The Rules of Contagion

you don’t list me. This suggests I am more influenced by you than
you are by me. However, if in reality we’re actually both influenced by
some shared environmental factor – like a new fast food restaurant –
our friendship direction shouldn’t affect who becomes obese.
Christakis and Fowler found evidence that it did matter, suggesting
that obesity could be contagious.
When the analysis was published, it received sharp criticism from
some researchers. Much of the debate came down to two main
points. The first was that the statistical evidence could have been
stronger: the result showing that obesity was contagious was not as
definitive as it would need to be for, say, a clinical trial showing
whether a new drug worked. The second criticism was that, given the
methods and data Christakis and Fowler had used, they could not
conclusively rule out other explanations. In theory, it was possible to
imagine a situation involving homophily and environment that could
have produced the same pattern.
In my view, these are both reasonable criticisms of the research.
But it doesn’t mean that the studies weren’t useful. Commenting on
the debate about Christakis and Fowler’s early papers, statistician
Tom Snijders suggested that the studies had limitations, but were still
important because they’d found an innovative way to put social
contagion on scientists’ agenda. ‘Bravo for the imagination and
braveness of Nick Christakis and James Fowler.’ [45]
In the decade since Christakis and Fowler published their initial
analysis of the Framingham data, evidence for social contagion has
accumulated. Several other research groups have also shown that
things like obesity, smoking, and happiness can be contagious. As
we’ve seen, it is notoriously difficult to study social contagion, but we
now have a much better understanding of what can spread.
The next step will be to move beyond simply saying that contagion
exists. Showing that behaviour can catch on is equivalent to knowing
that the reproduction number is above zero: on average, there will be
some transmission, but we don’t know how much. Of course, this is
still useful information, because it shows contagion is a factor we
need to think about. It tells us the behaviour is capable of spreading,
even if we can’t predict how big the outbreak might be. However, if