The Rules of Contagion

most,’ they noted, ‘but those that are only moderately popular’. This is
because a small first cascade leaves more people who haven’t seen
the meme yet. With a large initial outbreak, there aren’t enough
susceptible people left to sustain transmission. For a cascade to
recur, it also helps if there are several copies of the meme circulating.
This is consistent with what we’ve already seen for stuttering
outbreaks: having multiple sparks can make infections spread further.
Cheng looked at popular images, but what about other types of
content? Back in 2016, I gave a public talk at London’s Royal
Institution. Over the next couple of years, a video of the talk somehow
racked up over a million views on YouTube. Around the same time in
2016, I’d given a talk on a similar topic at Google, which had also
been posted on YouTube, on a channel with a similar number of
subscribers. During the same period, this one was viewed around
10,000 times. (Ideally, this popularity would have been the other way
round: it turns out that if you give two related talks, but screw up a live
demonstration in one of them, that’s the talk that will become popular
online.)
I hadn’t expected the Royal Institution talk to get so much
attention, but what really came as a surprise was how the views had
accumulated. For its first year online, the video had gained relatively
little interest, getting a hundred or so views per day. Then suddenly, in
the space of a few days, it picked up more attention than it had in an
entire year.