The Department of Human Behavior, Ecology and Culture investigates the role of culture in human evolution and adaptation. The primary goal is to theorize, design, and conduct longitudinal studies of human adaptation and cultural dynamics in ecological context. By integrating cross-cultural fieldwork with mathematical models and advanced quantitative methods, we hope to contribute to the interdisciplinary study of human evolution and human social dynamics.
Please see the Projects and Fieldwork links on the left for much more information.
Human marriage systems, characterized by long-term partnerships and extended windows of parental care, differ from the mating systems of pulsed or seasonally breeding non-human animals in which Bateman’s principles were originally tested. These features, paradigmatic of but not unique to humans, complicate the accurate measurement of mating success in evaluating Bateman’s three principles. Here, we unpack the concept of mating success into distinct components: number of partners, number of years partnered, the timing of partnerships, and the quality of partners. Drawing on longitudinal records of marriage and reproduction collected in a natural-fertility East African population over a 20-year period, we test and compare various models of the relationship between mating success and reproductive success (RS), and show that an accurate assessment of male and female reproductive behaviour requires consideration of all major components of mating success. Furthermore, ... Read full article here.
Long-lasting, romantic partnerships are a universal feature of human societies, but almost as ubiquitous is the risk of instability when one partner strays. Jealous response to the threat of infidelity is well studied, but most empirical work on the topic has focused on a proposed sex difference in the type of jealousy (sexual or emotional) that men and women find most upsetting, rather than on how jealous response varies1,2. This stems in part from the predominance of studies using student samples from industrialized populations, which represent a relatively homogenous group in terms of age, life history stage and social norms3,4. To better understand variation in jealous response, we conducted a 2-part study in 11 populations (1,048 individuals). In line with previous work, we find a robust sex difference in the classic forced-choice jealousy task. However, we also show substantial variation in jealous response across populations. Read full article here.
Human-induced rapid environmental change (HIREC) poses threats to a variety of species, and if or how it changes phenotypes is a question of central importance bridging evolutionary ecology and conservation management. Social learning is one type of phenotypic plasticity that can shape organismal responses to HIREC; it allows organisms to acquire phenotypes on a timescale that closely tracks environmental change while minimizing the costs of individual learning. A common assumption in behavioral ecology, is that social learning is generally an adaptive way to cope with HIREC by facilitating the rapid spread of innovative responses to change. While this can be true, social learning can also be maladaptive. It may hinder the spread of adaptive behavior by causing a carryover of old, no longer adaptive behaviors that slow the response to HIREC or even promote the spread of maladaptive behaviors. Here, we present a conceptual framework outlining how an organism's evolutionary history can shape cognitive mechanisms, social behavior, and population composition, which in turn affect how an organism responds to HIREC. Read full article here.
We present a statistical approach—a custom-built hidden Markov model (HMM)—that is broadly applicable to the analysis of temporally clustered display events, as found in many animals, including
birds, orthopterans, and anurans. This HMM can simultaneously estimate both the expected lengths of each animal’s display bouts and their within-bout display rates. We highlight the HMM’s ability to estimate changes in animals’ display effort over time and across different social contexts, using data from male greater sage grouse (Centrocercus urophasianus). Male display effort was modeled across three sites in two experimental treatments (robotic female simulating interested or uninterested behavior) and in the presence or absence of live females. Across contexts, we show that sage grouse males primarily adjust their bout lengths rather than their within-bout display rates. Males’ responses to female behavior were correlated with male mating success: males with more matings showed high display persistence regardless of female behavior, whilemales with fewer matings
tended to invest selectively in females that were already showing interest in mating. Additionally, males with higher mating success responded more to the presence of a female than males with fewer
matings did. We conclude with suggestions for adapting our HMM approach for use in other animal systems. Read the full article.
Human-induced rapid environmental change (HIREC) has recently led to alterations in the fitness and behavior of many organisms. Game theory is an important tool of behavioral ecology for analyzing evolutionary situations involving multiple individuals. However, game theory bypasses the details by which behavioral phenotypes are determined, taking the functional perspective straight from expected payoffs to predicted frequencies of behaviors. In contrast with optimization approaches, we identify that to use existing game theoretic models to predict HIREC effects, additional mechanistic details (or assumptions) will often be required. We illustrate this in relation to the hawk-dove game by showing that three different mechanisms, each of which support the same ESS prior to HIREC (fixed polymorphism, probabilistic choice, or cue dependency), can have a substantial effect on behavior (and success) following HIREC. Read the full article.
