Playing the political game: the coevolution of institutions with group size and political inequality.

All societies need to form institutional rules to regulate their social interactions. These specify what actions individuals should take in particular situations, and what sanctions will apply if individuals violate these rules. However, forming these institutional rules involves playing a political game-a process of negotiation between individuals that is costly and time-consuming. Intuitively, this cost should be expected to increase as a group becomes larger, which could then select for a transition to hierarchy to keep the cost of playing the political game down as group size increases. However, previous work has lacked a mechanistic yet general model of political games that could formalize this argument and test the conditions under which it holds. We address this by formalizing the political game using a standard consensus formation model. We show that the increasing cost of forming a consensus over institutional rules selects for a transition from egalitarian to hierarchical organization over a wide range of conditions. Playing a political game to form institutional rules in this way captures and unites a previously disparate set of voluntary theories for hierarchy formation, and can explain why the increasing group size in the Neolithic would lead to strong political inequality. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Modelling the role of environmental circumscription in the evolution of inequality.

Circumscription theory proposes that complex hierarchical societies emerged in areas surrounded by barriers to dispersal, e.g. mountains or seas. This theory has been widely influential but the lack of formal modelling has resulted in theoretical and empirical challenges. This theory shares parallels with reproductive skew models from evolutionary ecology where inequality depends on the capacity of subordinates to escape from despotic leaders. Building on these similarities, we extend reproductive skew models to simulate the concurrent evolution of inequality in many connected groups. Our results show that cost of migration does not directly limit inequality in the long term, but it does control the rate of increase in inequality. Second, we show that levels of inequality can be reduced if there are random errors made by dominants, as these lead to variations that propagate between polities. Third, our model clarifies the concept of circumscription by relating it to geographical features: the size of a region and the connectivity between polities. Overall, our model helps clarify some issues about how migration may affect inequality. We discuss our results in the light of anthropological and archaeological evidence and present the future extensions required to build towards a complete model of circumscription theory. This article is part of the theme issue 'Evolutionary ecology of inequality'.

Sequence alignment of folk song melodies reveals cross-cultural regularities of musical evolution.

Culture evolves,1-5 but the existence of cross-culturally general regularities of cultural evolution is debated.6-8 As a diverse but universal cultural phenomenon, music provides a novel domain to test for the existence of such regularities.9-12 Folk song melodies can be thought of as culturally transmitted sequences of notes that change over time under the influence of cognitive and acoustic/physical constraints.9-15 Modeling melodies as evolving sequences constructed from an "alphabet" of 12 scale degrees16 allows us to quantitatively test for the presence of cross-cultural regularities using a sample of 10,062 melodies from musically divergent Japanese and English (British/American) folk song traditions.17,18 Our analysis identifies 328 pairs of highly related melodies, finding that note changes are more likely when they have smaller impacts on a song's melody. Specifically, (1) notes with stronger rhythmic functions are less likely to change, and (2) note substitutions are most likely between neighboring notes. We also find that note insertions/deletions ("indels") are more common than note substitutions, unlike genetic evolution where the reverse is true. Our results are consistent across English and Japanese samples despite major differences in their scales and tonal systems. These findings demonstrate that even a creative art form such as music is subject to evolutionary constraints analogous to those governing the evolution of genes, languages, and other domains of culture.

Assessing different historical pathways in the cultural evolution of economic development.

A huge number of hypotheses have been put forward to explain the substantial diversity in economic performance we see in the present-day. There has been a growing appreciation that historical and ecological factors have contributed to social and economic development. However, it is not clear whether such factors have exerted a direct effect on modern productivity, or whether they influence economies indirectly by shaping the cultural evolution of norms and institutions. Here we analyse a global cross-national dataset to test between hypotheses involving a number of different ecological, historical, and proximate social factors and a range of direct and indirect pathways. We show that the historical timing of agriculture predicts the timing of the emergence of statehood, which in turn affects economic development indirectly through its effect on institutions. Ecological factors appear to affect economic performance indirectly through their historical effects on the development of agriculture and by shaping patterns of European settler colonization. More effective institutional performance is also predicted by lower-levels of in-group bias which itself appears related to the proportion of a nation's population that descends from European countries. These results support the idea that cultural evolutionary processes have been important in shaping the social norms and institutions that enable large-scale cooperation and economic growth in present-day societies.

Exploring correlations in genetic and cultural variation across language families in northeast Asia.

