CAMBRIDGE, Mass., March 4, 2009 — While a friend might convince you to try a new restaurant or buy a cup of coffee to support a charity, it's unlikely that the same friend alone can convince you to invest a significant amount of money in a stock or adopt a different health behavior. Multiple people in the same social circles must adopt that riskier or more complex behavior for it to spread, according to research by MIT Sloan School of Management Professor Damon Centola.
This contradicts the well known “small world” theory in which people with “long ties” who don't know each other very well can spread behaviors, information and diseases. For example, a few contagious people travelling between remote villages can sicken an entire population. And a villager with a cousin in the city can bring news of job openings at a factory. These long ties can help connect people in the larger world to make it smaller. All it takes is one person to serve as a bridge or connector to other social groups.
However, Centola's research found the theory doesn't apply when it comes to the spreading of “complex contagions” where there is a barrier to adopting the behavior like cost, risk or social awkwardness. Distantly knowing someone -- a long tie connection -- who invested in a specific stock won't convince you to invest as well. Likewise, just because someone you casually know in another city adopts a new fashion trend doesn't mean you are likely to do the same. Instead, these types of behaviors require social affirmation from multiple sources.
Centola's theory can be applied to a broad range of behaviors from financial investments and social movements to health behavior. For example, AIDS can be spread among people with casual or long tie connections. While it might seem logical to spread information through those same connections about the use of condoms, it would actually be more effective to target clustered groups of people who can reinforce each other's adoption of condom use. “That will work better than using the faster, more distant networks that allowed the disease to spread,” he said.
He found there are at least four factors that explain why complex contagions require exposure to multiple sources. The first is “strategic complementarity,” where simply knowing about an innovation is rarely sufficient for adoption. Many innovations such as new technologies are costly at first, but less so for those who wait. The second factor is credibility. The more people who adopt an innovation, the more credibility it gains. The third factor is legitimacy. Having close friends participate in a group action often increases a bystander's acceptance of the legitimacy of the action. This can be seen in decisions about what clothing to wear, body parts to pierce, or hairstyle to adopt. The last factor is “emotional contagion,” where expressive and symbolic impulses can be communicated and amplified in group gatherings.
In “Complex Contagions and the Weakness of Long Ties,” Centola and his coauthor wrote, “We show that for complex contagions, long ties can be weak …. A low level of trust and familiarity between socially distant persons means the relationship is weak, and this inhibits the ability of one person to influence the other. What is not at all obvious is that long ties can also have a structural weakness — they are nontransitive. For the spread of information, transitive ties between friends tend to be redundant, such that we hear the same thing from multiple friends.”
They continued, “However, when activation requires confirmation or reinforcement from two or more sources, the transitive structure that was redundant for the spread of information now becomes an essential pathway for diffusion. Thus while weak ties are beneficial for the spread of new information precisely because they are nonredundant, for complex contagions uniqueness becomes a weakness rather than a strength.”
The paper was coauthored by Centola and Michael Macy of Cornell University.
Full Paper (PDF)