Machines powered by artificial intelligence increasingly mediate our social, cultural, economic and political interactions. Understanding the behaviour of artificial intelligence systems is essential to our ability to control their actions, reap their benefits and minimize their harms. Here we argue that this necessitates a broad scientific research agenda to study machine behaviour that incorporates and expands upon the discipline of computer science and includes insights from across the sciences. We first outline a set of questions that are fundamental to this emerging field and then explore the technical, legal and institutional constraints on the study of machine behaviour. Read the full article.
The emergence of large-scale cooperation during the Holocene remains a central problem in the evolutionary literature. One hypothesis points to culturally evolved beliefs in punishing, interventionist gods that facilitate the extension of cooperative behaviour toward geographically distant coreligionists. Furthermore, another hypothesis points to such mechanisms being constrained to the religious ingroup, possibly at the expense of religious outgroups. To test these hypotheses, we administered two behavioural experiments and a set of interviews to a sample of 2228 participants from 15 diverse populations. These populations included foragers, pastoralists, horticulturalists, and wage labourers, practicing Buddhism, Christianity, and Hinduism, but also forms of animism and ancestor worship. Read the full article.
Identity fusion theory has become a popular psychological explanation of costly self-sacrifice. It posits that while maintaining one’s own individual identity, a deep affinity with one’s group can contribute to sacrifice for that group. We test this and related hypotheses using a behavioral economic experiment designed to detect biased, self-interested favoritism among eight different populations ranging from foragers and horticulturalists to the fully market-integrated. We find that while individuals favor themselves on average, those with higher ingroup fusion sacrifice more money to other members of their ingroup who are unable to reciprocate. We also find that positive outgroup relations has a similar effect. Additionally, we assess a recently-posited interaction between ingroup and outgroup relations and show no consistent effect at the individual or sub-sample levels. Read the full article.
Does the experience of war increase people’s religiosity? Much evidence supports the idea that particular religious beliefs and ritual forms can galvanize social solidarity and motivate in-group cooperation, thus facilitating a wide range of cooperative behaviours including—but not limited to—peaceful resistance and collective aggression. However, little work has focused on whether violent conflict, in turn, might fuel greater religious participation. Here, we analyse survey data from 1,709 individuals in three post-conflict societies—Uganda, Sierra Leone and Tajikistan. The nature of these conflicts allows us to infer, and statistically verify, that individuals were quasirandomly afflicted with different intensities of war experience—thus potentially providing a natural experiment. We then show that those with greater exposure to these wars were more likely to participate in Christian or Muslim religious groups and rituals, even several years after the conflict. Read the full article.
Acoustic signals, shaped by natural and sexual selection, reveal ecological and social selection pressures. Examining acoustic signals together with morphology can be particularly revealing. But this approach has rarely been applied to primates, where clues to the evolutionary trajectory of human communication may be found. Across vertebrate species, there is a close relationship between body size and acoustic parameters, such as formant dispersion and fundamental frequency (f0). Deviations from this acoustic allometry usually produce calls with a lower f0 than expected for a given body size, often due to morphological adaptations in the larynx or vocal tract. An unusual example of an obvious mismatch between fundamental frequency and body size is found in the two closest living relatives of humans, bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). Read the full article.
Adults sometimes disperse, while philopatric offspring inherit the natal site, a pattern known as bequeathal. Despite a decades‐old empirical literature, little theoretical work has explored when natural selection may favor bequeathal. We present a simple mathematical model of the evolution of bequeathal in a stable environment, under both global and local dispersal. We find that natural selection favors bequeathal when adults are competitively advantaged over juveniles, baseline mortality is high, the environment is unsaturated, and when juveniles experience high dispersal mortality. However, frequently bequeathal may not evolve, because the fitness cost for the adult is too large relative to inclusive fitness benefits. Read the full article.
