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

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”

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.

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.

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.

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

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

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

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

Example of triads

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

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

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

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

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

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

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

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

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.

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