The Physics of Diffusion: Complex Contagion and the Failure of Weak Ties
1. “Viral” always means success?
For the past half century, one of the most influential theoretical paradigms in network science and social network analysis (SNA) has been Mark Granovetter’s “the strength of weak ties.” This theory offers an intuitive and appealing idea. Rather than close friends or family (strong ties), it is our occasional acquaintances or distant connections (weak ties) that serve as important channels for new information and opportunities. From this perspective, shortcuts that reduce network diameter are always beneficial, and as the world becomes more connected (the small world phenomenon), diffusion accelerates. Marketers embraced this idea and built the myth of viral marketing, dreaming of campaigns that spread instantly.
However, recent network research suggests a different answer to the question, “Does everything spread like a virus?” Information or simple diseases (simple contagion) do travel quickly through weak ties. But changes in behavior, adoption of new technologies, and the spread of social norms follow very different rules. This is the logic of complex contagion.
In the world of complex contagion, weak ties are no longer a powerful asset and can even hinder diffusion.
The work of Centola and Macy in 2007, along with subsequent studies, shows why innovations spread not through weak ties but through wide bridges, why influencers can sometimes obstruct the diffusion of innovation, and how interacting contagions can display patterns similar to social behavior. These findings offer SNA researchers and practitioners a new insight: network structures should be designed differently depending on the nature of what is being diffused.