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