1. The Strength of Weak Ties
Mark S. Granovetter
The American Journal of Sociology, 1973

2. Introduction
“One of the most influential sociology papers ever written” (Barabasi)
One of the most cited (Current Contents, 1986)
Interviewed people and asked: “How did you find your job?”
Kept getting the the same answer: “through an acquaintance, not a friend”

3. Context macro patterns micro behavior
Social mobility, community organization
micro behavior
Interactions within small groups
It is interesting to offer explanations for how simple processes at the micro-level of individual nodes and links can have complex effects that ripple through a macro-level of population as a whole.

4. Triadic closure
(Main Idea) If two people in a social network have a friend in common, then there is an increased likelihood that they will become friends themselves at some point in the future
Edges BC, FG, ED are results of triadic closure.
DG is not a result of triadic closure.

5. Bridge and local bridge
AB is a bridge. The removal
of AB partitions the network
AB is a local bridge. The removal of AB
increases the distance between A, B.

6. Bridges and local bridges
Bridges play an important role of the micro => macro puzzle. For example,
Bridges allow diffusion of information between otherwise disconnected communities.
Local bridges bring otherwise distant communities together

7. Strong and Weak Ties Strong tie =>
Links can have a wide range of possible strengths, but for conceptual simplicity we’ll categorize all links in the social network as belonging to one of two types:
stronger links, correspond to friend, and
weaker links, correspond to acquaintances
Strong tie =>
lots of time together
lots of opportunity for B to interact with A’s friends
greater chance that B will be “compatible” with A’s friends

8. Forbidden Triad s s A A s s
(Granovetter said) The Strong Triadic Closure Property is violated if a node A has strong ties with two other nodes B and C, but there is no edge (strong or weak) between B and C. Otherwise the property is satisfied. New edge need not be strong
Any time strong tie A-B exists, then all of A’s strong ties will be at least weakly connected to B (Granovetter said) s s A A s s

9. Triadic closure with string and weak ties
Can AB be a local bridge?
Local bridges correspond to weak ties.

10. Example of triads

11. All Bridges are Weak Ties!
Proof:
If A-B and A-C are strong, then forbidden triad implies that B-C is at least weak
If A-B is deleted, then A can still reach B via A-C-B
All local bridges are also weak ties
Proof is identical

12. Implications
Removal of weak ties raises path lengths more than removal of strong ties
Assume: probability of info passing successfully between two node is proportional to the number of paths connecting the two nodes
Conclusion: Removal of a weak edge damages the connectivity more than the removal of a strong edge

13. Tipping Point
An individual’s uptake of a new technique depends on how many of those around him have “bought in”
The “Tipping Point” (Gladwell, 2000)
Quickly adopted techniques must be rapidly spread to many cliques

14. Tipping Point
People with many weak ties critical to spreading the idea
Example: Mass Hysteria in Textile Factory
Earliest people “infected” were:
friends with very few
acquaintances with many
Acted as “seeders”, rapidly disseminating idea to many friend circles at once

15. Community Co-ordination
Imagine a community organizing to defeat a common threat
Requires organization and leadership
Leadership requires trust in the leaders
Trust is difficult without a connection

16. Community Co-ordination
Without weak links, community exists as a set of strongly connected, but disjoint cliques
No one suitable to act as a leader for all
Example: Boston West End
Connections were mainly family-based
Few ways for weak links to be formed

17. Access to Resources
Our weak ties are with people whose ties are with those socially distant to us.
Weak ties bring us knowledge of our community not available through friends
Many weak ties => more access to wider community’s ideas, resources, etc.
Few weak ties => little information of outside world

18. Access to Resources Example: Academic Hiring
School’s reluctance to hire your own PhD’s
Want to prevent “intellectual inbreeding”

19. Finding a Job
Do leads for new jobs come through strong or weak contacts?
Strong: More motivation to help you, since they know you better
Weak: Likely less overlap with leads you can easily get elsewhere
Study by author shows that weak wins
Most job referrals come through those who we see rarely: old school friends, former co-workers, etc.

20. Conclusion Opposite to what you might expect:
Weak personal relationships bind communities together
Exclusively strong ties lead to global fragmentation