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The 'Dunbar Number', Evolution and Cognition
Friday 13 November 2015
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The 'Dunbar Number', Evolution and Cognition

Friday 13 November 2015

Archaeology G6 LT, Institute of Archaeology, 31-34 Gordon Square, London WC1H 0PY

This event is free, but tickets must be booked. To book tickets please go to

Chair: Professor Rob Foley (Cambridge)

10.00am - Welcome and opening remarks

Paper One

“Primate social evolution and the anti-social myth”
Dr Susanne Shultz (Manchester)

Human social cognition underpins our ultra-sociality by promoting and stabilizing cooperation, collaboration, social norms, and division of labor. In contrast, animal social cognition is often couched in Machiavellian terms involving self-regarding behaviors such as social maneuvering or deception. This talk explored this apparent disparity and how it deemphasises the importance of prosocial behaviours in animal social cognition, including coordination, consensus building, and efficient information use. Moreover, the strong association between primate brain size, group size and prosocial behaviours, rather than technical competence or deception, suggests that the historic interest in anti-social behaviours misses the most cognitively challenging aspects of group life.

Paper Two

“How Many Homo Heidelbergensis does it take to hunt a lightbulb?
Group size, co-operative hunting, and the evolution of modern intelligence.”
Dr Sam Smith & Dr Simon Underdown (Oxford Brookes)

Markers of behavioural modernity in the fossil and archaeological records are the subject of much debate in human evolution. Tick lists of ‘human only’ abilities are subject to almost continual revision as hitherto unobserved abilities are observed in non-human primates, dolphins and other species. Anatomical modernity and behavioural modernity are still seen by many as distinct events in the evolution of Homo sapiens despite an over reliance on a patchy and late archaeological record. In this paper we argue that behavioural modernity was present by at least Homo heidelbergensis (sensu lato) around 500,000 years ago and is evidenced by this species’ ability to plan co-operative hunting events that would have required theory of mind and probably rudimentary symbolic language. In light of this we posit that behavioural modernity was a shared primitive trait in both Homo sapiens and the Neanderthals and can be seen clearly in the archaeological and fossils records of the Middle Pleistocene.

11.30-11.50am - Coffee and tea

Paper Three

“Ego-Centred Networks, Community Size and Cohesion: Dunbar’s Number and a Mandara Mountains Conundrum”.
James H. Wade

Dunbar’s contention that ‘In dispersed societies, individuals will meet less often and will thus be less familiar with each, so group sizes should be smaller in consequence’ rings true for the montagnard sorghum farmers of the Mandara Mountains. However, the further contention made by some proponents of Dunbar’s Number that groups significantly larger than Dunbar's number require bureaucracy and/or force to maintain social cohesion is more questionable. A combination of dispersed powers and ego-centred networks generated within the trajectory of the Fali of the southern Mandaras is shown to achieve a high degree of voluntary cohesion underpinning large nucleated settlements, in the absence of both significant force and anything approaching a bureaucracy.

Paper Four

“About the curious power of Dialogue”
Dr Esther Goody (Cambridge)

Ever since spending a year at the Wissenschafts Kolleg in Berlin in 1990-1, reading around classic social anthropological material, I have been fascinated by the puzzle of how did only Homo sapiens, and not other primates, come to have spoken language. I was intrigued by Robin Dunbar's ideas about shifts in primate group size, leading to augmenting grooming with vocality. Why not?  Since there seems to be agreement that Homo sapiens' ability to speak is unique among mammals, this raises the question:  How has using language influenced the shaping of human societies? My own research has looked at ways that routinization of dialogue both expresses and shapes patterns of interaction. As S.F. Nadel argued, the key to the link between systematic interaction patterns and social structure is roles. Of course dialogue frames role interaction differently in each society. The paper seeks to give depth to understanding dialogue by using two kinds of research findings: (i) Published detailed accounts of routinized dialogue – questions; greetings; joking relationships; prayer, etc. (ii) Observations and findings from a longitudinal study of L1/L2 literacy learning in nine Ghanaian government primary schools. Observations included different kinds of dialogue between teachers and pupils.

