Wilson’s target article illustrates how evolutionary hypotheses are advancing the science of complex cultural systems. We agree. The following extends the conversation to consider the benefits of evolutionary methods. We restrict our review to computational phylogenetic methods as these are being used to test evolutionary hypotheses about religions.
Why cultural phylogenetics?
Offspring resemble their parents because offspring share parental genes. Computational phylogenetic modeling of genetic lineages has enabled researchers to test hypotheses about the timing, sequence, and rate of evolutionary change in genetic sequences, advancing understanding about how complex biological systems evolve and function (Yang and Rannala 2012).
In humans, offspring also resemble their parents because offspring acquire parental culture (Boyd & Richerson 1985; Sterelny 2005). Consider language dialects. A child raised in a community of exclusive Canadian speakers will not utter “rot” for “red,” though matters differ in German-speaking communities. Yet the resemblance between “rot” and “red” is not arbitrary. Both dialects trace their origins about 1,500 years ago to Old High German. The properties of cultural evolution enable the modeling of cultural lineages using bayesian approaches developed to model genetic lineages. A compelling virtue of phylogenetic approaches is their capacity to deal with the statistical non-independence of culturally acquired traits –“Galton’s problem” – which invalidates standard statistical tests (Mace & Pagel, 1994). Comparative phylogenetic methods account for variation explained by shared ancestry, avoiding Galton’s problem (Felsenstein, 1985). Preliminary applications to the evolutionary study of culture have produced striking results (Mace, Holden et al. 2005, Pagel 2009, Gray, Atkinson et al. 2011). Phylogenetic studies have explained the ancestral roots of modern languages (Atkinson 2011), social influences on rates of word change (Pagel, Atkinson et al. 2007), the evolution of grammars (Dunn, Greenhill et al. 2011), and the geographical location of ancestral language homelands (Bouckaert, Lemey et al. 2012). Applications have been far-reaching, bringing new understanding to ancestral migrations in the Pacific (Gray, Drummond et al. 2009), the rise and fall of social complexity (Currie, Greenhill et al. 2010), and the evolution and diffusion of social norms (Fortunato, Holden et al. 2006, Jordan, Gray et al. 2009).
Cultural Phylogenetics and Religious Cooperation
Can computational phylogenetic methods be used to test functional hypotheses about religion? Evolutionary researchers hypothesize that religious cultures are exquisite designs for cooperation, which both motivate and signal cooperative intentions. In a recent article, Luke Matthews used computational phylogenetic methods to test model for religious cooperation that he calls “recognition signaling” (Matthews 2012). According to recognition signaling, culturally acquired traits enable religious partners to discriminate between cooperators who belong to a group and defectors who might imitate belonging. Discrimination is possible because it is difficult to simultaneously acquire knowledge of many religious characters at one time [for a similar model see (Mahoney 2008).] Cultural phylogenetics is appropriate for testing recognition signaling predicts the model predicts that religious group schisms will be associated with an increase in religious character change. The expected increase in character change arises from the demands of religious brand differentiation. By hypothesis, the more traits that distinguish religious groups, the easier for members to identify who belongs. Matthews tested recognition signaling’s prediction for increasing differentiation at historical schisms by first producing a database of sixty-four religious characters that existing Christian denominations might have or not have. Examples of such characters are whether a denomination supports iconography, whether the sick are anointed, and whether Jesus is believed to have been immaculately conceived. The histories of Christian denominations are known, which enabled Matthews to reconstruct their phylogenetic ancestries on exact trees. Matthews then used comparative phylogenetic methods to model rates of change in religious characters over time. Correlational analysis empowered Matthews to evaluate whether higher rates of change occurred at schism events. Matthews found that the rate of change in culturally acquired religious traits tended to become faster precisely at the point where religious groups divide. In line with the recognition signaling hypothesis, diversification in characters tended to occur near historical schisms.
Importantly, Matthews identified the religious characters for his study in consultation with a religious studies expert, and his phylogenetic tree was populated with data gathered from historical texts. Rather than consigning historians to unemployment, cultural phylogenetics reveals their central importance for addressing basic scientific questions about how religious cultures work.
Cultural Phylogenetics and Religious Violence
The extent to which religious people acquire religious traits from “horizontal” influences is an empirical question that comparative phylogenetic methods can help to answer. In the case of language evolution, cultural phylogenetic methods have proved remarkably robust to horizontal transmission (Currie, Greenhill et al. 2010). Yet perhaps the transmission properties of languages and of religions differ. To better disentangle the contributions of inherited influences from those of horizontal influences, Matthews and colleagues conducted a second study examining sixteenth century Anabaptist groups (Matthews, Edmonds et al. 2012). The authors identified forty-four characters in eighteen Anabaptist groups, and coded these characters using binary variables (has or does not have the character). Phylogenies were developed for this period from known schism events in Anabaptist denominations. Comparative phylogenetics enabled the team to estimate the amount of character change along each branch of these known phylogenies. Forty-seven leaders were identified from historical sources, and leader networks were reconstructed. The authors were especially interested in the dark side of within-group religious cooperation: between-group religious violence. To examine how attitudes to violence and religious orientations are transmitted, the team used logistic regression mixed models to compare the effects of inherited culture and of leader networks for practical and abstract religious orientations.
Results were intriguing. The best-fitting models showed that phylogenetic inheritance was a better predictor of denominational attitudes to violence, yet leader networks were a better predictor of similarities in theological beliefs and practices. Although this is a small study, which was restricted to a relatively brief episode in religious history, the team’s finding suggests that religious violence might be subject to adaptive lag. Strategies from a violent past appear to be transmitted to contexts where they might prove harmful or even lethal. This finding repeats the important point that evolutionary approaches to religion do not imply that religions are always and everywhere functional (Richerson and Boyd 2005, Wilson 2008, Sosis 2009). That attitudes to violence are vertically transmitted may also hold important lessons for peacemakers. Instead of wire tapping the lines of religious leaders, the intelligence community might better spend their time consulting historians about the violent histories (Matthews, per. comm.)
As Wilson argues, evolutionary hypotheses must be tested on a case by case basis. Though evolutionary hypotheses for religion abound, they are only beginning to be put to the test (Bulbulia and Slingerland 2012). To fully understand the role that religion plays in the emergence of human societies we need to trace religious, social, and environmental variation between societies, and assess the functional role that religion played over the course of human history. Cross-cultural comparisons of religions, moreover, must account for the statistical non-independence of cultural traits (Galton’s problem). Computational phylogenetics affords powerful toolkit for addressing evolutionary questions about religion precisely because computational phylogenetics relates the historical and functional properties of religious cultures using statistically appropriate methods. Preliminary applications have proved encouraging, showing that religions harbor exquisite functional designs for within-group cooperation. Yet early findings also reveal constraints on these designs. Religious conflict might be inherited from a denomination’s past. Far from rendering classically trained historians of religion obsolete, computational phylogenetics discloses their central importance. Early studies herald the beginnings of what we believe will amount to phylogenetic revolution in the study of religious cultures. By testing evolutionary hypotheses for religion using phylogenetic methods, a ubiquitous, ancient, and still mysterious dimension of the human condition is slowly divulging its secrets.
This research was supported by the John F. Templeton Foundation: Testing the functional roles of religion in human society – ID: 28745 and The Royal Society of New Zealand: “The cultural evolution of religion:” 11-UOA-239.
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Victoria University of Wellington
Wellington, New Zealand
Australian National University
School of Culture, History & Language
University of Auckland
School of Psychology
Auckland New Zealand