Cultural transmission and the evolution of human behaviour: a general approach based on the Price equation

Transmitted culture can be viewed as an inheritance system somewhat independent of genes that is subject to processes of descent with modification in its own right. Although many authors have conceptualized cultural change as a Darwinian process, there is no generally agreed formal framework for defining key concepts such as natural selection, fitness, relatedness and altruism for the cultural case. Here, we present and explore such a framework using the Price equation. Assuming an isolated, independently measurable culturally transmitted trait, we show that cultural natural selection maximizes cultural fitness, a distinct quantity from genetic fitness, and also that cultural relatedness and cultural altruism are not reducible to or necessarily related to their genetic counterparts. We show that antagonistic coevolution will occur between genes and culture whenever cultural fitness is not perfectly aligned with genetic fitness, as genetic selection will shape psychological mechanisms to avoid susceptibility to cultural traits that bear a genetic fitness cost. We discuss the difficulties with conceptualizing cultural change using the framework of evolutionary theory, the degree to which cultural evolution is autonomous from genetic evolution, and the extent to which cultural change should be seen as a Darwinian process. We argue that the nonselection components of evolutionary change are much more important for culture than for genes, and that this and other important differences from the genetic case mean that different approaches and emphases are needed for cultural than genetic processes.     

Modelling the evolution and diversity of cumulative culture

Previous work on mathematical models of cultural evolution has mainly focused on the diffusion of simple cultural elements. However, a characteristic feature of human cultural evolution is the seemingly limitless appearance of new and increasingly complex cultural elements. Here, we develop a general modelling framework to study such cumulative processes, in which we assume that the appearance and disappearance of cultural elements are stochastic events that depend on the current state of culture. Five scenarios are explored: evolution of independent cultural elements, stepwise modification of elements, differentiation or combination of elements and systems of cultural elements. As one application of our framework, we study the evolution of cultural diversity (in time as well as between groups).     

Cultural niche construction and the evolution of small family size

A model of cultural niche construction with two culturally transmitted traits is examined. The frequency of individuals with a certain general predisposition, which is transmitted vertically, plays a role as the cultural background, or the cultural niche, of the population. The cultural background determines the rate of oblique, relative to vertical, transmission of another cultural trait that affects fertility of individuals. It is assumed that individuals with fewer offspring are more likely to achieve social roles that influence the succeeding generation and are therefore overrepresented as transmitters in the process of oblique transmission. Our model suggests that even a slight overrepresentation of those with fewer offspring can drive the evolution of small family size, provided that the rate of oblique transmission depends strongly on the cultural background. In addition, our model may help to explain the time lag between the decrease in death rates and the subsequent decrease in birth rates during the demographic transition of industrializing societies.     

Variable cultural acquisition costs constrain cumulative cultural evolution

One of the hallmarks of the human species is our capacity for cumulative culture, in which beneficial knowledge and technology is accumulated over successive generations. Yet previous analyses of cumulative cultural change have failed to consider the possibility that as cultural complexity accumulates, it becomes increasingly costly for each new generation to acquire from the previous generation. In principle this may result in an upper limit on the cultural complexity that can be accumulated, at which point accumulated knowledge is so costly and time-consuming to acquire that further innovation is not possible. In this paper I first review existing empirical analyses of the history of science and technology that support the possibility that cultural acquisition costs may constrain cumulative cultural evolution. I then present macroscopic and individual-based models of cumulative cultural evolution that explore the consequences of this assumption of variable cultural acquisition costs, showing that making acquisition costs vary with cultural complexity causes the latter to reach an upper limit above which no further innovation can occur. These models further explore the consequences of different cultural transmission rules (directly biased, indirectly biased and unbiased transmission), population size, and cultural innovations that themselves reduce innovation or acquisition costs.     

The swashbuckling anthropologist: Henrich on The Secret of Our Success

In The Secret of Our Success, Joseph Henrich claims that human beings are unique—different from all other animals—because we engage in cumulative cultural evolution. It is the technological and social products of cumulative cultural evolution, not the intrinsic rationality or ‘smartness’ of individual humans, that enable us to live in a huge range of different habitats, and to dominate most of the creatures who share those habitats with us. We are sympathetic to this general view, the latest expression of the ‘California school’s’ view of cultural evolution, and impressed by the lively and interesting way that Henrich handles evidence from anthropology, economics, and many fields of biology. However, because we think it is time for cultural evolutionists to get down to details, this essay review raises questions about Henrich’s analysis of both the cognitive processes and the selection processes that contribute to cumulative cultural evolution. In the former case, we argue that cultural evolutionists need to make more extensive use of cognitive science, and to consider the evidence that mechanisms of cultural learning are products as well as processes of cultural evolution. In the latter case, we ask whether the California school is really serious about selection, or whether it is offering a merely ‘kinetic’ view of cultural evolution, and, assuming the former, outline four potential models of cultural selection that it would be helpful to distinguish more clearly.     

