Structural and Cognitive Mechanisms of Group Cohesion in Primates.

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.

Relating size and functionality in human social networks through complexity.

Extensive empirical evidence suggests that there is a maximal number of people with whom an individual can maintain stable social relationships (the Dunbar number). We argue that this arises as a consequence of a natural phase transition in the dynamic self-organization among N individuals within a social system. We present the calculated size dependence of the scaling properties of complex social network models to argue that this collective behavior is an enhanced form of collective intelligence. Direct calculation establishes that the complexity of social networks as measured by their scaling behavior is nonmonotonic, peaking around 150, thereby providing a theoretical basis for the value of the Dunbar number. Thus, we establish a theory-based bridge spanning the gap between sociology and psychology.

Blocking mu-opioid receptors inhibits social bonding in rituals.

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.

Trade-off between fertility and predation risk drives a geometric sequence in the pattern of group sizes in baboons.

Group-living offers both benefits (protection against predators, access to resources) and costs (increased ecological competition, the impact of group size on fertility). Here, we use cluster analysis to detect natural patternings in a comprehensive sample of baboon groups, and identify a geometric sequence with peaks at approximately 20, 40, 80 and 160. We suggest (i) that these form a set of demographic oscillators that set habitat-specific limits to group size and (ii) that the oscillator arises from a trade-off between female fertility and predation risk.

Primate social group sizes exhibit a regular scaling pattern with natural attractors.

Primate groups vary considerably in size across species. Nonetheless, the distribution of mean species group size has a regular scaling pattern with preferred sizes approximating 2.5, 5, 15, 30 and 50 individuals (although strepsirrhines lack the latter two), with a scaling ratio of approximately 2.5 similar to that observed in human social networks. These clusters appear to form distinct social grades that are associated with rapid evolutionary change, presumably in response to intense environmental selection pressures. These findings may have wider implications for other highly social mammal taxa.

Stay or stray? Evidence for alternative mating strategy phenotypes in both men and women.

In all comparative analyses, humans always fall on the borderline between obligate monogamy and polygamy. Here, we use behavioural indices (sociosexuality) and anatomical indices (prenatal testosterone exposure indexed by 2D : 4D digit ratio) from three human populations to show that this may be because there are two distinct phenotypes in both sexes. While males are more promiscuous and display higher prenatal testosterone exposure than females overall, our analyses also suggest that the within-sex variation of these variables is best described by two underlying mixture models, suggesting the presence of two phenotypes with a monogamous/promiscuous ratio that slightly favours monogamy in females and promiscuity in males. The presence of two phenotypes implies that mating strategy might be under complex frequency-dependent selection.

Causal evidence for social group sizes from Wikipedia editing data.

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.

Lifehistory Trade-Offs Influence Women's Reproductive Strategies.

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.

How the size and structure of egocentric networks change during a life transition.

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.

New insights into differences in brain organization between Neanderthals and anatomically modern humans.

Previous research has identified morphological differences between the brains of Neanderthals and anatomically modern humans (AMHs). However, studies using endocasts or the cranium itself are limited to investigating external surface features and the overall size and shape of the brain. A complementary approach uses comparative primate data to estimate the size of internal brain areas. Previous attempts to do this have generally assumed that identical total brain volumes imply identical internal organization. Here, we argue that, in the case of Neanderthals and AMHs, differences in the size of the body and visual system imply differences in organization between the same-sized brains of these two taxa. We show that Neanderthals had significantly larger visual systems than contemporary AMHs (indexed by orbital volume) and that when this, along with their greater body mass, is taken into account, Neanderthals have significantly smaller adjusted endocranial capacities than contemporary AMHs. We discuss possible implications of differing brain organization in terms of social cognition, and consider these in the context of differing abilities to cope with fluctuating resources and cultural maintenance.

Processing power limits social group size: computational evidence for the cognitive costs of sociality.

Sociality is primarily a coordination problem. However, the social (or communication) complexity hypothesis suggests that the kinds of information that can be acquired and processed may limit the size and/or complexity of social groups that a species can maintain. We use an agent-based model to test the hypothesis that the complexity of information processed influences the computational demands involved. We show that successive increases in the kinds of information processed allow organisms to break through the glass ceilings that otherwise limit the size of social groups: larger groups can only be achieved at the cost of more sophisticated kinds of information processing that are disadvantageous when optimal group size is small. These results simultaneously support both the social brain and the social complexity hypotheses.

Why did doctrinal religions first appear in the Northern Subtropical Zone?

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.

Orbital prefrontal cortex volume predicts social network size: an imaging study of individual differences in humans.