Habitual reliance on tool use is a marked behavioural difference between wild robust (genus Sapajus) and gracile (genus Cebus) capuchin monkeys. Despite being well studied and having a rich repertoire of social and extractive foraging traditions, Cebus sp. rarely use tools and have never been observed using stone tools. By contrast, habitual tool use by Sapajus is widespread. We review theory and discuss factors which might explain these differences in patterns of tool use between Cebus and Sapajus. We then report the first case of habitual stone tool use in a gracile capuchin: a population of whitefaced capuchins (Cebus capucinus imitator) in Coiba National Park, Panama who habitually rely on hammerstone and anvil tool use to access structurally protected food items in coastal areas including Terminalia catappa seeds, hermit crabs, marine snails, terrestrial crabs and other items. Read the full article.
Monogamy appears to have become the predominant human mating system with the emergence of highly unequal agricultural populations that replaced relatively egalitarian horticultural populations, challenging the conventional idea—based on the polygyny threshold model—that polygyny should be positively associated with wealth inequality. To address this polygyny paradox, we generalize the standard polygyny threshold model to a mutual mate choice model predicting the fraction of women married polygynously. We then demonstrate two conditions that are jointly sufficient to make monogamy the predominant marriage form, even in highly unequal societies. We assess if these conditions are satisfied using individual-level data from 29 human populations. Read the full article.
It is often observed that human culture, unlike most other animal culture, is cumulative: Human technology and behavior is more complex than any individual could invent in their own lifetime. Cumulative culture is often explained by appeal to a combination of highfidelity social learning and innovation, the “ratchet effect.” What is often overlooked is that both human and other animal cultures are supported by a more primary ratchet effect that retains and increases the prevalence of adaptive behavior. This primary ratchet can arise without appeal to specialized cognitive adaptations and is plausibly more widespread in animal societies. We use a simple model to highlight how simple forms of contingent social learning can create the primary ratchet effect, dramatically increasing the prevalence of adaptive, hard-to-invent behavior. Read the full article.
Analyses of racial disparities in police use-of-force against unarmed individuals are central to public policy interventions; however, recent studies have come to apparently paradoxical findings concerning the existence and form of such disparities. Although anti-black racial disparities in U.S. police shootings have been consistently documented at the population level, new work has suggested that racial disparities in encounter-conditional use of lethal force by police are reversed relative to expectations, with police being more likely to: (1) shoot white relative to black individuals, and (2) use non-lethal as opposed to lethal force on black relative to white individuals. Encounter- and use-of-force-conditional results, however, can be misleading if the rates with which police encounter and use non-lethal force vary across officers and depend on suspect race. We find that all currently described empirical patterns in the structuring of police use-of-force—including the “reversed” racial disparities in encounter-conditional use of lethal force—are explainable under a generative model in which there are consistent and systemic biases against black individuals. Read the full article.
Studies of eusocial invertebrates regard complex societies as those where there is a clear division of labour and extensive cooperation between breeders and helpers. In contrast, studies of social mammals identify complex societies as those where differentiated social relationships influence access to resources and reproductive opportunities. We show here that, while traits associated with social complexity of the first kind occur in social mammals that live in groups composed of close relatives, traits associated with the complexity of social relationships occur where average kinship between female group members is low. Read the full article.
Sizing up human brain evolution
An innovative computational analysis of factors that might have influenced human brain evolution suggests that ecological, rather than social, factors had a key role in the evolution of large, rapidly developing brains. Read the full article.
Human sociality depends upon the benefits of mutual aid and extensive communication. However, diverse norms and preferences complicate mutual aid, and ambiguity in meaning hinders communication. Here we demonstrate that these two problems can work together to enhance cooperation through the strategic use of deliberately ambiguous signals: covert signaling. Covert signaling is the transmission of information that is accurately received by its intended audience but obscured when perceived by others. Such signals may allow coordination and enhanced cooperation while also avoiding the alienation or hostile reactions of individuals with different preferences. Read the full article.
Researchers have recently proposed that “moralistic” religions—those with moral doctrines, moralistic supernatural punishment, and lower emphasis on ritual—emerged as an effect of greater wealth and material security. One interpretation appeals to life history theory, predicting that individuals with “slow life history” strategies will be more attracted to moralistic traditions as a means to judge those with “fast life history” strategies. As we had reservations about the validity of this application of life history theory, we tested these predictions with a data set consisting of 592 individuals from eight diverse societies. Read the full article.