Branching,blending, and the evolution of cultural similarities and differences among human populations

It has been claimed that blending processes such as trade and exchange have always been more important in the evolution of cultural similarities and differences among human populations than the branching process of population fissioning. In this paper, we report the results of a novel comparative study designed to shed light on this claim. We fitted the bifurcating tree model that biologists use to represent the relationships of species to 21 biological data sets that have been used to reconstruct the relationships of species and/or higher level taxa and to 21 cultural data sets. We then compared the average fit between the biological data sets and the model with the average fit between the cultural data sets and the model. Given that the biological data sets can be confidently assumed to have been structured by speciation, which is a branching process, our assumption was that, if cultural evolution is dominated by blending processes, the fit between the bifurcating tree model and the cultural data sets should be significantly worse than the fit between the bifurcating tree model and the biological data sets. Conversely, if cultural evolution is dominated by branching processes, the fit between the bifurcating tree model and the cultural data sets should be no worse than the fit between the bifurcating tree model and the biological data sets. We found that the average fit between the cultural data sets and the bifurcating tree model was not significantly different from the fit between the biological data sets and the bifurcating tree model. This indicates that the cultural data sets are not less tree-like than are the biological data sets. As such, our analysis does not support the suggestion that blending processes have always been more important than branching processes in cultural evolution. We conclude from this that, rather than deciding how cultural evolution has proceeded a priori, researchers need to ascertain which model or combination of models is relevant in a particular case and why.     

DIRECTIONAL CULTURAL CHANGE BY MODIFICATION AND REPLACEMENT OF MEMES

Evolutionary approaches to culture remain contentious. A source of contention is that cultural mutation may be substantial and, if it drives cultural change, then current evolutionary models are not adequate. But we lack studies quantifying the contribution of mutations to directional cultural change. We estimated the contribution of one type of cultural mutations—modification of memes—to directional cultural change using an amenable study system: learned birdsongs in a species that recently entered an urban habitat. Many songbirds have higher minimum song frequency in cities, to alleviate masking by low‐frequency noise. We estimated that the input of meme modifications in an urban songbird population explains about half the extent of the population divergence in song frequency. This contribution of cultural mutations is large, but insufficient to explain the entire population divergence. The remaining divergence is due to selection of memes or creation of new memes. We conclude that the input of cultural mutations can be quantitatively important, unlike in genetic evolution, and that it operates together with other mechanisms of cultural evolution. For this and other traits, in which the input of cultural mutations might be important, quantitative studies of cultural mutation are necessary to calibrate realistic models of cultural evolution.     

A two level mutation-selection model of cultural evolution and diversity

Cultural evolution is a complex process that can happen at several levels. At the level of individuals in a population, each human bears a set of cultural traits that he or she can transmit to its offspring (vertical transmission) or to other members of his or her society (horizontal transmission). The relative frequency of a cultural trait in a population or society can thus increase or decrease with the relative reproductive success of its bearers (individual’s level) or the relative success of transmission (called the idea’s level). This article presents a mathematical model on the interplay between these two levels. The first aim of this article is to explore when cultural evolution is driven by the idea’s level, when it is driven by the individual’s level and when it is driven by both. These three possibilities are explored in relation to (a) the amount of interchange of cultural traits between individuals, (b) the selective pressure acting on individuals, (c) the rate of production of new cultural traits, (d) the individual’s capacity to remember cultural traits and to the population size. The aim is to explore the conditions in which cultural evolution does not lead to a better adaptation of individuals to the environment. This is to contrast the spread of fitness-enhancing ideas, which make individual bearers better adapted to the environment, to the spread of “selfish” ideas, which spread well simply because they are easy to remember but do not help their individual bearers (and may even hurt them). At the same time this article explores in which conditions the adaptation of individuals is maximal. The second aim is to explore how these factors affect cultural diversity, or the amount of different cultural traits in a population. This study suggests that a larger interchange of cultural traits between populations could lead to cultural evolution not improving the adaptation of individuals to their environment and to a decrease of cultural diversity.     

The advantage of multiple cultural parents in the cultural transmission of stories

Recent mathematical modeling of repeated cultural transmission has shown that the rate at which culture is lost (due to imperfect transmission) will crucially depend on whether individuals receive transmissions from many cultural parents or only from one. However, the modeling assumptions leading up to this conclusion have so far not been empirically assessed. Here we do this for the special case of transmission chains where each individual either receives the same story twice from one cultural parent (and retransmits twice to a cultural child) or receives possibly different versions of the story from two cultural parents (and then retransmits to two cultural children). For this case, we first developed a more general mathematical model of cultural retention that takes into account the possibility of dependence of error rates between transmissions. In this model, under quite plausible assumptions, chains with two cultural parents will have superior retention of culture. This prediction was then tested in two experiments using both written and oral modes of transmission. In both cases, superior retention of culture was found in chains with two cultural parents. Estimation of model parameters indicated that error rates were not identical and independent between transmissions; instead, a primacy effect was suggested, such that the first transmission tends to have higher fidelity than the second transmission.     

