Mark Granovetter
No strong tie is a bridge
The people closest to you know what you know
Granovetter 1973, on bridges
It sounds like a turn of phrase. It's a structural result, and here is what it rests on.
What actually happened?
Granovetter states it flatly: no strong tie is a bridge. The reason isn't sentiment, it's geometry. If you're close to A and close to B, then A and B probably know each other — close people introduce each other, and the circle closes. Once it's closed, the three of you know largely the same things, and none of you is a route to anywhere outside. Only rarely-used ties can be bridges, because only they have non-overlapping networks at each end.
Closure is what strong ties inevitably produce
This isn't a question of social strategy, it's geometry: closeness means frequent contact, and frequent contact makes your friends meet each other. Closure itself is good — it's where support and trust come from — but it also means information circulates rather than arrives.
A tight team is superbly informed internally and the slowest in the building to notice what's changing outside.
To test whether a tie is a bridge, look at the overlap
The test is simple: if this person knows most of the people you know, he isn't a bridge; if you share very few mutual contacts, he probably is. A bridge's value has nothing to do with how close you are and everything to do with the overlap at each end.
The person you share two mutual friends with is likelier to bring you something new than the one you share thirty with.
A group's health shows in how many bridges it has
Applied to a team: an organisation made entirely of strong ties is warm inside and slowest to react to anything outside. What needs protecting is the few people who don't quite fit in and spend time elsewhere. They are the bridges, and bridges are what cohesion squeezes out first.
The one who's always off with another department is usually the door news comes through.
How do I use it today?
Where you are: you want yourself or your team to hear sooner what's happening outside.
Ask first: how many lines do I have into circles that don't overlap with mine? Name them. If you can't name one, build a bridge before deepening what you already have.
Where it goes wrong: using this to dismiss close ties — they supply support and trust, a different job entirely; or setting out to 'network outward', which makes the tie a task, and those don't hold.
Lines to keep
No strong tie is a bridge.
Close to A and close to B? A and B probably know each other.
Bridges are what a group squeezes out first when it wants cohesion.