What is Social contagion?
Social contagion is the spread of behaviors, emotions, or ideas through social networks, where exposure to others’ actions increases the probability that an observer will adopt the same behavior. It explains how trends, habits, and even health conditions propagate through populations.
How it works
Christakis and Fowler’s analysis of the Framingham Heart Study showed that obesity spread through social networks like an infectious process: if your friend becomes obese, your own risk increases by 57%. The effect extends to three degrees of separation (friends of friends of friends) and operates through changes in social norms about acceptable body size rather than shared environments.
Applied example
A teenager is more likely to start smoking if their close friends smoke, not because of direct pressure but because seeing peers smoke normalizes the behavior and shifts the perceived social norm about what is acceptable.
Why it matters
Social contagion means that interventions targeting influential individuals in a network (network-based targeting) can produce outsized effects, because behavior changes ripple outward through connected people.