The role of cultural practices in the emergence of modern human intelligence

Innate cognitive capacities are orchestrated by cultural practices to produce high-level cognitive processes. In human activities, examples of this phenomenon range from everyday inferences about space and time to the most sophisticated reasoning in scientific laboratories. A case is examined in which chimpanzees enter into cultural practices with humans (in experiments) in ways that appear to enable them to engage in symbol-mediated thought. Combining the cultural practices perspective with the theories of embodied cognition and enactment suggests that the chimpanzees' behaviour is actually mediated by non-symbolic representations. The possibility that non-human primates can engage in cultural practices that give them the appearance of symbol-mediated thought opens new avenues for thinking about the coevolution of human culture and human brains.     

Review. The multiple roles of cultural transmission experiments in understanding human cultural evolution

In this paper, we explore how experimental studies of cultural transmission in adult humans can address general questions regarding the 'who, what, when and how' of human cultural transmission, and consequently inform a theory of human cultural evolution. Three methods are discussed. The transmission chain method, in which information is passed along linear chains of participants, has been used to identify content biases in cultural transmission. These concern the kind of information that is transmitted. Several such candidate content biases have now emerged from the experimental literature. The replacement method, in which participants in groups are gradually replaced or moved across groups, has been used to study phenomena such as cumulative cultural evolution, cultural group selection and cultural innovation. The closed-group method, in which participants learn in groups with no replacement, has been used to explore issues such as who people choose to learn from and when they learn culturally as opposed to individually. A number of the studies reviewed here have received relatively little attention within their own disciplines, but we suggest that these, and future experimental studies of cultural transmission that build on them, can play an important role in a broader science of cultural evolution.     

Cultural Transmission and the Diffusion of Innovations: Adoption Dynamics Indicate That Biased Cultural Transmission Is the Predominate Force in Behavioral Change

In challenging the pervasive model of individual actors as cost-benefit analysts who adapt their behavior by learning from the environment, this article analyzes the temporal dynamics of both environmental (individual) learning and biased cultural transmission processes by comparing these dynamics with the robust "5-shaped" curves that emerge from the diffusion of innovations literature. The analysis shows three things: (1) that environmental learning alone never produces the 5-shaped adoption dynamics typically observed in the spread of novel practices, ideas, and technologies; (2) that biased cultural transmission always produces the S-shaped temporal dynamics; and (3) that a combination of environmental learning and biased cultural transmission can generate 5-dynamics but only when biased cultural transmission is the predominate force in the spread of new behaviors. These findings suggest that biased cultural transmission processes are much more important to understanding the diffusion of innovations and sociocultural evolution than is often assumed by most theorists. [ diffusion of innovations, cultural transmission, learning, cultural evolution ]     

Cultural consonance and psychological distress: examining the associations in multiple cultural domains

Cultural consonance is the degree to which individuals, in their own beliefs and behaviors, approximate widely shared cultural models. In previous research in Brazil and the United States we found that higher cultural consonance in the cultural domains of lifestyle and social support was associated with lower psychological distress. The aim of this paper is to expand on these results in two ways. First, the measurement of cultural consonance has been improved through a closer link of cultural domain analyses and survey research. Second, the number of domains in which cultural consonance has been examined has been expanded to include—along with lifestyle and social support—family life, national identity, and food. We found that cultural consonance in these five domains can be conceptualized as two latent variables of generalized cultural consonance, and that this generalized cultural consonance is associated with lower psychological distress. These results continue to support the usefulness of cultural consonance as a theoretical construct in the explanation of human social suffering.     

Theoretical aspects of the mode of transmission in cultural inheritance.

This study investigates how evolutionary factors interact to determine the relative importance of vertical versus nonvertical mode of transmission in cultural inheritance. Simple mathematical models are provided to study the joint evolution of two cultural characters, one determining the viability and the fertility of individuals, and the other determining the vertical transmission rate of the first trait. Ordinary local stability analyses indicate that intrademic processes should lead to a greater reliance on vertical cultural transmission. On the other hand, when newly arisen variants are adaptive and favored in biased cultural transmission, interdemic processes may lead to a decrease in vertical transmission. This is because biased nonvertical transmission may effectively propagate the adaptive variants, further increasing the average growth rate of the population. These results are verified under several distinct sets of assumptions. It is also inferred that the degree and intensity of transmission bias may be the important determinants of cultural processes.     

Secondary theorem of natural selection in biocultural populations.

