Laura Fortunato

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IMBS Colloquium

“The Evolution of the Human Family”

Thursday, April 19, SSPA 2112, 4:00 – 5:00 p.m.

Compared to other species, humans show a remarkable degree of variation in family organization. This talk presents recent advances in the application of evolutionary thinking to the study of the human family, focusing on the evolution of monogamous marriage.

First, I present the results of a game-theoretic model investigating the co-evolution of marriage and wealth inheritance strategies. The analysis shows that where resources are transferred across generations, monogamous marriage may be advantageous because it "concentrates" wealth in a limited number of heirs. It may also be advantageous because a female may grant her husband higher probability of paternity if he marries monogamously, leading to exclusive investment of his resources in her offspring. This may explain why monogamous marriage prevailed across societies of Europe and Asia practicing intensive agriculture, and why it first emerged in these regions: here land was limited and the partitioning of estates depleted their value. Consistently, cultural norms promoting high paternity, such as ideologies of virginity and sexual fidelity, were common in these societies.

Second, I present the results of two case studies testing specific predictions of the theoretical model. One case study focuses on evaluating the prediction of an "early" origin of monogamous marriage, linked to the development of intensive modes of production such as plough agriculture. In agreement with this prediction, phylogenetic comparative analysis of marriage strategies across Indo-European-speaking societies reconstructs monogamous marriage as the ancestral state. The other case study focuses on the predicted association between monogamous marriage and norms stipulating the transfer of wealth to a man’s wife’s offspring, as opposed to alternative inheritance strategies. This prediction is supported by analysis of variation in marriage and inheritance strategies across a world-wide sample of societies, while controlling for the confounding effects of Christianization.

Finally, I discuss implications of these findings for our understanding of the evolution of human family systems.

University College London

Laura Fortunatocompleted a PhD student with Ruth Mace at the

Department of Anthropology
University College London
14 Taviton Street
London WC1H 0BW, UK
tel: +44 (0) 20 7679 5463

She is now an Omidyar Fellow at SFI

http://www.santafe.edu/news/item/fortunato-origin-monogamous-marriage/

Selected publications

Abstract: Accurate reconstruction of prehistoric social organization is important if we are to put together satisfactory multidisciplinary scenarios about, for example, the dispersal of human groups. Such considerations apply in the case of Indo-European and Austronesian, two large-scale language families that are thought to represent Neolithic expansions. Ancestral kinship patterns have mostly been inferred through reconstruction of kin terminologies in ancestral proto-languages using the linguistic comparative method, and through geographical or distributional arguments based on the comparative patterns of kin terms and ethnographic kinship ‘facts’. While these approaches are detailed and valuable, the processes through which conclusions have been drawn from the data fail to provide explicit criteria for systematic testing of alternative hypotheses. Here, we use language trees derived using phylogenetic tree-building techniques on Indo-European and Austronesian vocabulary data. With these trees, ethnographic data and Bayesian phylogenetic comparative methods, we statistically reconstruct past marital residence and infer rates of cultural change between different residence forms, showing Proto-Indo-European to be virilocal and Proto-Malayo-Polynesian uxorilocal. The instability of uxorilocality and the rare loss of virilocality once gained emerge as common features of both families.
For the Eurasian language tree, see Pagel, Mark, and Andrew Meade. 2005. Chapter 13: Bayesian Estimation of Correlated Evolution across Cultures: A Case Study of Marriage Systems and Wealth Transfer at Marriage, pp. 235-256, in, The Evolution of Cultural Diversity: A Phylogenetic Approach. Eds Ruth Mace, Claire J. Holden and Stephen Shennan. Walnut Creek, CA: Left Coast Press.
Abstract: The majority of human societies allow polygynous marriage, and the prevalence of this practice is readily understood in evolutionary terms. Why some societies prescribe monogamous marriage is however not clear: current evolutionary explanations—that social monogamy increases within-group co-operation, giving societies an advantage in competition with other groups—conflict with the historical and ethnographic evidence. We show that, within the framework of inclusive fitness theory, monogamous marriage can be viewed as the outcome of the strategic behaviour of males and females in the allocation of resources to the next generation. Where resources are transferred across generations, social monogamy can be advantageous if partitioning of resources among the offspring of multiple wives causes a depletion of their fitness value, and/or if females grant husbands higher fidelity in exchange for exclusive investment of resources in their offspring. This may explain why monogamous marriage prevailed among the historical societies of Eurasia: here, intensive agriculture led to scarcity of land, with depletion in the value of estates through partitioning among multiple heirs. Norms promoting high paternity were common among ancient societies in the region, and may have further facilitated the establishment of social monogamy. In line with the historical and ethnographic evidence, this suggests that monogamous marriage emerged in Eurasia following the adoption of intensive agriculture, as ownership of land became critical to productive and reproductive success.
Introduction: Behavioural ecology is the branch of biology dealing with the study of animalbehavioural variation within and across taxa; it addresses questions about the functionof behaviour, focusing on its survival value in relation to the environment(Tinbergen’s (1963) ‘why’ questions; Krebs and Davies 1993: 382). Humanbehavioural ecology investigates variation in human behaviour, including culturalbehaviour, within and across societies (Winterhalder and Smith 2000).Investigation into the function of behaviour involves making hypotheses based onobservations, deriving testable predictions from the hypotheses, and testing. Threestrategies are available for testing functional hypotheses: examination of variationamong individuals within a group, experiments, and examination of variation amonggroups (Krebs and Davies 1993: 24). Given that human behaviour is generally notamenable to experimental manipulation, the study of human behavioural variation islargely restricted to comparison within and among groups.In this paper we provide an example of the application of the behavioural ecologyapproach to the study of variation in cultural practices across groups, focusing onwealth transfers at marriage. In the first part, we review the observations that led tothe evolutionary interpretation of these practices, and previous analyses of theirdistribution; in the second part, we illustrate the use of a phylogenetic comparativemethod with data on wealth transfers at marriage in Indo-European groups
  • Laura Fortunato, 2009. Evolution of monogamous marriage Presentation. IGERT Program in Evolutionary Modeling. Model-Based Approaches to Biological and Cultural Evolution.
  • Laura Fortunato, 2008. A phylogenetic approach to the history of cultural practices.Allen, N. J., Callan, H., Dunbar, R. & James, W. (eds.). Early human kinship: from sex to social reproduction. Blackwell Publishing Ltd.: Malden, MA. 189-199
Abstract: Significant amounts of wealth have been exchanged as part of marriage settlements throughout history. Although various models have been proposed for interpreting these practices, their development over time has not been investigated systematically. In this paper we use a Bayesian MCMC phylogenetic comparative approach to reconstruct the evolution of two forms of wealth transfers at marriage, dowry and bridewealth, for 51 Indo-European cultural groups. Results indicate that dowry is more likely to have been the ancestral practice, and that a minimum of four changes to bridewealth is necessary to explain the observed distribution of the two states across the cultural groups.
Key words Ancestral states - Bayesian MCMC phylogenetic and comparative methods - Bridewealth - Dowry - Indo-European - Marriage transfers