Dr Anne Kandler and her co-author Dr Adam Powell have published a review paper on methods for inferring cultural evolutionary mechanisms from data. One of the major challenges in cultural evolution is to understand why and how various forms of social learning are used in human populations, both now and in the past. The authors demonstrate the applicability and utility of generative inference approaches to the field of cultural evolution. The framework advocated uses observed population-level frequency data directly to establish the likely presence or absence of particular hypothesized learning strategies. Read the full article.
New work from Dr Anne Kandler and colleagues sheds light on the cultural evolutionary dynamics of cryptocurrencies. Kandler and colleagues consider the history of 1469 cryptocurrencies introduced between April 2013 and May 2017. They reveal that, while new cryptocurrencies appear and disappear continuously and their market capitalization is increasing (super-)exponentially, several statistical properties of the market have been stable for years. These include the number of active cryptocurrencies, market share distribution and the turnover of cryptocurrencies. Adopting an ecological perspective, they show that the so-called neutral model of evolution is able to reproduce a number of key empirical observations. Read the full article.
Neutral evolution assumes that there are no selective forces distinguishing different variants in a population. Many studies have sought to assess whether neutrality can provide a good description of different episodes of cultural change. One approach has been to test whether neutral predictions are consistent with observed progeny distributions, recording the number of variants that have produced a given number of new instances within a specified time interval: a classic example is the distribution of baby names. New work from HBEC scientist Dr Anne Kandler and colleague Dr James O'Dwyer shows that these distributions consist of two phases: a power-law phase followed by an exponential cut-off for variants with very large numbers of progeny. They show that analyses based on only the most popular variants, as is often the case in studies of cultural evolution, can provide misleading evidence. While neutrality provides a plausible description of progeny distributions of abundant variants, rare variants deviate from neutrality. Read the full article.
Despite the global headlines emphasizing division and conflict, humans actually have a long history of forming friendships across group boundaries. But which criteria do they use for picking friends from a different group? In collaboration with three populations of horticulturalists in Bolivia, researchers of the Max Planck Institute for Evolutionary Anthropology and the University of California Santa Barbara found that we use very similar criteria when choosing friends from among in-group and out-group strangers – individual cooperative qualities are most important in both cases. Only when it comes to dividing limited resources, qualities associated with a group can affect partner preference. Full press release. Access the paper.
Dr Anne Kandler has published a new analysis of the potential for inferring learning mechanisms from population-level data. Patterns of cultural change depend upon individual-level strategies. Inferring these strategies from data is complicated by the paucity of individual-level data. Researchers commonly attempt to infer strategies instead of available population-level data. Kandler and her co-authors show that such data are useful in limited, but valuable, ways. These results represent a substantial advance and refine thinking about both the limits and potential of using aggregate data to infer individual processes.
Jeremy Koster and Richard McElreath have published a new statistical framework for analyzing behavioral data, applying the technique to human data from Koster's field site. The method accepts the multinomial nature of behavior data and also accounts for repeated observations. It is also capable of revealing individual-level trade-offs across behaviors, allowing for models that reveal the extent to which individuals who regularly engage in one behavior also exhibit relatively more or less of another behavior. These models can potentially be applied to a broad class of statistical analyses by behavioral ecologists, focusing on other polytomous response variables, such as behavior, habitat choice, or emotional states.
New publication by Dr John Bunce, Interethnic Interaction, Strategic Bargaining Power, and the Dynamics of Cultural Norms.
Dr Bunce addresses processes of ethnic boundary maintenance and change, using ethnographic data from his fieldwork in lowland Amazonia, in combination with Bayesian models of normative behavior. Code and data.
New analysis published on a collaboration between HBEC and the Edward Grey Institute at Oxford: Conformity does not perpetuate suboptimal traditions in a wild population of songbirds. Conformist learning can be adaptive, but it may be maladaptive when environments change. Using a field experiment, the researchers document the social learning strategies that allow a wild population to adapt to environmental change. A mix of conformity and innovation is found in the analysis and demonstrated in theory to be broadly adaptive. Code and data.
New analysis of social learning in a wild primate, by Brendan Barrett, Richard McElreath, and Susan Perry. Using field experiments, they document the spread of behavioral traditions and analyze the learning strategies that may account for the spread. The analysis suggests that payoff-biased social learning strongly influences behavior. Data and code.