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Human genetic and phenotypic diversity declines with distance from Africa, as predicted by a serial founder effect in which successive population bottlenecks during range expansion progressively reduce diversity, underpinning support for an African origin of modern humans. Recent work suggests that a similar founder effect may operate on human culture and language. Here I show that the number of phonemes used in a global sample of 504 languages is also clinal and fits a serial founder—effect model of expansion from an inferred origin in Africa. This result, which is not explained by more recent demographic history, local language diversity, or statistical non-independence within language families, points to parallel mechanisms shaping genetic and linguistic diversity and supports an African origin of modern human languages.

The persistent threat of natural disasters and their attendant resource shocks has likely shaped our prosocial drives throughout human evolution. However, it remains unclear how specific experiences during these events might impact cooperative decision making. We conducted two waves of four modified dictator-game experiments with the same individuals in Vanuatu (N = 164), before and after Cyclone Pam in 2015. After the cyclone, participants were generally less likely to show prosocial motives toward both in-group and out-group members and more likely to show parochialism when sharing between groups. Experiencing greater property damage predicted a general decrease in prosocial allocations and preference for participants’ in-group. By contrast, exposure to other people in distress predicted increased prosocial allocations to both participants’ in-group and out-groups. Our results suggest that people adjust their prosocial behavior in response to natural disasters but that the nature and direction of the effect depend on the type and severity of their experiences.

Cross-national analyses test hypotheses about the drivers of variation in national outcomes. However, since nations are connected in various ways, such as via spatial proximity and shared cultural ancestry, cross-national analyses often violate assumptions of non-independence, inflating false positive rates. Here, we show that, despite being recognised as an important statistical pitfall for over 200 years, cross-national research in economics and psychology still does not sufficiently account for non-independence. In a review of the 100 highest-cited cross-national studies of economic development and values, we find that controls for non-independence are rare. When studies do control for non-independence, our simulations suggest that most commonly used methods are insufficient for reducing false positives in non-independent data. In reanalyses of twelve previous cross-national correlations, half of the estimates are compatible with no association after controlling for non-independence using global proximity matrices. We urge social scientists to sufficiently control for non-independence in cross-national research. Nations are connected in many ways, yet cross-national analyses often assume they are independent. Here, the authors show that previous studies may not have sufficiently accounted for this non-independence of nations.

There are two competing hypotheses for the origin of the Indo-European language family. The conventional view places the homeland in the Pontic steppes about 6000 years ago. An alternative hypothesis claims that the languages spread from Anatolia with the expansion of farming 8000 to 9500 years ago. We used Bayesian phylogeographic approaches, together with basic vocabulary data from 103 ancient and contemporary Indo-European languages, to explicitly model the expansion of the family and test these hypotheses. We found decisive support for an Anatolian origin over a steppe origin. Both the inferred timing and root location of the Indo-European language trees fit with an agricultural expansion from Anatolia beginning 8000 to 9500 years ago. These results highlight the critical role that phylogeographic inference can play in resolving debates about human prehistory.

You scratch my back... Despite long-standing interest in explaining and describing diversity in primate social grouping patterns, the evolutionary history of primate sociality has received little attention. Recent advances in statistical methods allow trait changes to be explicitly modelled on phylogenetic trees and competing evolutionary hypotheses to be tested. Shultz et al . use Bayesian comparative phylogenetic methods to test competing theories for the evolution of social behaviour in primates. They conclude that large groups evolved directly from solitary foraging, with pair living and single-male harems being subsequently derived from the large groups. The shift from nocturnal to diurnal living is linked to the origin of sociality. Although much attention has been focused on explaining and describing the diversity of social grouping patterns among primates 1 , 2 , 3 , less effort has been devoted to understanding the evolutionary history of social living 4 . This is partly because social behaviours do not fossilize, making it difficult to infer changes over evolutionary time. However, primate social behaviour shows strong evidence for phylogenetic inertia, permitting the use of Bayesian comparative methods to infer changes in social behaviour through time, thereby allowing us to evaluate alternative models of social evolution. Here we present a model of primate social evolution, whereby sociality progresses from solitary foraging individuals directly to large multi-male/multi-female aggregations (approximately 52 million years (Myr) ago), with pair-living (approximately 16 Myr ago) or single-male harem systems (approximately 16 Myr ago) derivative from this second stage. This model fits the data significantly better than the two widely accepted alternatives (an unstructured model implied by the socioecological hypothesis or a model that allows linear stepwise changes in social complexity through time). We also find strong support for the co-evolution of social living with a change from nocturnal to diurnal activity patterns, but not with sex-biased dispersal. This supports suggestions that social living may arise because of increased predation risk associated with diurnal activity. Sociality based on loose aggregation is followed by a second shift to stable or bonded groups. This structuring facilitates the evolution of cooperative behaviours 5 and may provide the scaffold for other distinctive anthropoid traits including coalition formation, cooperative resource defence and large brains.