The social brain hypothesis, an explanation for the unusually large brains of primates, posits that the size of social group typical of a species is directly related to the volume of its neocortex. To test whether this hypothesis also applies at the within-species level, we applied the Cavalieri method of stereology in conjunction with point counting on magnetic resonance images to determine the volume of prefrontal cortex (PFC) subfields, including dorsal and orbital regions. Path analysis in a sample of 40 healthy adult humans revealed a significant linear relationship between orbital (but not dorsal) PFC volume and the size of subjects' social networks that was mediated by individual intentionality (mentalizing) competences. The results support the social brain hypothesis by indicating a relationship between PFC volume and social network size that applies within species, and, more importantly, indicates that the relationship is mediated by social cognitive skills.

Social laughter is correlated with an elevated pain threshold.

Although laughter forms an important part of human non-verbal communication, it has received rather less attention than it deserves in both the experimental and the observational literatures. Relaxed social (Duchenne) laughter is associated with feelings of wellbeing and heightened affect, a proximate explanation for which might be the release of endorphins. We tested this hypothesis in a series of six experimental studies in both the laboratory (watching videos) and naturalistic contexts (watching stage performances), using change in pain threshold as an assay for endorphin release. The results show that pain thresholds are significantly higher after laughter than in the control condition. This pain-tolerance effect is due to laughter itself and not simply due to a change in positive affect. We suggest that laughter, through an endorphin-mediated opiate effect, may play a crucial role in social bonding.

Fission-fusion and the evolution of hominin social systems.

The course of hominin evolution has involved successive migrations towards higher absolute latitudes over the past three million years. Poorer habitat quality further from the equator has led to the necessity for groups occupying higher latitudes to live at lower population densities. Coupled with a trend towards increasing group size over this time period, this tendency towards expansion has led to exponential increases in the area requirements of hominin groups, and a concomitant need to adjust foraging patterns. The current analyses suggest that the development of increasingly complex, multi-level fission-fusion social systems could have freed hominins of the foraging constraints imposed by large group sizes and low population densities. Analyses of the fossil record suggest latitudinally-driven differences in area requirements of the australopithecines from East and South Africa, and African and Asian Homo erectus. In contrast, chronologically-driven differences appear between H. erectus as a whole and Homo heidelbergensis, and between H. heidelbergensis and the Neanderthals. These results are discussed in relation to studies of the foraging patterns of primates and hunter-gatherers.

Cooperation, behavioural synchrony and status in social networks.

In this paper we present a new approach to modelling group coordination, based on dyadic synchronisation in a non-panmictic, structured network (a problem that applies widely to all species that live in medium to large groups). Using this approach, we present three models with three new theoretical results. (1) Multi-layered networks are optimal for groups that face costs associated with maintaining relationships among the members, combined with costs associated with information flows. (2) The presence of a social hierarchy can be an adaptive feature of the community: the steeper the optimal social hierarchy is, the fewer relationships group members need to have. (3) Falling communication costs lead to a less steep optimal social hierarchy in communities in which socially useful information is evenly distributed, but to an even steeper social hierarchy in groups in which socially useful information is uneven. Thus we show how, when communication is costly, cooperation can give rise to communities that are socially highly structured.

Predation as a determinant of minimum group size in baboons.

Predation risk places a pressure on animals to adopt mechanisms by which they reduce their individual risk of being preyed on. However, a consensus on methods of determining predation risk has yet to be reached. One of the most widespread ways in which animals respond to predation risk is by living in groups. Minimum permissible group size is the smallest group size that animals are able to live in, given the habitat-specific predation risk they face. We explore ways in which predation risk can be measured and analyse its effect on minimum observed group size in baboons. Using data on predator density, habitat composition and baboon body size, we investigate the impact of the components of predation risk on baboon group size, and derive an equation that best predicts minimum group size. Minimum group size in baboons is related to predator density and female body mass. Both of these elements can, in turn, be estimated from environmental variables. These findings present support for the argument that group living in primates is a response to predation risk and offer potentially new ways of investigating carnivore and primate ecology.

Predation by mammalian carnivores on nocturnal primates: is the lack of evidence support for the effectiveness of nocturnality as an antipredator strategy?

The apparent paucity of accounts of predation, particularly by felids, on nocturnal primates is confirmed by a quasi-systematic review of 1,939 publications which revealed just 1 case of a felid eating a nocturnal primate. This instance was amongst only 51 direct reports of predation by vertebrates on nocturnal primates (90% were on Madagascar, where 56% of approx. 110 nocturnal primate species occur), of which 41% were by birds of prey. These findings prompt discussion of two possibilities: (a) nocturnality is, in part, an effective antipredator adaptation, and (b) knowledge of nocturnal primates is so biased by their elusiveness and, for predation, underreporting (e.g. inadequate mechanisms to publish opportunistic observations) that understanding of their biology urgently necessitates both the collation of field observations and innovative research. Interspecific comparisons facilitate deductions about the role of predation in the evolution of primate nocturnality and associated traits, but intraspecific comparisons of changing activity rhythms in response to different levels of predation risk offer the most compelling insights into the functional significance of these adaptations.

Laughter and its role in the evolution of human social bonding.

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'.