Social Networks, Dunbar’s Number and Terrorism Robin Dunbar Institute of Cognitive & Evolutionary Anthropology University of Oxford Social Brain Hypothesis Primates have unusually big brains because they live in unusually complex societies Social group size [and a lot of social behaviour as well] correlates with relative neocortex volume On this basis, predicted group size for humans is ~150 “Dunbar’s Number” Dunbar (1992); Dunbar & Shultz (2007) “Reverse” Small World Experiments These all have mean sizes of 100-200 Neolithic villages 6500 BC 150-200 military units (company) (N=10) 180 Hutterite communities (N=51) 107 Nebraska Amish parishes (N=8) 113 business organisation <200 ideal church congregations <200 Doomsday Book villages 150 C18th English villages 160 GoreTex Inc’s structure 150 Research sub-disciplines (N=13) 100-200 Small world experiments (N=2) 134 Hunter-Gatherer communities 148 Xmas card networks 154 Killworth et al (1984) Hunter-Gatherer Societies Dunbar (1993) Individual Tribes Xmas Card Networks Hill & Dunbar (2003) Sizes of Hunter-Gatherer Groupings The Fractal Periodicity of Human Group Sizes Peak at ω=5.4 Social Groupings Database [N=60] Hamilton et al (2007) Peak at ω=5.2 Xmas Card Database Scaling ratio = exp(2π/ω) = 3.2 and 3.3 Zhou, Sornette, Hill & Dunbar (2005) Our Social World is Very Small Circles of Aquaintanceship 150 represents the limit on the number of reciprocated personalised relationships Beyond that, relationships are oneway or formalised Intensity EGO 5 15 50 150 500 1500 How We Get Higher Levels of Organisation War of Spanish Succession [1901-1714] Family up to company level Hierarchy and discipline thereafter Modern Army Organisation Section Platoon Company Battalion Brigade/Regiment Division USA [1994] Australia [2010] 10 30 126 650 4000 12,500 12 45 168 775 3750 15,000 How Primates [and Humans] Bond Groups A two-process mechanism involving: • Endorphins create a sense of belonging • Cognition [mentalising] An experimental creates a sense study with monkeys of obligation Opiates block social drive; Sal Opiate-blockers enhance social drive Keverne et al (1979) Mentalising [Theory of Mind] allows us to manage small intimate groups The Cognitive Limits to Sociality [Stiller & Dunbar 2007] Achievable intentionality level correlates with clique size …and with volume of core areas in frontal lobe associated with managing emotions and social behaviour Powell et al (in press); Powell et al (in review); Lewis et al (in review) Stay KIN Friends Move Change in Network Layer Change over Time 0 9 months Roberts & Dunbar (2010) 18 Kin Friends How to Prevent Relationships Decaying: A Tale of Two [very different] Genders Change in contact frequency Kin Friends Roberts & Dunbar (2010) Change in activities done together Face-to-Face Contact Assuming knowledge is rare Cumulative probability of finding out: Cum(Pk) = Σ[pi x Ni] Asymptotic value at 1500 Optimal network size [defined by point of inflexion] is N=50 1/e Dunbar (2010) Very small and very large foundations don’t survive well Optimum size is 50-150 For secular communes, optimal size may even be smaller (~50) Dunbar & Sosis (in prep) 50 Conclusions The human social world is VERY small scale (~150) This small world is highly structured in terms of both kinship and friendship [which are very different] On the small scale, organisations can work effectively informally because members are committed to the common project ….but on the large scale, they have to have line management (and imposed discipline?) Line management is easily disrupted, but commitment is not
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