The Rules of Contagion

it. ‘A one-man riot is a tantrum.’[54] So how does a riot grow from a
single person? In 1978, Mark Granovetter published a now classic
study looking at how trouble might take off. He suggested that people
might have different thresholds for rioting: a radical person might riot
regardless of what others were doing, whereas a conservative
individual might only riot if many others were. As an example,
Granovetter suggested we imagine 100 people hanging around in a
square. One person has a threshold of 0, meaning they’ll riot (or
tantrum) even if nobody else does; the next person has a threshold of
1, so they will only riot if at least one other person does; the next
person has a threshold of 2, and so on, increasing by one each time.
Granovetter pointed out that this situation would lead to an inevitable
domino effect: the person with a 0 threshold would start rioting,
triggering the person with a threshold of 1, which would trigger the
person with a threshold of 2. This would continue until the entire
crowd was rioting.
But what if the situation were slightly different? Say the person with
a threshold of 1 had a threshold of 2. This time, the first person would
start rioting, but there would be nobody else with a low enough
threshold to be triggered. Although the crowds in each situation are
near identical, the behaviour of one person could be the difference
between a riot and a tantrum. Granovetter suggested personal
thresholds could apply to other forms of collective behaviour too, from
going on strike to leaving a social event.[55]
The emergence of collective behaviour can also be relevant to
counter-terrorism. Are potential terrorists recruited into an existing
hierarchy, or do they form groups organically? In 2016, physicist Neil
Johnson led an analysis looking at how support for the so-called
Islamic State grew online. Combing through discussions on social
networks, his team found that supporters aggregated in progressively
larger groups, before breaking apart into smaller ones when the
authorities shut them down. Johnson has compared the process to a
school of fish splitting and reforming around predators. Despite
gathering into distinct groups, Islamic State supporters didn’t seem to
have a consistent hierarchy.[56] In their studies of global insurgency,
Johnson and his collaborators have argued that these collective