Ethnographic Database Project

Caution: Many of the links here do not have return links so they are dead ends. Work did not proceed once the PI took a 3-year Postdoc at SFI.

Her Ethnographic Database Project is hosted at UCL EDP: "The EDP is a web-based tool for the collection of comparative ethnographic data. It allows anthropologists to enter data about their field research using a set of standard codes (also see the SCCS) developed for cross-cultural application.

At present, the codes included in the EDP relate to a society’s organisation, kinship and marriage practices, subsistence economy, and pattern of sexual division of labour."

On-line questionnaire (for ethnographers)

opinio learning tools -UCL


Bayesian phylogenetic comparative methods

John P. Huelsenbeck, Bret Larget, Richard E. Miller and Fredrik Ronquist. 2002. Potential Applications and Pitfalls of Bayesian Inference of Phylogeny. Syst Biol (2002) 51 (5): 673-688.

Absract: Although many of the statistical techniques used in comparative biology were originally developed in quantitative genetics, subsequent development of comparative techniques has progressed in relative isolation. Consequently, many of the new and planned developments in comparative analysis already have well-tested solutions in quantitative genetics. In this paper, we take three recent publications that develop phylogenetic meta-analysis, either implicitly or explicitly, and show how they can be considered as quantitative genetic models. We highlight some of the difficulties with the proposed solutions, and demonstrate that standard quantitative genetic theory and software offer solutions. We also show how results from Bayesian quantitative genetics can be used to create efficient Markov chain Monte Carlo algorithms for phylogenetic mixed models, thereby extending their generality to non-Gaussian data. Of particular utility is the development of multinomial models for analysing the evolution of discrete traits, and the development of multi-trait models in which traits can follow different distributions. Meta-analyses often include a nonrandom collection of species for which the full phylogenetic tree has only been partly resolved. Using missing data theory, we show how the presented models can be used to correct for nonrandom sampling and show how taxonomies and phylogenies can be combined to give a flexible framework with which to model dependence.
Hadfield, J. 2010. MCMC methods for multi-response generalised linear mixed models: The MCMCglmm R package. Journal of Statistical Software (in press).
Hadfield, J.D. 2008. Estimating evolutionary parameters when viability selection is operating. Proc. R. Soc. Biol. Sci. Ser. B 275: 723–734.
Abstract:Some individuals die before a trait is measured or expressed (the invisible fraction), and some relevant traits are not measured in any individual (missing traits). This paper discusses how these concepts can be cast in terms of missing data problems from statistics. Using missing data theory, I show formally the conditions under which a valid evolutionary inference is possible when the invisible fraction and/or missing traits are ignored. These conditions are restrictive and unlikely to be met in even the most comprehensive long-term studies. When these conditions are not met, many selection and quantitative genetic parameters cannot be estimated accurately unless the missing data process is explicitly modelled. Surprisingly, this does not seem to have been attempted in evolutionary biology. In the case of the invisible fraction, viability selection and the missing data process are often intimately linked. In such cases, models used in survival analysis can be extended to provide a flexible and justified model of the missing data mechanism. Although missing traits pose a more difficult problem, important biological parameters can still be estimated without bias when appropriate techniques are used. This is in contrast to current methods which have large biases and poor precision. Generally, the quantitative genetic approach is shown to be superior to phenotypic studies of selection when invisible fractions or missing traits exist because part of the missing information can be recovered from relatives.