How Behavior Spreads: The Science of Complex Contagions — A Review

By Yuan Hsiao

How Behavior Spreads: The Science of Complex Contagions by Damon Centola


The role of social networks in mobilization processes has been a fundamental concern for both scholars and activists. How can connections between individuals facilitate or inhibit participation in collective action? Damon Centola’s book How Behavior Spreads: The Science of Complex Contagions draws from a decade’s research on behavioral diffusion, and is a significant theoretical and practical reference for anyone interested in network effects. While the book is not specifically about social movements, with a brief glance, one can find that Centola draws heavily from examples and literature in the field of social movements. What is particularly relevant is that Centola not only engages in scholarly dialogue with social theories, but also provides advice for practitioners who wish to boost participation in their organization. Furthermore, Centola writes in a style that transforms complex concepts into easily understandable sentences, and is suitable for a wide range of audiences.

This is a book about how network topologies (i.e., the configuration of how social networks are structured) affect participation. The assumption throughout the book is that behaviors such as movement participation can be described as “contagious.” Being connected to a participant increases the likelihood of participation, just like being connected to an infected person increases the possibility of contracting a disease. From this assumption, Centola starts by describing the traditional insight by Granovetter, Watts, and Strogatz: the more “weak ties” that span across network clusters (e.g., across different towns), the broader the reach of information, and thus the better the outcome of participation. However, in reality we often see participation diffusing not via these spanning weak ties, but gradually alongside networks clusters where individuals have a lot of overlapping ties.

If the traditional insight is correct, it is puzzling why so often diffusion spreads via such inefficient pathways. Starting from this puzzle, Centola distinguishes between processes of simple contagion and complex contagion. Simple contagions are scenarios where a single contact suffices for motivating participation, whereas complex contagions require multiple contacts to activate participation. Centola finds that spanning/weak ties benefit simple contagions and clustered ties inhibit simple contagions, the opposite is true for complex contagions. Complex contagions, such as movement participation, require clustered, dense, and overlapping ties to spread. In contrast, weak ties reduce clustering but lack the social reinforcement needed to activate participation. Thus, for complex contagions to spread, instead of bridges originating from isolated weak ties, Centola proposes the concept of wide bridges – multiple shared network ties across clusters. Wide bridges create the social reinforcement for the behavior to spread from one cluster to another, such as the diffusion of a rebellion from one town to another.  These findings have implications for social movement organizations, as the construction of wide bridges is a strategy that is applicable for building mobilization structures. Without bridges, a movement can only mobilize a confined area of supporters. However, with too many narrow bridges the movement not only cannot spread, but also may take away the social fabric within the social movement organization. Wide bridges provide the optimal strategy that both incubates a movement but also facilitates diffusion.

From the theory of complex contagions, Centola moves on to the question about how diffusion occurs in the face of opposition. Decades of research in collective action has shown while ties to participants facilitate participation, ties to nonparticipants often inhibit the incentive to join. For the initiation of a movement, since most people are nonparticipants, how can diffusion occur in such scenarios? For Centola, the answer lies in the clustering of the early participants. In other words, compared to the scenario where early participants are placed randomly in the network, connections among early participants can facilitate a “clustered seed” that sparks diffusion. These participants create a group with strong social ties that can resist the opposition of nonparticipants. Furthermore, they can persuade neighbors who are connected via wide bridges to join, which snowballs into a mass movement. For an application in social movements, rather than trying to recruit randomly with a broad reach (as is often the case on the internet), Centola’s ideas suggest that clustered recruiting may be a better strategy for complex contagions.

What is interesting about this book is that Centola also offers recommendations for “social design,” or how to shape networks within organizations. Much of the research on networks has been on how different network topologies affect participation. However, for practitioners the key is not only to understand but how to apply these ideas in social settings. It is hard to imagine how one could build or change people’s social ties, but Centola nicely outlines how social contexts increase the odds of tie formation. By restructuring the physical space of organizations or arranging inter-group activities, it is possible to increase or decrease the odds that groups of people meet, and in turn, nurture social ties. Since most people tend to form ties to people with similar characteristics (e.g., gender, race, or hierarchy within an organization), such arrangements allow groups of different traits to build multiple cross-group ties and hence wide bridges. These wide bridges can then be transformed into channels of diffusion for complex contagion processes.

How Behavior Spreads provides a fresh perspective on how to enhance participation for processes of complex contagions such as high-risk collective action. It is certainly important to pay attention to structural factors (e.g., political opportunities) or symbolic reasons (e.g., framing tactics). However, altering the topology of social networks may equally be important. In the book, Centola demonstrated through numerous social experiments how little changes in network topology can drastically affect the outcome of participation. These experiments not only have scholarly value in showing the causal relationship between networks and behavior, but many also can be used as prototypes for redesigning mobilization efforts. For those interested in how social connections affect macro outcomes of engagement, How Behavior Spreads is a nicely-written piece for the summer.

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