Culture evolves in ways that are analogous to, but distinct from, genomes. Previous studies examined similarities between cultural variation and genetic variation (population history) at small scales within language families, but few studies have empirically investigated these parallels across language families using diverse cultural data. We report an analysis comparing culture and genomes from in and around northeast Asia spanning 11 language families. We extract and summarize the variation in language (grammar, phonology, lexicon), music (song structure, performance style), and genomes (genome-wide SNPs) and test for correlations. We find that grammatical structure correlates with population history (genetic history). Recent contact and shared descent fail to explain the signal, suggesting relationships that arose before the formation of current families. Our results suggest that grammar might be a cultural indicator of population history while also demonstrating differences among cultural and genetic relationships that highlight the complex nature of human history.

The cultural evolution and ecology of institutions.

Human societies are structured by what we refer to as 'institutions', which are socially created and culturally inherited proscriptions on behaviour that define roles and set expectations about social interactions. The study of institutions in several social science fields has provided many important insights that have not been fully appreciated in the evolutionary human sciences. However, such research has often lacked a shared understanding of general processes of change that shape institutional diversity across space and time. We argue that evolutionary theory can provide a useful framework for synthesizing information from different disciplines to address issues such as how and why institutions change over time, how institutional rules co-evolve with other culturally inherited traits, and the role that ecological factors might play in shaping institutional diversity. We argue that we can gain important insights by applying cultural evolutionary thinking to the study of institutions, but that we also need to expand and adapt our approaches to better handle the ways that institutions work, and how they might change over time. In this paper, we illustrate our approach by describing macro-scale empirical comparative analyses that demonstrate how evolutionary theory can be used to generate and test hypotheses about the processes that have shaped some of the major patterns we see in institutional diversity over time and across the world today. We then go on to discuss how we might usefully develop micro-scale models of institutional change by adapting concepts from game theory and agent-based modelling. We end by considering current challenges and areas for future research, and the potential implications for other areas of study and real-world applications. This article is part of the theme issue 'Foundations of cultural evolution'.

Complex societies precede moralizing gods throughout world history.

The origins of religion and of complex societies represent evolutionary puzzles1-8. The 'moralizing gods' hypothesis offers a solution to both puzzles by proposing that belief in morally concerned supernatural agents culturally evolved to facilitate cooperation among strangers in large-scale societies9-13. Although previous research has suggested an association between the presence of moralizing gods and social complexity3,6,7,9-18, the relationship between the two is disputed9-13,19-24, and attempts to establish causality have been hampered by limitations in the availability of detailed global longitudinal data. To overcome these limitations, here we systematically coded records from 414 societies that span the past 10,000 years from 30 regions around the world, using 51 measures of social complexity and 4 measures of supernatural enforcement of morality. Our analyses not only confirm the association between moralizing gods and social complexity, but also reveal that moralizing gods follow-rather than precede-large increases in social complexity. Contrary to previous predictions9,12,16,18, powerful moralizing 'big gods' and prosocial supernatural punishment tend to appear only after the emergence of 'megasocieties' with populations of more than around one million people. Moralizing gods are not a prerequisite for the evolution of social complexity, but they may help to sustain and expand complex multi-ethnic empires after they have become established. By contrast, rituals that facilitate the standardization of religious traditions across large populations25,26 generally precede the appearance of moralizing gods. This suggests that ritual practices were more important than the particular content of religious belief to the initial rise of social complexity.

Effects of supplementary feeding on interspecific dominance hierarchies in garden birds.

Individuals often differ in competitive ability, which can lead to the formation of a dominance hierarchy that governs differential access to resources. Previous studies of dominance have predominently focussed on within-species interactions, while the drivers of between-species competitive hierarchies are poorly understood. The increasing prevalence of predictable anthropogenic food subsidies, such as that provided by garden bird feeders, is likely to intensify between-species competition. However, the consequences for resource acquisition await detailed study, and in particular, whether competitive interactions are influenced by food quality is not known. Here, we examine competitive interactions amongst ten passerine species of birds utilising supplementary food sources of differing quality. We show that dominance rank is strongly predicted by body mass across species. Socially dominant, heavier species monopolised access to a food that had a relatively short handling time (sunflower hearts), spent longer on supplementary feeders, and pecked at lower rates. In contrast subordinate, lighter species were constrained to feed on a food that had a relatively long handling time (sunflower seeds with the hull intact). Our findings suggest that differences in body mass may result in between-species dominance hierarchies that place the heaviest species in the greatest control of supplementary feeding sites, gaining superior access to higher value foods. This may have important implications for the use of supplementary feeding as a conservation tool.

Analyses of ovarian activity reveal repeated evolution of post-reproductive lifespans in toothed whales.

