A model for the evolutionary diversification of religions

Abstract

We address the problem of cultural diversification by studying selection on cultural ideas that colonize human hosts and using diversification of religions as a conceptual example. In analogy to studying the evolution of pathogens or symbionts colonizing animal hosts, we use models for host–pathogen dynamics known from theoretical epidemiology. In these models, religious content colonizes individual humans. Rates of transmission of ideas between humans, i.e., transmission of cultural content, and rates of loss of ideas (loss of belief) are determined by the phenotype of the cultural content, and by interactions between hosts carrying different ideas. In particular, based on the notion that cultural non-conformism can be negative frequency-dependent (for example, religion can lead to oppression of lower classes and emergence of non-conformism and dissent once a religious belief has reached dominance), we assume that the rate of loss of belief increases as the number of humans colonized by a particular religious phenotype increases. This generates frequency-dependent selection on cultural content, and we use evolutionary theory to show that this frequency dependence can lead to the emergence of coexisting clusters of different cultural types. The different clusters correspond to different cultural traditions, and hence our model describes the emergence of distinct descendant cultures from a single ancestral culture in the absence of any geographical isolation.

Introduction

Evolution can occur whenever there are units of reproduction that produce other such units which inherit some characteristics of the parent units. If the units of reproduction vary in their reproductive output, there will be evolutionary change. “Intellectual content” can satisfy these simple requirements. An idea or a theory can be viewed as colonizing the brain of an individual human (or animal). It can develop or mutate within that brain, and it can be passed on to the brains of other individuals, thereby reproducing itself. Such reproduction typically occurs with modification and through mixing and amalgamation with other ideas present in the human population. For a multitude of potential reasons, both cognitive and selective in nature (Henrich, 2009, Henrich et al., 2008, Richerson and Boyd, 2005), some ideas and theories are more successful at such reproduction through transmission than others, hence there is typically differential reproductive success. As a consequence, there is cultural evolution of intellectual content such as ideas and theories.

Such a perspective of cultural evolution has been lucidly advocated by Cavalli-Sforza and Feldman (1981) and Boyd and Richerson (1985), and by now there is a rather large body of literature on cultural evolution (see e.g. Richerson and Boyd, 2005). In particular, population genetic models have been adopted early on to study cultural evolution (Boyd and Richerson, 1985, Cavalli-Sforza and Feldman, 1981). On the other hand, an epidemiological perspective of cultural content colonizing human brains has been advocated in various verbal narratives by a number of researchers (e.g. Dietz, 1967, Sperber, 1996). However, despite some early attempts (e.g. Cavalli-Sforza and Feldman, 1981), it seems that to date this epidemiological perspective has not been rigorously adopted as a basis for the mathematical modeling of cultural evolution. There is a large body of theoretical and mathematical literature on the modeling of the epidemiological dynamics of pathogens in humans and many other animals. While one naturally has to be careful in adopting such models for cultural evolution, we believe that such epidemiological models might be well suited to study cultural dynamics (Dietz, 1967).

Here we apply mathematical epidemiological models to address the problem of cultural diversification. It seems perhaps relatively easy to understand how cultural differentiation can develop between human populations that live in isolation from each other (e.g. on different continents), because different climatic and biological circumstances may lead different cultural content to thrive in different areas of the globe. Much thought has been given to understanding what happens when cultures come into contact after they have diversified in isolation. Such an approach would attempt to determine the “winners” among a preexisting set of different cultures (e.g. Diamond, 2005, Lim et al., 2007). This approach is roughly equivalent to studying “species selection” between already established species and foregoes the question of how diversity arose in the first place within a single culture.

However, cultural differentiation also seems to occur when people adopting diverging cultures live together, which appears to have been an important mode of cultural diversification throughout history. Several instances of diversification in religion may serve as paradigms for such processes. For example, the split of the protestant from the catholic church in the 16th century occurred from within an essentially entirely catholic culture, and despite some subsequent spatial segregation of the diverging religions (due, among other things, to violent conflicts), the two religions essentially coexisted since the split. It has been argued that this split was caused by a decline in the moral authority of the catholic leadership (Tuchman, 1985), i.e., by processes occurring within the catholic church that led certain people to overcome the peer pressure to conform to the existing doctrine, and generate and/or become susceptible to new religious ideas. Thus, processes within the catholic church may have created conditions that favoured the emergence of a dissident religious strain. In a more recent example, Whitehouse (1995) has observed an ongoing splitting off of minor sectarian movements from a mainstream religious organization in Papua New Guinea. In the sociological literature, such a new branch of religion that is formed from within and in coexistence with the old religion is often referred to as a “sect” (Stark, 1996), while a following generated by an original set of ideas, sufficiently unrelated to any existing religion (at all, or typical to a particular geographical area) is defined as a “cult”. In this language, our the models presented here would (loosely) refer to the formation of new sects rather than new cults.

In particular, we propose to model cultural diversification arising without spatial segregation. We use the emergence of new religions, or sects, as a schematic backdrop for informing our modeling assumptions, but conceptually, our approach can be used to describe general scenarios for the evolution of new sets of cultural content (e.g. languages or ideologies) from a single ancestral culture. Borrowing from the epidemiological literature, our models incorporate human individuals as hosts for religious content. Just as in traditional epidemiological models, religious content can be lost by a human host, and it can be transmitted between human hosts. We use established mathematical techniques from evolutionary theory to describe the dynamics of host populations that harbour a variable set of religious ideas that are, for simplicity, described by a continuous real variable. The trait value of a religious idea determines the idea's propensity of being lost by their human hosts, as well as their success in colonizing susceptible hosts. The basic question we address concerns the distribution of religious values: when does a unimodal distribution, corresponding to a human population adopting a single religion, split into different modes, corresponding to coexisting subpopulations adopting different religions. Overall, the purposes of our paper are twofold. On the one hand, following mostly verbal narratives of epidemiological terminology in the theory of human culture, we advocate a more explicit use of quantitative epidemiological modeling to understand the dynamics of cultural evolution. On the other hand, we illustrate the potential usefulness of such modeling by presenting models for gradual cultural diversification of an ancestral culture into different and coexisting descendant lineages. In analogy to organismic evolution, where the theory of adaptive diversification in the absence of geographical isolation has received considerable attention in the past decade (Dieckmann and Doebeli, 1999), such processes of cultural diversification occur due to frequency-dependent selection on cultural content. Our models are both general and simplistic, and we view them as a conceptual proof of principal. Nevertheless, we hope that these models serve the purpose of illustrating how the general mechanism of frequency-dependent non-conformism can give rise to cultural diversity without spatial segregation. Studying potential diversification as a consequence of non-conformism appears to be interesting, because seeds of non-conformism seem to be present in almost any cultural setting, ranging from languages to politics, economics, and religion.