The Rules of Contagion

adopt a harmful activity like smoking on the off chance it will help us
understand social behaviour.
Rather than randomly introducing smoking, we could instead look
at how existing smoking behaviour spreads through new social
connections. But this would mean rearranging people’s friendships
and locations at random and tracking whether people adopt their new
friends’ behaviour. Again, this is generally not feasible: who wants to
reshuffle their entire friendship network for a research project?
When it comes to designing social experiments, Aplin’s work on
birds had some big advantages over studies of humans. Whereas
humans may keep similar social links for years or decades, birds
have a relatively short lifespan, which meant new networks of
interactions would form each year. The team could also tag most of
the birds in the area, making it possible to track the network in real-
time. This meant the researchers could introduce a new idea – the
puzzle solution – and watch how it spread through the newly formed
networks.
There are some circumstances in which new human friendships
randomly form all at once, for example when recruits are assigned to
military squadrons or students are allocated to university halls.[35]
Unfortunately for researchers, these are rare examples. In most real-
life situations, scientists can’t meddle with behaviour or friendship
dynamics to see what might happen. Instead, they must try and gain
insights from what they can observe naturally. ‘Though a lot of the
best strategies involve randomisation or some plausible source of
randomness, for many things we really care about as social scientists
and citizens, we’re not going to be able to randomise,’ said Dean
Eckles, a social scientist at MIT.[36] ‘So we should do the best job we
can with purely observational research.’
Much of epidemiology relies on observational analysis: in general,
reseachers can’t deliberately start outbreaks or give people severe
illnesses to understand how they work. This has led to some
suggestions that epidemiology is closer to journalism than science,
because it just reports on the situation as it happens, instead of
running experiments.[37] But such claims ignore the huge
improvements in health that have come from observational studies.