In most species the reproductive system ages at the same rate as somatic tissue and individuals continue reproducing until death. However, females of three species - humans, killer whales and short-finned pilot whales - have been shown to display a markedly increased rate of reproductive senescence relative to somatic ageing. In these species, a significant proportion of females live beyond their reproductive lifespan: they have a post-reproductive lifespan. Research into this puzzling life-history strategy is hindered by the difficulties of quantifying the rate of reproductive senescence in wild populations. Here we present a method for measuring the relative rate of reproductive senescence in toothed whales using published physiological data. Of the sixteen species for which data are available (which does not include killer whales), we find that three have a significant post-reproductive lifespan: short-finned pilot whales, beluga whales and narwhals. Phylogenetic reconstruction suggests that female post-reproductive lifespans have evolved several times independently in toothed whales. Our study is the first evidence of a significant post-reproductive lifespan in beluga whales and narwhals which, when taken together with the evidence for post-reproductive lifespan in killer whales, doubles the number of non-human mammals known to exhibit post-reproductive lifespans in the wild.

Quantitative historical analysis uncovers a single dimension of complexity that structures global variation in human social organization.

Do human societies from around the world exhibit similarities in the way that they are structured, and show commonalities in the ways that they have evolved? These are long-standing questions that have proven difficult to answer. To test between competing hypotheses, we constructed a massive repository of historical and archaeological information known as "Seshat: Global History Databank." We systematically coded data on 414 societies from 30 regions around the world spanning the last 10,000 years. We were able to capture information on 51 variables reflecting nine characteristics of human societies, such as social scale, economy, features of governance, and information systems. Our analyses revealed that these different characteristics show strong relationships with each other and that a single principal component captures around three-quarters of the observed variation. Furthermore, we found that different characteristics of social complexity are highly predictable across different world regions. These results suggest that key aspects of social organization are functionally related and do indeed coevolve in predictable ways. Our findings highlight the power of the sciences and humanities working together to rigorously test hypotheses about general rules that may have shaped human history.

Cultural group selection is plausible, but the predictions of its hypotheses should be tested with real-world data.

The evidence compiled in the target article demonstrates that the assumptions of cultural group selection (CGS) theory are often met, and it is therefore a useful framework for generating plausible hypotheses. However, more can be said about how we can test the predictions of CGS hypotheses against competing explanations using historical, archaeological, and anthropological data.

Statistical universals reveal the structures and functions of human music.

Music has been called "the universal language of mankind." Although contemporary theories of music evolution often invoke various musical universals, the existence of such universals has been disputed for decades and has never been empirically demonstrated. Here we combine a music-classification scheme with statistical analyses, including phylogenetic comparative methods, to examine a well-sampled global set of 304 music recordings. Our analyses reveal no absolute universals but strong support for many statistical universals that are consistent across all nine geographic regions sampled. These universals include 18 musical features that are common individually as well as a network of 10 features that are commonly associated with one another. They span not only features related to pitch and rhythm that are often cited as putative universals but also rarely cited domains including performance style and social context. These cross-cultural structural regularities of human music may relate to roles in facilitating group coordination and cohesion, as exemplified by the universal tendency to sing, play percussion instruments, and dance to simple, repetitive music in groups. Our findings highlight the need for scientists studying music evolution to expand the range of musical cultures and musical features under consideration. The statistical universals we identified represent important candidates for future investigation.

Broad supernatural punishment but not moralizing high gods precede the evolution of political complexity in Austronesia.

Supernatural belief presents an explanatory challenge to evolutionary theorists-it is both costly and prevalent. One influential functional explanation claims that the imagined threat of supernatural punishment can suppress selfishness and enhance cooperation. Specifically, morally concerned supreme deities or 'moralizing high gods' have been argued to reduce free-riding in large social groups, enabling believers to build the kind of complex societies that define modern humanity. Previous cross-cultural studies claiming to support the MHG hypothesis rely on correlational analyses only and do not correct for the statistical non-independence of sampled cultures. Here we use a Bayesian phylogenetic approach with a sample of 96 Austronesian cultures to test the MHG hypothesis as well as an alternative supernatural punishment hypothesis that allows punishment by a broad range of moralizing agents. We find evidence that broad supernatural punishment drives political complexity, whereas MHGs follow political complexity. We suggest that the concept of MHGs diffused as part of a suite of traits arising from cultural exchange between complex societies. Our results show the power of phylogenetic methods to address long-standing debates about the origins and functions of religion in human society.

Evolution of cultural traits occurs at similar relative rates in different world regions.

