Why do ideas and protests spread like a fad, not like a flu?

Because joining a protest is not like catching a cold. A disease can pass on after a single contact, but a behaviour often needs several. Damon Centola and Michael Macy found that information and disease spread as "simple contagions", requiring only one contact for transmission, while behaviours typically spread as "complex contagions", requiring multiple sources of reinforcement to induce adoption.

Why would one contact not be enough? The sources give reasons. Hearing the same story from different people makes it less likely to be an invention. Knowing that a movement exists is rarely enough to induce bystanders to join, whereas several close friends taking part greatly raises the likelihood, especially for high-risk movements, since innovators risk being shunned as deviants until there is a critical mass of early adopters.

This changes which kind of network helps. Mark Granovetter showed that weak ties, acquaintances rather than close friends, bring new information: most jobs were found through them. Centola and Macy show that weak ties and small-world networks are very good for spreading simple contagions, but for complex contagions they can slow diffusion.

Centola ran experiments to test this. In 2018 his team found that once 25% of people have adopted a new social norm, a committed minority can rapidly change an entire population's opinion. That figure comes from experiments in online communities, so it is one experimental result, not a law of every movement.