Optimising human community sizes

Income and Sex Moderate the Association Between Population Density and Reproduction: A Multilevel Analysis of Life History Strategies Across 23 Nations

While previous studies guided by evolutionary life history theory have revealed several important socioecological moderators of the influence of population density (PD) on reproduction, absent is an understanding of how individual-level factors such as personal resources and sex differences might interact and play a role. Using data from a large sample of clients (N = 4,432,440) of an online dating company spanning 317 states nested within 23 countries, we contributed a robust multilevel analysis of life history effects by assessing the interaction between state-level PD and individual-level income on offspring quantity, and we further qualified this analysis by sex. Consistent with previous research, PD was negatively correlated with having children. Consistent with our novel hypotheses, this negative relationship was moderated by income such that the link between PD and low fertility became weaker with increasing levels of income and these patterns were stronger for men than for women. These results held despite controlling for a variety of country-level, state-level, and individual-level confounds. Findings are discussed together with theoretical and practical implications for the management of fertility based on evolutionary life history perspectives.

The origins and function of musical performance

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.

Four errors and a fallacy: pitfalls for the unwary in comparative brain analyses

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.

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.

Structural invariants and semantic fingerprints in the "ego network" of words

Well-established cognitive models coming from anthropology have shown that, due to the cognitive constraints that limit our "bandwidth" for social interactions, humans organize their social relations according to a regular structure. In this work, we postulate that similar regularities can be found in other cognitive processes, such as those involving language production. In order to investigate this claim, we analyse a dataset containing tweets of a heterogeneous group of Twitter users (regular users and professional writers). Leveraging a methodology similar to the one used to uncover the well-established social cognitive constraints, we find regularities at both the structural and semantic levels. In the former, we find that a concentric layered structure (which we call ego network of words, in analogy to the ego network of social relationships) very well captures how individuals organise the words they use. The size of the layers in this structure regularly grows (approximately 2-3 times with respect to the previous one) when moving outwards, and the two penultimate external layers consistently account for approximately 60% and 30% of the used words, irrespective of the number of layers of the user. For the semantic analysis, each ring of each ego network is described by a semantic profile, which captures the topics associated with the words in the ring. We find that ring #1 has a special role in the model. It is semantically the most dissimilar and the most diverse among the rings. We also show that the topics that are important in the innermost ring also have the characteristic of being predominant in each of the other rings, as well as in the entire ego network. In this respect, ring #1 can be seen as the semantic fingerprint of the ego network of words.

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 Homoca 2.5 Ma.

This article is part of the theme issue ‘Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience’.

Chimpanzees organize their social relationships like humans

Human relationships are structured in a set of layers, ordered from higher (intimate relationships) to lower (acquaintances) emotional and cognitive intensity. This structure arises from the limits of our cognitive capacity and the different amounts of resources required by different relationships. However, it is unknown whether nonhuman primate species organize their affiliative relationships following the same pattern. We here show that the time chimpanzees devote to grooming other individuals is well described by the same model used for human relationships, supporting the existence of similar social signatures for both humans and chimpanzees. Furthermore, the relationship structure depends on group size as predicted by the model, the proportion of high-intensity connections being larger for smaller groups.

Managing the stresses of group-living in the transition to village life

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.

Beyond Dunbar circles: a continuous description of social relationships and resource allocation

We discuss the structure of human relationship patterns in terms of a new formalism that allows to study resource allocation problems where the cost of the resource may take continuous values. This is in contrast with the main focus of previous studies where relationships were classified in a few, discrete layers (known as Dunbar's circles) with the cost being the same within each layer. We show that with our continuum approach we can identify a parameter [Formula: see text] that is the equivalent of the ratio of relationships between adjacent circles in the discrete case, with a value [Formula: see text]. We confirm this prediction using three different datasets coming from phone records, face-to-face contacts, and interactions in Facebook. As the sample size increases, the distributions of estimated parameters smooth around the predicted value of [Formula: see text]. The existence of a characteristic value of the parameter at the population level indicates that the model is capturing a seemingly universal feature on how humans manage relationships. Our analyses also confirm earlier results showing the existence of social signatures arising from having to allocate finite resources into different relationships, and that the structure of online personal networks mirrors those in the off-line world.

