Even the smallest changes in a context can determine whether an epidemic takes off
The emergence of epidemics depends largely on external circumstances and can frequently be traced back to small changes.
This was apparent to the authorities in New York City when, in the mid‑1990s, the city’s crime spiralled out of control, and they blamed a number of seemingly harmless details. They thought that things like graffitied subway cars, or subway-fare evaders going unpunished, sent out signals to people that nobody was taking care of the decaying situation – and that anyone and everyone could do whatever they wanted.
In order to get a handle on this crime epidemic, authorities began focusing on these more minor details. Graffiti was removed, seemingly overnight, and fare-evasion became a punishable crime. By showing zero tolerance for what seemed like trivialities, it became clear to the public that reckless behaviour was no longer acceptable. The crime rate dropped rapidly in the following years; i.e., the epidemic was reversed thanks to these small interventions.
Another subtle factor that plays a role in the emergence of social epidemics is the size of a group. The rule of 150 states that only in groups of no more than 150 people can a dynamic develop that can later extend beyond the group.
In other words, if you want groups, e.g., clubs, communities, companies or schools, to be incubators for contagious messages, make sure to keep them small.