide new insights into the cognitive skills involved in forming social bonds in primates and the evolution of communication systems in both primates and humans. This article is part of the theme issue 'Cognition, communication and social bonds in primates'.

Group living is stressful for all mammals, and these stresses limit the size of their social groups. Humans live in very large groups by mammal standards, so how have they solved this problem? I use homicide rates as an index of within-community stress for humans living in small-scale ethnographic societies, and show that the frequency of homicide increases linearly with living-group size in hunter-gatherers. This is not, however, the case for cultivators living in permanent settlements, where there appears to be a 'glass ceiling' below which homicide rates oscillate. This glass ceiling correlates with the adoption of social institutions that allow tensions to be managed. The results suggest (a) that the transition to a settled lifestyle in the Neolithic may have been more challenging than is usually assumed and (b) that the increases in settlement size that followed the first villages necessitated the introduction of a series of social institutions designed to manage within-community discord., Competing Interests: The author declares no conflicts of interest., (© The Author(s) 2022.)

Humans are social animals and the interpersonal bonds formed between them are crucial for their development and well being in a society. These relationships are usually structured into several layers (Dunbar's layers of friendship) depending on their significance in an individual's life with closest friends and family being the most important ones taking major part of their time and communication effort. However, we have little idea how the initiation and termination of these relationships occurs across the lifespan. Mobile phones, in particular, have been used extensively to shed light on the different types of social interactions between individuals and to explore this, we analyse a national cellphone database to determine how and when changes in close relationships occur in the two genders. In general, membership of this inner circle of intimate relationships is extremely stable, at least over a three-year period. However, around 1-4% of alters change every year, with the rate of change being higher among 17-21 year olds than older adults. Young adult females terminate more of their opposite-gender relationships, while older males are more persistent in trying to maintain relationships in decline. These results emphasise the variability in relationship dynamics across age and gender, and remind us that individual differences play an important role in the structure of social networks. Overall, our study provides a holistic understanding of the dynamic nature of close relationships during different stages of human life., (© 2022. The Author(s).)

Professional religious specialists centralised religious authority in early human societies and represented some of the earliest instances of formalised social leadership. These individuals played a central role in the emergence of organised religion and transitions to more stratified human societies. Evolutionary theories highlight a range of environmental, economic and social factors that are potentially causally related to the emergence of professional religious specialists in human history. There remains little consensus over the relative importance of these factors and whether professional religious specialists were the outcome or driver of increased socio-cultural complexity. We built a global dataset of hunter-gatherer societies and developed a novel method of exploratory phylogenetic path analysis. This enabled us to systematically identify the factors associated with the emergence of professional religious specialists and infer the directionality of causal dependencies. We find that environmental predictability, environmental richness, pathogen load, social leadership and food storage systems are all correlated with professional religious specialists. However, only food storage is directly related to the emergence of professional religious specialists. Our findings are most consistent with the claim that the early stages of organised religion were the outcome rather than driver of increased socio-economic complexity., Competing Interests: The authors are not aware of any conflicts of interest., (© The Author(s) 2022.)

The claim that nonverbal cues provide more information than the linguistic content of a conversational exchange (the Mehrabian Conjecture) has been widely cited and equally widely disputed, mainly on methodological grounds. Most studies that have tested the Conjecture have used individual words or short phrases spoken by actors imitating emotions. While cue recognition is certainly important, speech evolved to manage interactions and relationships rather than simple information exchange. In a cross-cultural design, we tested participants' ability to identify the quality of the interaction (rapport) in naturalistic third party conversations in their own and a less familiar language, using full auditory content versus audio clips whose verbal content has been digitally altered to differing extents. We found that, using nonverbal content alone, people are 75-90% as accurate as they are with full audio cues in identifying positive vs negative relationships, and 45-53% as accurate in identifying eight different relationship types. The results broadly support Mehrabian's claim that a significant amount of information about others' social relationships is conveyed in the nonverbal component of speech., Supplementary Information: The online version contains supplementary material available at 10.1007/s10919-021-00386-y., (© The Author(s) 2021.)

Fictional storytelling has played an important role in human cultural life since earliest times, and we are willing to invest significant quantities of time, mental effort and money in it. Nonetheless, the psychological mechanisms that make this possible, and how they relate to the mechanisms that underpin real-world social relationships, remain understudied. We explore three factors: identification (the capacity to identify with a character), moral approval and causal attribution with respect to a character's behaviour in live performances of two plays from the European literary canon. There were significant correlations between the extent to which subjects identified with a character and their moral approval of that character's behaviour that was independent of the way the play was directed. However, the subjects' psychological explanations for a character's behaviour (attribution) were independent of whether or not they identified with, or morally approved of, the character. These data extend previous findings by showing that moral approval plays an important role in facilitating identification even in live drama. Despite being transported by an unfolding drama, audiences do not necessarily become biased in their psychological understanding of why characters behaved as they did. The psychology of drama offers significant insights into the psychological processes that underpin our everyday social world., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Teasdale, Maguire, Budelmann and Dunbar.)