The "Secondary Theorem of Natural Selection," an extension of Fisher's fundamental theorem, states that the rate of change in the mean of an arbitrary character in response to selection is proportional to the additive genetic covariance between the character and fitness. Here I derive an expression for the change in the mean value of a trait subject to both genetic and cultural transmission. I start with the one-locus case under generalized mating and cultural transmission from parents to offspring, then proceed to the two-locus case. My results support previous work on the effects of nongenetic inheritance by showing that (i) cultural transmission introduces a timelag in the population response to selection; (ii) with cultural transmission the effects of selection persist even after selection is relaxed; and (iii) cultural transmission can either enhance or retard phenotypic evolution relative to that obtained under purely genetic transmission.     

Evolution of cultural communication systems: the coevolution of cultural signals and genes encoding learning preferences

In several communication systems that rely on social learning, such as bird song, and possibly human language, the range of signals that can be learned is limited by perceptual biases--predispositions--that are presumably based on genes. In this paper, we examine the coevolution of such genes with the culturally transmitted communication traits themselves, using deterministic population genetic models. We argue that examining how restrictive genetic predispositions are is a useful way of examining the evolutionary origin and maintenance of learning. Under neutral cultural evolution, where no cultural trait has any inherent advantage over another, there is selection in favour of less restrictive genes (genes that allow a wider range of signals to recognized). In contrast, cultural conformity (where the most common cultural trait is favoured) leads to selection in favour of more restrictive genes.     

The Health Consequences of Cultural Consonance: Cultural Dimensions of Lifestyle, Social Support, and Arterial Blood Pressure in an African American Community

Cultural dimensions of health and behavior have been difficult to study because of limited theoretical and methodological models linking the cultural, the individual, and the biological. We employ a cognitive theory of culture to understand culture and health in an African American community in the southern United States. First, cultural consensus analysis is used to test for shared cultural models of lifestyles and social supports within the community. Then, the theoretical and operational construct of "cultural consonance" is used to assess the degree to which individuals behave in a way consistent with cultural models. Findings indicate that cultural consonance in lifestyle and social support combine synergistically in association with blood pressure. These associations of cultural consonance and health are not altered by taking into account a variety of other variables, indicating an independent association of cultural dimensions of behavior with health status. Implications of these results for culture theory are discussed, [culture theory, culture consensus analysis, cultural consonance, African American community, arterial blood pressure]     

Behavioural variation in 172 small-scale societies indicates that social learning is the main mode of human adaptation

The behavioural variation among human societies is vast and unmatched in the animal world. It is unclear whether this variation is due to variation in the ecological environment or to differences in cultural traditions. Underlying this debate is a more fundamental question: is the richness of humans’ behavioural repertoire due to non-cultural mechanisms, such as causal reasoning, inventiveness, reaction norms, trial-and-error learning and evoked culture, or is it due to the population-level dynamics of cultural transmission? Here, we measure the relative contribution of environment and cultural history in explaining the behavioural variation of 172 Native American tribes at the time of European contact. We find that the effect of cultural history is typically larger than that of environment. Behaviours also persist over millennia within cultural lineages. This indicates that human behaviour is not predominantly determined by single-generation adaptive responses, contra theories that emphasize non-cultural mechanisms as determinants of human behaviour. Rather, the main mode of human adaptation is social learning mechanisms that operate over multiple generations.     

Studying Cultural Evolution at the Tips: Human Cross-cultural Ecology

While human genetic variation is limited due to a bottleneck on the origin of the species ～200 kya, cultural traits can change more rapidly, and may do so in response to the variation in human habitats. Does cultural diversification simulate a natural experiment in evolution much like biodiversity so that cultural divergences and convergences can be interpreted in terms of the differences and similarities of local environments? Or is cultural diversity simply the result of human behavioral flexibility? Although the majority of cultural data comes from the tips of the hominin phylogeny, anthropologists can follow the example of evolutionary ecologists, who often compare the endpoints of phylogenies when that is all that is available. This article compares 97 contemporary indigenous language communities from around the world, and 24 of their cultural traditions, to help determine whether human cultures and their cultural traits are proportionately dispersed, as predicted by the neutral theory of biodiversity, or whether they show non-proportionalities that could be explained with evolutionary reasoning.     

The cultural contagion of conflict

Anecdotal evidence abounds that conflicts between two individuals can spread across networks to involve a multitude of others. We advance a cultural transmission model of intergroup conflict where conflict contagion is seen as a consequence of universal human traits (ingroup preference, outgroup hostility; i.e. parochial altruism) which give their strongest expression in particular cultural contexts. Qualitative interviews conducted in the Middle East, USA and Canada suggest that parochial altruism processes vary across cultural groups and are most likely to occur in collectivistic cultural contexts that have high ingroup loyalty. Implications for future neuroscience and computational research needed to understand the emergence of intergroup conflict are discussed.