The nature of social networks and how they affect us has been explored over the last two centuries, but American sociologist Mark Granovetter's paper The Strength of Weak Ties, published in 1973, transformed the way that scientists thought about networks.
Granovetter interviewed a group of people to find out how social networks are used to land a new job, and discovered that the best leads for job opportunities are more likely to come from more distant acquaintances (weak ties) rather than close friends (strong ties). The reason is that information flow is reduced within your closest circle of friends - they are more likely to be similar to you and have access to roughly the same information.
Granovetter concludes that weak ties are indispensable to individuals' opportunities and integration into communities. Strong ties, on the other hand, breed local cohesion but lead to overall fragmentation - confining opportunities to a much smaller network.
"Seen from a more macroscopic vantage, weak ties play a role in effecting social cohesion," he states. Weak 'bridging links' between different networks increases information flow and opens up new opportunities.
The Strength of Weak Ties pointed the way towards methods for quantifying many characteristics of large networks. This has, in turn, fed into the analysis of a wide range of network structures.