1. Peter Sheridan Dodds Roby Muhamad Duncan J. Watts Columbia University An Experimental Study of Search in Global Social Networks

2. Motivation and Method “Small World” hypothesis is poorly understood Individuals in real social networks have only limited, local information about the global social network Finding short paths represents a nontrivial search effort Experimental evidence for short global chain lengths is extremely limited Studies specific to e-mail networks is limited to within single institutions Internet-based social search experiment Participants registered online and were allocated one target There were 18 target persons from 13 countries Task 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

3. Information Statistics 98,847 individuals registered 25% provided their personal information and initiated message chains the participation rate after the first step increased to an average of 37% data were recorded on 61,168 individuals from 166 countries 24,163 distinct message chains Information collected about the chosen contact Name and e-mail address How he or she had come to know the person Type and strength of the relationship Why they considered their nominated acquaintance a suitable recipient

4. 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

5. Observations Men passed messages more frequently to other men (57%), and women to other women (61%) Geography dominated earlier in the chain but after the 3 rd step occupation was the most No evidence of message “funneling” through a single acquaintance of the target The compounding effects of attrition over multiple links resulted in exponential attenuation of chains Chains 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

6. Observations

7. Summary Experiment suggests a mixed picture of search in global social networks. All targets may be reachable from random initial senders in only a few steps Small differences in either participation rates or the chain lengths can have a dramatic impact on reachability Target 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.

8. R. A. Hill University of Durham and R. I. M. Dunbar University of Liverpool SOCIAL NETWORK SIZE IN HUMANS

9. Motivation Social 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 primates Easy to define an individual’s “network” Relationships are characterized by intense social grooming Neocortex Part of the brain that is involved in higher functions Sensory perception, motor commands, spatial reasoning, conscious thought and language Relationship between Social network and Neocortex Primate group size and grooming clique size are a function of neocortical volume Similar relationships have been reported for carnivores and cetaceans

10. Motivation Cognitive 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 known Relationship between group size and neocortex size in primates is known Humans should live in social groups of 150 individuals Tribal 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?

11. Method Questionnaire Filled in by individuals while sending out Christmas cards Small study group due to complex/lengthy questionnaire One questionnaire per household All respondents were British Caucasians Data collected from Respondent Age and Gender Identity of individuals living in the household Individuals to whom they regularly send cards Individuals they expect to see at Christmas Note Gender was not taken into account in the experiment

12. Table 1: Information Requested on Questionnaire for Each Christmas Card

13. Results 43 questionnaires were returned Total of 2,984 Christmas cards Mean network size Couples and Families: 68.19 (range 11–149) Total individuals: 153.5 (± 84.5) Actively contacted: 124.9 (± 68.0)

14. Results Relationship between maximum network size and age. Mean proportion of total network made up of different household types for five age categories.

15. Time since last contact increases as distance to the individual increases decreases as emotional closeness increases decreases if the individual is a work colleague decreases if the contact is overseas decreases as the coefficient of relatedness increases increases with age Results

16. The probability of a letter being included with a Christmas card increases with distance to contact increases with emotional closeness decreases with genetic relatedness Results

17. “Passive” factors that influence frequency of contact between network members independent of any intentional contact distance to individual whether 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

18. Emotional closeness is related to frequency of contact Time since last contact declines as the level of emotional closeness increases Results

19. Summary Christmas card networks provide useful insights into human social networks are utilized to make contact with all those important individuals Total 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