A fundamental issue in understanding human diversity is whether or not there are regular patterns and processes involved in cultural change. Theoretical and mathematical models of cultural evolution have been developed and are increasingly being used and assessed in empirical analyses. Here, we test the hypothesis that the rates of change of features of human socio-cultural organization are governed by general rules. One prediction of this hypothesis is that different cultural traits will tend to evolve at similar relative rates in different world regions, despite the unique historical backgrounds of groups inhabiting these regions. We used phylogenetic comparative methods and systematic cross-cultural data to assess how different socio-cultural traits changed in (i) island southeast Asia and the Pacific, and (ii) sub-Saharan Africa. The relative rates of change in these two regions are significantly correlated. Furthermore, cultural traits that are more directly related to external environmental conditions evolve more slowly than traits related to social structures. This is consistent with the idea that a form of purifying selection is acting with greater strength on these more environmentally linked traits. These results suggest that despite contingent historical events and the role of humans as active agents in the historical process, culture does indeed evolve in ways that can be predicted from general principles.

War, space, and the evolution of Old World complex societies.

How did human societies evolve from small groups, integrated by face-to-face cooperation, to huge anonymous societies of today, typically organized as states? Why is there so much variation in the ability of different human populations to construct viable states? Existing theories are usually formulated as verbal models and, as a result, do not yield sharply defined, quantitative predictions that could be unambiguously tested with data. Here we develop a cultural evolutionary model that predicts where and when the largest-scale complex societies arose in human history. The central premise of the model, which we test, is that costly institutions that enabled large human groups to function without splitting up evolved as a result of intense competition between societies-primarily warfare. Warfare intensity, in turn, depended on the spread of historically attested military technologies (e.g., chariots and cavalry) and on geographic factors (e.g., rugged landscape). The model was simulated within a realistic landscape of the Afroeurasian landmass and its predictions were tested against a large dataset documenting the spatiotemporal distribution of historical large-scale societies in Afroeurasia between 1,500 BCE and 1,500 CE. The model-predicted pattern of spread of large-scale societies was very similar to the observed one. Overall, the model explained 65% of variance in the data. An alternative model, omitting the effect of diffusing military technologies, explained only 16% of variance. Our results support theories that emphasize the role of institutions in state-building and suggest a possible explanation why a long history of statehood is positively correlated with political stability, institutional quality, and income per capita.

Cultural phylogeography of the Bantu Languages of sub-Saharan Africa.

There is disagreement about the routes taken by populations speaking Bantu languages as they expanded to cover much of sub-Saharan Africa. Here, we build phylogenetic trees of Bantu languages and map them onto geographical space in order to assess the likely pathway of expansion and test between dispersal scenarios. The results clearly support a scenario in which groups first moved south through the rainforest from a homeland somewhere near the Nigeria-Cameroon border. Emerging on the south side of the rainforest, one branch moved south and west. Another branch moved towards the Great Lakes, eventually giving rise to the monophyletic clade of East Bantu languages that inhabit East and Southeastern Africa. These phylogenies also reveal information about more general processes involved in the diversification of human populations into distinct ethnolinguistic groups. Our study reveals that Bantu languages show a latitudinal gradient in covering greater areas with increasing distance from the equator. Analyses suggest that this pattern reflects a true ecological relationship rather than merely being an artefact of shared history. The study shows how a phylogeographic approach can address questions relating to the specific histories of certain groups, as well as general cultural evolutionary processes.

Analyses do not support the parasite-stress theory of human sociality.

Re-analysis of the data provided in the target article reveals a lack of evidence for a strong, universal relationship between parasite stress and the variables relating to sociality. Furthermore, even if associations between these variables do exist, the analyses presented here do not provide evidence for Fincher & Thornhill's (F&T's) proposed causal mechanism.

Mode and tempo in the evolution of socio-political organization: reconciling 'Darwinian' and 'Spencerian' evolutionary approaches in anthropology.

Traditional investigations of the evolution of human social and political institutions trace their ancestry back to nineteenth century social scientists such as Herbert Spencer, and have concentrated on the increase in socio-political complexity over time. More recent studies of cultural evolution have been explicitly informed by Darwinian evolutionary theory and focus on the transmission of cultural traits between individuals. These two approaches to investigating cultural change are often seen as incompatible. However, we argue that many of the defining features and assumptions of 'Spencerian' cultural evolutionary theory represent testable hypotheses that can and should be tackled within a broader 'Darwinian' framework. In this paper we apply phylogenetic comparative techniques to data from Austronesian-speaking societies of Island South-East Asia and the Pacific to test hypotheses about the mode and tempo of human socio-political evolution. We find support for three ideas often associated with Spencerian cultural evolutionary theory: (i) political organization has evolved through a regular sequence of forms, (ii) increases in hierarchical political complexity have been more common than decreases, and (iii) political organization has co-evolved with the wider presence of hereditary social stratification.