Political conservatives' opposition to COVID-19 restrictions is puzzling given the well-documented links between conservatism and conformity, threat sensitivity, and pathogen aversion. We propose a resolution based on the Dual Foundations Theory of ideology, which holds that ideology comprises two dimensions, one reflecting trade-offs between threat-driven conformity and individualism, and another reflecting trade-offs between empathy-driven cooperation and competition. We test predictions derived from this theory in a UK sample using individuals' responses to COVID-19 and widely-used measures of the two dimensions – ‘right-wing authoritarianism’ (RWA) and ‘social dominance orientation’ (SDO), respectively. Consistent with our predictions, we show that RWA, but not SDO, increased following the pandemic and that high-RWA conservatives do display more concerned, conformist, pro-lockdown attitudes, while high-SDO conservatives display less empathic, cooperative attitudes and are anti-lockdown. This helps explain paradoxical prior results and highlights how a focus on unidimensional ideology can mask divergent motives across the ideological landscape.

Understanding the psychological causes of variation in climate change belief and pro-environmental behaviour remains an urgent challenge for the social sciences. The “cooperative phenotype” is a stable psychological preference for cooperating in social dilemmas that involve a tension between individual and collective interest. Since climate change poses a social dilemma on a global scale, this issue may evoke similar psychological processes as smaller social dilemmas. Here, we investigate the relationships between the cooperative phenotype and climate change belief and behaviour with a representative sample of New Zealanders ( N  = 897). By linking behaviour in a suite of economic games to self-reported climate attitudes, we show robust positive associations between the cooperative phenotype and both climate change belief and pro-environmental behaviour. Furthermore, our structural equation models support a motivated reasoning account in which the relationship between the cooperative phenotype and pro-environmental behaviour is mediated by climate change belief. These findings suggest that common psychological mechanisms underlie cooperation in both micro-scale social dilemmas and larger-scale social dilemmas like climate change.

Using economic games, the authors examine the role of religion in the persistence of human cooperation; individuals who claim that their gods are moralizing, punitive and knowledgeable about human affairs are more likely to play fairly towards geographically distant co-religionists. Moralizing gods as force for stability How has cooperation persisted in human societies that are increasingly large and complex, where simple relatedness is not necessarily sufficient? These authors examine the role of religion and gods that are moralizing, punitive, and take a specific interest in human affairs. Using economic games and ethnographic interviews, the authors tested more than 500 individuals following Christianity, Hinduism, Buddhism, and local traditions including animism and ancestor worship. They find that those who believe in gods they perceive as moralistic, punitive and knowing are most likely to be generous to geographically distant co-religionists. Since the origins of agriculture, the scale of human cooperation and societal complexity has dramatically expanded 1 , 2 . This fact challenges standard evolutionary explanations of prosociality because well-studied mechanisms of cooperation based on genetic relatedness, reciprocity and partner choice falter as people increasingly engage in fleeting transactions with genetically unrelated strangers in large anonymous groups. To explain this rapid expansion of prosociality, researchers have proposed several mechanisms 3 , 4 . Here we focus on one key hypothesis: cognitive representations of gods as increasingly knowledgeable and punitive, and who sanction violators of interpersonal social norms, foster and sustain the expansion of cooperation, trust and fairness towards co-religionist strangers 5 , 6 , 7 , 8 . We tested this hypothesis using extensive ethnographic interviews and two behavioural games designed to measure impartial rule-following among people ( n  = 591, observations = 35,400) from eight diverse communities from around the world: (1) inland Tanna, Vanuatu; (2) coastal Tanna, Vanuatu; (3) Yasawa, Fiji; (4) Lovu, Fiji; (5) Pesqueiro, Brazil; (6) Pointe aux Piments, Mauritius; (7) the Tyva Republic (Siberia), Russia; and (8) Hadzaland, Tanzania. Participants reported adherence to a wide array of world religious traditions including Christianity, Hinduism and Buddhism, as well as notably diverse local traditions, including animism and ancestor worship. Holding a range of relevant variables constant, the higher participants rated their moralistic gods as punitive and knowledgeable about human thoughts and actions, the more coins they allocated to geographically distant co-religionist strangers relative to both themselves and local co-religionists. Our results support the hypothesis that beliefs in moralistic, punitive and knowing gods increase impartial behaviour towards distant co-religionists, and therefore can contribute to the expansion of prosociality.