Compared to most other mammals and birds, anthropoid primates have unusually complex societies characterised by bonded social groups. Among primates, this effect is encapsulated in the social brain hypothesis: the robust correlation between various indices of social complexity (social group size, grooming clique size, tactical behaviour, coalition formation) and brain size. Hitherto, this has always been interpreted as a simple, unitary relationship. Using data for five different indices of brain volume from four independent brain databases, we show that the distribution of group size plotted against brain size is best described as a set of four distinct, very narrowly defined grades which are unrelated to phylogeny. The allocation of genera to these grades is highly consistent across the different data sets and brain indices. We show that these grades correspond to the progressive evolution of bonded social groups. In addition, we show, for those species that live in multilevel social systems, that the typical sizes of the different grouping levels in each case coincide with different grades. This suggests that the grades correspond to demographic attractors that are especially stable. Using five different cognitive indices, we show that the grades correlate with increasing social cognitive skills, suggesting that the cognitive demands of managing group cohesion increase progressively across grades. We argue that the grades themselves represent glass ceilings on animals' capacity to maintain social and spatial coherence during foraging and that, in order to evolve more highly bonded groups, species have to be able to invest in costly forms of cognition., (© 2021 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.)

Humans deploy a number of specific behaviours for forming social bonds, one of which is laughter. However, two questions have not yet been investigated with respect to laughter: (1) Does laughter increase the sense of bonding to those with whom we laugh? and (2) Does laughter facilitate prosocial generosity? Using changes in pain threshold as a proxy for endorphin upregulation in the brain and a standard economic game (the Dictator Game) as an assay of prosociality, we show that laughter does trigger the endorphin system and, through that, seems to enhance social bonding, but it does not reliably influence donations to others. This suggests that social bonding and prosociality may operate via different mechanisms, or on different time scales, and relate to different functional objectives., Competing Interests: The authors have declared that no competing interests exist.

Human behaviour follows a 24-h rhythm and is known to be governed by the individual chronotypes. Due to the widespread use of technology in our daily lives, it is possible to record the activities of individuals through their different digital traces. In the present study we utilise a large mobile phone communication dataset containing time stamps of calls and text messages to study the circadian rhythms of anonymous users in a European country. After removing the effect of the synchronization of East-West sun progression with the calling activity, we used two closely related approaches to heuristically compute the chronotypes of the individuals in the dataset, to identify them as morning persons or "larks" and evening persons or "owls". Using the computed chronotypes we showed how the chronotype is largely dependent on age with younger cohorts being more likely to be owls than older cohorts. Moreover, our analysis showed how on average females have distinctly different chronotypes from males. Younger females are more larkish than males while older females are more owlish. Finally, we also studied the period of low calling activity for each of the users which is considered as a marker of their sleep period during the night. We found that while "extreme larks" tend to sleep more than "extreme owls" on the weekends, we do not observe much variation between them on weekdays. In addition, we have observed that women tend to sleep even less than males on weekdays while there is not much difference between them on the weekends., (© 2021. The Author(s).)

The C-tactile (CLTM) peripheral nervous system is involved in social bonding in primates and humans through its capacity to trigger the brain's endorphin system. Since the mammalian cochlea has an unusually high density of similar neurons (type-II spiral ganglion neurons, SGNs), we hypothesise that their function may have been exploited for social bonding by co-opting head movements in response to music and other rhythmic movements of the head in social contexts. Music provides one of many cultural behavioural mechanisms for 'virtual grooming' in that it is used to trigger the endorphin system with many people simultaneously so as to bond both dyadic relationships and large groups. Changes in pain threshold across an activity are a convenient proxy assay for endorphin uptake in the brain, and we use this, in two experiments, to show that pain thresholds are higher when nodding the head than when sitting still., (© 2021. The Author(s).)

Introduction: Previous studies have shown that engaging in formal social participation may protect against declining mental health, but social network size (the number of close social ties a person has) may moderate the relationship. We assessed the potential moderating role of social network size using longitudinal data., Methods: Nationally representative data from two consecutive waves (2011, 2013) of the SHARE survey were analyzed. The data consisted of 38,300 adults from 13 European countries aged 50 years and older in 2011. Measures pertaining to formal social participation, social network size, quality of life, and depression symptoms were used. Multivariable linear regression models were conducted., Results: The majority of participants (over 70% of the sample) had a social network size of four or less close social ties. We identified significant moderations in both models. Individuals with relatively few close social ties may have benefitted from formal social participation both in terms of reductions in depression symptoms and increases in quality of life, while formal social participation among those with many social ties did not appear to be beneficial, and may even to some extent have been detrimental., Conclusions: Declines in mental health specifically among those with relatively few close social ties could potentially be prevented through the promotion of formal social participation. It is possible that such strategies could have a greater impact by specifically targeting individuals that are otherwise socially isolated. High levels of formal participation among those with relatively many close social ties may not be pragmatically beneficial.

