Peter Sheridan DoddsRoby MuhamadDuncan J. Watts

Columbia University

An Experimental Study of Search

in Global Social Networks

Motivation and Method

“Small World” hypothesis is poorly understoodIndividuals in real social networks have only limited, local

information about the global social networkFinding short paths represents a nontrivial search effortExperimental evidence for short global chain lengths is

extremely limitedStudies specific to e-mail networks is limited to within single

institutions

Internet-based social search experimentParticipants registered online and were allocated one targetThere were 18 target persons from 13 countriesTask was to relay a message to their target by passing the

message to a social acquaintance whom they considered “closer” than themselves to the target

InformationStatistics

98,847 individuals registered25% provided their personal information and initiated

message chainsthe participation rate after the first step increased to an

average of 37%data were recorded on 61,168 individuals from 166 countries24,163 distinct message chains

Information collected about the chosen contactName and e-mail addressHow he or she had come to know the personType and strength of the relationshipWhy they considered their nominated acquaintance a suitable

recipient

Observations

Senders typically used friendships in preference to business or family ties

Half of these friendships were formed through either work or school affiliations

In successful chains “casual” and “not close” ties were chosen more frequently than in unsuccessful chains . “Weak” ties are responsible for social connectivity

ObservationsMen passed messages more frequently to other

men (57%), and women to other women (61%)Geography dominated earlier in the chain but

after the 3rd step occupation was the mostNo evidence of message “funneling” through a

single acquaintance of the targetThe compounding effects of attrition over multiple

links resulted in exponential attenuation of chainsChains may have terminated

Randomly, individual apathy or disinclination to participate

Chains get “lost” or are otherwise unable to reach their targets

Preferentially at short chain lengths, the longer the better

Observations

SummaryExperiment suggests a mixed picture of search in

global social networks.All targets may be reachable from random initial

senders in only a few stepsSmall differences in either participation rates or the

chain lengths can have a dramatic impact on reachabilityTarget 5 (a professor) had lowest attrition rate

Slight increase in incentives can make social searches successful

Network structure alone is not everything. Actions, strategies, and even perceptions of the individuals embedded in the network are significant.

R. A. HillUniversity of Durham

andR. I. M. Dunbar

University of Liverpool

SOCIAL NETWORK SIZE IN HUMANS

MotivationSocial network in humans

Hard to define an individual’s “network”Earlier studies focused on total network size (250 - 5,000 individuals)Little attention paid to the interactions in the network

Social network in primatesEasy to define an individual’s “network”Relationships are characterized by intense social grooming

NeocortexPart of the brain that is involved in higher functions Sensory perception, motor commands, spatial reasoning, conscious

thought and language

Relationship between Social network and NeocortexPrimate group size and grooming clique size are a function of

neocortical volumeSimilar relationships have been reported for carnivores and cetaceans

MotivationCognitive constraint

Volume of neocortical neurons limits an organism’s information processing capacity

Affects the number of social relationships that an individual can monitor simultaneously

Cognitive group size for humans?Size of the human neocortex is knownRelationship between group size and neocortex size in

primates is knownHumans should live in social groups of 150 individualsTribal census data confirms this prediction

Will modern, postindustrial societies exhibit a similar pattern?Studies have estimated the maximum network size

Little information on which relationships are valued The way in which networks are maintained

What is the way in which humans may actively maintain contact?

MethodQuestionnaire

Filled in by individuals while sending out Christmas cards

Small study group due to complex/lengthy questionnaire

One questionnaire per householdAll respondents were British Caucasians

Data collected from RespondentAge and GenderIdentity of individuals living in the householdIndividuals to whom they regularly send cardsIndividuals they expect to see at Christmas

NoteGender was not taken into account in the experiment

Table 1: Information Requested on Questionnaire for Each Christmas Card

Results

43 questionnaires were returnedTotal of 2,984 Christmas cardsMean network size

Couples and Families: 68.19 (range 11–149)Total individuals: 153.5 (± 84.5)Actively contacted: 124.9 (± 68.0)

Results• Relationship between

maximum network size and age.

• Mean proportion of total network made up of different household types for five age categories.

Time since last contact increases as distance to the individual increasesdecreases as emotional closeness increasesdecreases if the individual is a work colleaguedecreases if the contact is overseasdecreases as the coefficient of relatedness

increasesincreases with age

Results

The probability of a letter being included with a Christmas card increases with distance to contact increases with emotional closenessdecreases with genetic relatedness

Results

“Passive” factors that influence frequency of contact between network members independent of any intentional contactdistance to individualwhether the contact is a work colleague whether the contact lives overseas

Results

hierarchy on the basis of frequency of contact

• inflection points at 7, 20, and 35 individuals• further grouping at 70 and 100 individuals

Emotional closeness is related to frequency of contact

Time since last contact declines as the level of emotional closeness increases

Results

SummaryChristmas card networks

provide useful insights into human social networksare utilized to make contact with all those

important individualsTotal network sizes estimated are close to 150

Similar to the one predicted for humans based on the brain size across primates

Even in contemporary egocentric networks, size and differentiation are similar to traditional societies

Cognitive constraints on network size apply to all modern humans