The Rules of Contagion

Even so, the sort of network structure that amplifies disease
transmission won’t always have the same effect on social contagion.
Sociologist Damon Centola points to the example of , which has
spread widely through networks of sexual partners. If biological and
social contagion work in the same way, ideas about preventing the
disease should also have spread widely via these networks. And yet
they have not. Something must be slowing the information down.
During an infectious disease outbreak, infection typically spreads
through a series of single encounters. If you get the infection, it will
usually have come from a specific person.[48] Things aren’t always
so simple for social behaviour. We might only start doing something
after we’ve seen multiple other people doing it, in which case there is
no single clear route of transmission. These behaviours are known as
‘complex contagions’, because transmission requires multiple
exposures. For example, in Christakis and Fowler’s analysis of
smoking, they noted that people were more likely to quit if lots of their
contacts stopped as well. Researchers have also identified complex
contagion in behaviours ranging from exercise and health habits to
the uptake of innovations and political activism. Whereas a pathogen
like can spread through a single long-range contact, complex
contagions need multiple people to transmit them, so can’t pass
through single links. While small-world networks might help diseases
spread, these same networks could limit the transmission of complex
contagions.
Why do complex contagions occur? Damon Centola and his
colleague Michael Macy have proposed four processes that might
explain what’s happening. First, there can be benefits to joining
something that has existing participants. From social networks to
protests, new ideas are often more appealing if more people have
already adopted them. Second, multiple exposures can generate
credibility: people are more likely to believe in something if they get
confirmation from several sources. Third, ideas can depend on social
legitimacy: knowing about something isn’t the same as seeing others
acting – or not acting – on it. Take fire alarms. As well as signaling
there might be a fire, alarms make it acceptable for everyone to leave
the building. One classic 1968 experiment had students sit working in