Religious rituals are associated with health benefits, potentially produced via social bonding. It is unknown whether secular rituals similarly increase social bonding. We conducted a field study with individuals who celebrate secular rituals at Sunday Assemblies and compared them with participants attending Christian rituals. We assessed levels of social bonding and affect before and after the rituals. Results showed the increase in social bonding taking place in secular rituals is comparable to religious rituals. We also found that both sets of rituals increased positive affect and decreased negative affect, and that the change in positive affect predicted the change in social bonding observed. Together these results suggest that secular rituals might play a similar role to religious ones in fostering feelings of social connection and boosting positive affect., Competing Interests: The authors have declared that no competing interests exist.

Humans survive and thrive through social exchange. Yet, social dependency also comes at a cost. Perceived social isolation, or loneliness, affects physical and mental health, cognitive performance, overall life expectancy, and increases vulnerability to Alzheimer's disease-related dementias. Despite severe consequences on behavior and health, the neural basis of loneliness remains elusive. Using the UK Biobank population imaging-genetics cohort (n = ~40,000, aged 40-69 years when recruited, mean age = 54.9), we test for signatures of loneliness in grey matter morphology, intrinsic functional coupling, and fiber tract microstructure. The loneliness-linked neurobiological profiles converge on a collection of brain regions known as the 'default network'. This higher associative network shows more consistent loneliness associations in grey matter volume than other cortical brain networks. Lonely individuals display stronger functional communication in the default network, and greater microstructural integrity of its fornix pathway. The findings fit with the possibility that the up-regulation of these neural circuits supports mentalizing, reminiscence and imagination to fill the social void.

Religious rituals are universal human practices that play a seminal role in community bonding. In two experiments, we tested the role of mu-opioids as the active factor fostering social bonding. We used a mu-opioid blocker (naltrexone) in two double-blind studies of rituals from different religious traditions. We found the same effect across both studies, with naltrexone leading to significantly lower social bonding compared with placebo. These studies suggest that mu-opioids play a significant role in experiences of social bonding within ritual contexts.

Microbes colonise all multicellular life, and the gut microbiome has been shown to influence a range of host physiological and behavioural phenotypes. One of the most intriguing and least understood of these influences lies in the domain of the microbiome's interactions with host social behaviour, with new evidence revealing that the gut microbiome makes important contributions to animal sociality. However, little is known about the biological processes through which the microbiome might influence host social behaviour. Here, we synthesise evidence of the gut microbiome's interactions with various aspects of host sociality, including sociability, social cognition, social stress, and autism. We discuss evidence of microbial associations with the most likely physiological mediators of animal social interaction. These include the structure and function of regions of the 'social' brain (the amygdala, the prefrontal cortex, and the hippocampus) and the regulation of 'social' signalling molecules (glucocorticoids including corticosterone and cortisol, sex hormones including testosterone, oestrogens, and progestogens, neuropeptide hormones such as oxytocin and arginine vasopressin, and monoamine neurotransmitters such as serotonin and dopamine). We also discuss microbiome-associated host genetic and epigenetic processes relevant to social behaviour. We then review research on microbial interactions with olfaction in insects and mammals, which contribute to social signalling and communication. Following these discussions, we examine evidence of microbial associations with emotion and social behaviour in humans, focussing on psychobiotic studies, microbe-depression correlations, early human development, autism, and issues of statistical power, replication, and causality. We analyse how the putative physiological mediators of the microbiome-sociality connection may be investigated, and discuss issues relating to the interpretation of results. We also suggest that other candidate molecules should be studied, insofar as they exert effects on social behaviour and are known to interact with the microbiome. Finally, we consider different models of the sequence of microbial effects on host physiological development, and how these may contribute to host social behaviour., (© 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.)

Never before have we experienced social isolation on such a massive scale as we have in response to coronavirus disease 2019 (COVID-19). However, we know that the social environment has a dramatic impact on our sense of life satisfaction and well-being. In times of distress, crisis, or disaster, human resilience depends on the richness and strength of social connections, as well as on active engagement in groups and communities. Over recent years, evidence emerging from various disciplines has made it abundantly clear: perceived social isolation (i.e., loneliness) may be the most potent threat to survival and longevity. We highlight the benefits of social bonds, the choreographies of bond creation and maintenance, as well as the neurocognitive basis of social isolation and its deep consequences for mental and physical health., (Copyright © 2020 Elsevier Ltd. All rights reserved.)