Psychology Within and Without the State

Psychological research in small-scale societies is crucial for what it stands to tell us about human psychological diversity. However, people in these communities, typically Indigenous communities in the global South, have been underrepresented and sometimes misrepresented in psychological research. Here I discuss the promises and pitfalls of psychological research in these communities, reviewing why they have been of interest to social scientists and how cross-cultural comparisons have been used to test psychological hypotheses. I consider factors that may be undertheorized in our research, such as political and economic marginalization, and how these might influence our data and conclusions. I argue that more just and accurate representation of people from small-scale communities around the world will provide us with a fuller picture of human psychological similarity and diversity, and it will help us to better understand how this diversity is shaped by historical and social processes. Expected final online publication date for the Annual Review of Psychology, Volume 73 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

An agent-based model of the female rivalry hypothesis for concealed ovulation in humans

After half a century of debate and few empirical tests, there remains no consensus concerning why ovulation in human females is considered concealed. The predominant male investment hypothesis states that females were better able to obtain material investment from male partners across those females' ovulatory cycles by concealing ovulation. We build on recent work on female competition to propose and investigate an alternative-the female rivalry hypothesis-that concealed ovulation benefited females by allowing them to avoid aggression from other females. Using an agent-based model of mating behaviour and paternal investment in a human ancestral environment, we did not find strong support for the male investment hypothesis, but found support for the female rivalry hypothesis. Our results suggest that concealed ovulation may have benefitted females in navigating their intrasexual social relationships. More generally, this work implies that explicitly considering female-female interactions may inspire additional insights into female behaviour and physiology.

Proximate and ultimate causes of ritual behavior

Ritual behaviour, intended as a specific, repetitive and rigid form of action flow, appears both in social and non-social environmental contexts, representing an ubiquitous phenomenon in animal life including human individuals and cultures. The purpose of this contribution is to investigate an evolutionary continuum in proximate and ultimate causes of ritual behavior. A phylogenetic homology in proximal mechanisms can be found, based on the repetition of genetically programmed and/or epigenetically acquired action patterns of behavior. As far as its adaptive significance, ethological comparative studies show that the tendency to ritualization is driven by the unpredictability of social or ecological environmental stimuli. In this perspective, rituals may have a "homeostatic" function over unpredictable environments, as further highlighted by psychopathological compulsions. In humans, a circular loop may have occurred among ritual practices and symbolic activity to deal with a novel culturally-mediated world. However, we suggest that the compulsion to action patterns repetition, typical of all rituals, has a genetically inborn motor foundation, thus precognitive and pre-symbolic. Rooted in such phylogenetically conserved motor structure (proximate causes), the evolution of cognitive and symbolic capacities have generated the complexity of human rituals, though maintaining the original adaptive function (ultimate causes) to cope with unpredictable environments.

Scaling human sociopolitical complexity

Human societies exhibit a diversity of social organizations that vary widely in size, structure, and complexity. Today, human sociopolitical complexity ranges from stateless small-scale societies of a few hundred individuals to complex states of millions, most of this diversity evolving only over the last few hundred years. Understanding how sociopolitical complexity evolved over time and space has always been a central focus of the social sciences. Yet despite this long-term interest, a quantitative understanding of how sociopolitical complexity varies across cultures is not well developed. Here we use scaling analysis to examine the statistical structure of a global sample of over a thousand human societies across multiple levels of sociopolitical complexity. First, we show that levels of sociopolitical complexity are self-similar as adjacent levels of jurisdictional hierarchy see a four-fold increase in population size, a two-fold increase in geographic range, and therefore a doubling of population density. Second, we show how this self-similarity leads to the scaling of population size and geographic range. As societies increase in complexity population density is reconfigured in space and quantified by scaling parameters. However, there is considerable overlap in population metrics across all scales suggesting that while more complex societies tend to have larger and denser populations, larger and denser populations are not necessarily more complex.

The fractal structure of communities of practice: Implications for business organization

Communities of practice (COP) are informal (sometimes formal) groupings of professionals with shared interests that form to facilitate the exchange of expertise and shared learning or to function as professional support networks. We analyse a dataset on the size of COPs and show that their distribution has a fractal structure similar to that found in huntergatherer social organisation and the structure of human personal social networks. Small communities up to about 40 in size can be managed democratically, but all larger communities require a leadership team structure. We show that frequency of interaction declines as size increases, as is the case in personal social networks. This suggests that professional work-oriented organisations may be subject to the same kinds of constraint imposed on human social organisation by the social brain. We discuss the implications for business management structure.

Cognitive resource allocation determines the organization of personal networks

The typical human personal social network contains about 150 relationships including kin, friends, and acquaintances, organized into a set of hierarchically inclusive layers of increasing size but decreasing emotional intensity. Data from a number of different sources reveal that these inclusive layers exhibit a constant scaling ratio of [Formula: see text] While the overall size of the networks has been connected to our cognitive capacity, no mechanism explaining why the networks present a layered structure with a consistent scaling has been proposed. Here we show that the existence of a heterogeneous cost to relationships (in terms of time or cognitive investment), together with a limitation in the total capacity an individual has to invest in them, can naturally explain the existence of layers and, when the cost function is linear, explain the scaling between them. We develop a one-parameter Bayesian model that fits the empirical data remarkably well. In addition, the model predicts the existence of a contrasting regime in the case of small communities, such that the layers have an inverted structure (increasing size with increasing emotional intensity). We test the model with five communities and provide clear evidence of the existence of the two predicted regimes. Our model explains, based on first principles, the emergence of structure in the organization of personal networks and allows us to predict a rare phenomenon whose existence we confirm empirically.

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.