Words on the brink As a language evolves, grammatical rules emerge and exceptions die out. Lieberman et al . have calculated the rate at which a language grows more regular, based on 1,200 years of English usage. Of 177 irregular verbs, 79 became regular in the last millennium. And the trend follows a simple rule: a verb's half-life scales as the square root of its frequency. Irregular verbs that are 100 times as rare regularize 10 times faster. The emergence of a rule (such as adding – ed for the past tense) spells death for exceptional forms. The cover graphic makes the point: verb size corresponds to usage frequency, so large verbs stay at the top, and small verbs fall to the bottom. ' Wed ', the next irregular verb to go, is on the brink. In a separate study, Pagel et al . looked at changing word meanings. Across the Indo-European languages, words like ' tail ' or ' bird ' evolve rapidly and are expressed by many unrelated words. Others, like ' two ', are expressed by closely related word forms across the whole language family. Data from over 80 modern languages show that the more a word is used, the less it changes. Statistical modelling techniques and concepts from biology are applied to linguistic data to show a general and law-like relationship between the frequency with which meanings are used in everyday language and their rate of evolution throughout Indo-European history: the more a word is used, the less likely it is to change over time. The findings demonstrate a fundamental aspect of language evolution that is predicted to apply to all languages. Greek speakers say “ο υρ ”, Germans “ schwanz ” and the French “ queue ” to describe what English speakers call a ‘tail’, but all of these languages use a related form of ‘two’ to describe the number after one. Among more than 100 Indo-European languages and dialects, the words for some meanings (such as ‘tail’) evolve rapidly, being expressed across languages by dozens of unrelated words, while others evolve much more slowly—such as the number ‘two’, for which all Indo-European language speakers use the same related word-form 1 . No general linguistic mechanism has been advanced to explain this striking variation in rates of lexical replacement among meanings. Here we use four large and divergent language corpora (English 2 , Spanish 3 , Russian 4 and Greek 5 ) and a comparative database of 200 fundamental vocabulary meanings in 87 Indo-European languages 6 to show that the frequency with which these words are used in modern language predicts their rate of replacement over thousands of years of Indo-European language evolution. Across all 200 meanings, frequently used words evolve at slower rates and infrequently used words evolve more rapidly. This relationship holds separately and identically across parts of speech for each of the four language corpora, and accounts for approximately 50% of the variation in historical rates of lexical replacement. We propose that the frequency with which specific words are used in everyday language exerts a general and law-like influence on their rates of evolution. Our findings are consistent with social models of word change that emphasize the role of selection, and suggest that owing to the ways that humans use language, some words will evolve slowly and others rapidly across all languages.

Phylogenetic methods were applied to a cross-cultural database of traditional Austronesian societies to test the link between ritual human sacrifice and the origins of social hierarchy—the presence of sacrifice in a society stabilized social stratification and promoted inherited class systems. Human sacrifice and social stratification Attempts to explain the evidence for widespread ritual human sacrifice in early human societies include the social control hypothesis, which holds that human sacrifice legitimizes and stabilizes a stratified society. In this paper, Joseph Watts et al . apply phylogenetic methods to a database of Austronesian traditional societies and find strong support for models in which human sacrifice stabilizes social stratification once stratification has arisen, and promotes a shift to strictly inherited class systems. The authors come to the uncomfortable conclusion that that ritual killing may have helped humans make the transition from life in small egalitarian groups to the large stratified societies that we see today. Evidence for human sacrifice is found throughout the archaeological record of early civilizations 1 , the ethnographic records of indigenous world cultures 2 , 3 , 4 , 5 , and the texts of the most prolific contemporary religions 6 . According to the social control hypothesis 2 , 7 , 8 , human sacrifice legitimizes political authority and social class systems, functioning to stabilize such social stratification. Support for the social control hypothesis is largely limited to historical anecdotes of human sacrifice 2 , 8 , where the causal claims have not been subject to rigorous quantitative cross-cultural tests. Here we test the social control hypothesis by applying Bayesian phylogenetic methods to a geographically and socially diverse sample of 93 traditional Austronesian cultures. We find strong support for models in which human sacrifice stabilizes social stratification once stratification has arisen, and promotes a shift to strictly inherited class systems. Whilst evolutionary theories of religion have focused on the functionality of prosocial and moral beliefs 9 , 10 , our results reveal a darker link between religion and the evolution of modern hierarchical societies 11 , 12 .