Reference: Social Intelligence and Interaction, E. N Goody (Ed), CUP, 1995

Lunch – 1.10pm-2.00pm

Paper Five

“Time: An ‘obvious’ constraint on the social organisation of primates”
Dr Russell Hill (Durham)

In exploring the evolution of sociality in primates (including humans), Robin Dunbar’s work has focussed on understanding both the constraints on social group size and the structure and dynamics of contemporary human social networks.  Here I show how these themes have shaped my own research agenda.  Initially I review how Dunbar’s time budget models have helped us understand patterns of primate group size, and show how these time budget models may perform better than the traditional approaches to mapping species’ distributions used in many branches of ecology.  In so doing, the behavioural data underpinning these models provides a more detailed understanding of the ways in which ecology limits social organisation.  Time also imposes important constraints on the structure of social networks within groups, with natural networks structured in a series of layers with each subsequent level of the network approximately 3 times the size of the preceding (smaller, more intimate) grouping level.  Evidence from traditional and contemporary human populations supports this network structuring, but I also illustrate how this holds true for many non-human species in hierarchical societies (including gelada, the focus of Robin’s PhD thesis).

Paper Six

“The Prehistory of the Social Brain”
Dr Mat Grove (Liverpool)

Research into the Social Brain Hypothesis has demonstrated that contemporary human societies are structured according to a hierarchically inclusive series of group sizes, with each larger group approximately three times the size of the last. That this pattern is found in both agricultural and hunter-gather societies suggests that it may be a fundamental organizing principle persisting despite differences in subsistence, settlement, and mobility. Whilst some other mammals show broadly similar patterns, the antiquity of the human inclusive hierarchy remains unclear. Using statistics originally developed to assess the validity of the postulated ‘Neolithic Yard’, this paper examines the size distribution of British Late Neolithic and Early Bronze Age stone circles in an attempt to infer prehistoric group size distributions. Results suggest that circles occur at frequencies inversely proportional to their size and that, where discrete size clusters can be identified, a constant scaling ratio describes the relationship between clusters. These results are consistent with the conclusion that hierarchically inclusive grouping patterns were present in later prehistoric British societies, and suggest further refinements of the Social Brain Hypothesis for use with archaeological data.

3.20-3.40pm - Coffee and tea

Paper Seven

“Religion and psychosis: a common evolutionary trajectory?”
Dr Simon Dein (QMUL), Prof Roland Littlewood (UCL)

In this presentation we propose that schizophrenia and religious cognition engage cognate mental modules in the over-attribution of agency and the overextension of theory of mind. We argue similarities and differences between assumptions of ultrahuman agents with omniscient minds and certain ''pathological'' forms of thinking in schizophrenia: thought insertion, withdrawal and broadcasting, and delusions of reference. In everyday religious cognition agency detection and theory of mind modules function ''normally,'' whereas in schizophrenia both modules are impaired. It is suggested that religion and schizophrenia have perhaps had a related evolutionary trajectory.

Paper Eight

“Dunbar’s other number: The evolution of primate monogamy”
Dr Kit Opie (University College London)

Dunbar started work on monogamy much earlier than his work on the relationship between groups and brains, but first in klipspringer (1974). He then switched to primates, showing that neither space use nor paternal care could be the explanation for monogamy in Gibbons (1990) or Callitrichids (1995), leaving infanticide as the only feasible driver. Other researchers continued to suggest that female space use could be a general explanation for monogamy across all mammals. But primates are unusual among mammals because monogamy has evolved in all the major clades and is present in 30% of extant species compared to 5% for mammals as a whole. Something different is happening in primates, just as Dunbar suggested. By combining trait data across 230 primate species with Bayesian phylogenetic analyses it has been possible to test for correlated evolution between mating systems and a number of traits to evaluate the differing drivers of monogamy. I will show evidence for correlated evolution between mating systems and both female ranging patterns and bi-parental care, but the most compelling explanation for the appearance of monogamy is male infanticide. Monogamy evolves only in the presence of infanticide, but once it is established there is a subsequent shift to paternal care and discrete female ranges. The evolution of primate monogamy can be explained by long lactation periods due to slow development, making primate infants vulnerable to infanticide by males. Just as Dunbar argued, these analyses support a key role for infanticide in the evolution of primate social systems.

5.00pm-6.00pm - Closing session: Robin Dunbar

6.00pm - Reception: All welcome!