friends have influenced you over time. Alternatively, it may be the
other way around: you may have chosen to become friends because
you already shared certain characteristics. This is known as
‘homophily’, the idea that ‘birds of a feather flock together’. Of course,
your behaviour might be nothing to do with social connections at all.
You may just happen to share the same environment, which
influences your behaviour. Sociologist Max Weber used the example
of a crowd of people opening umbrellas when it starts to rain. They
aren’t necessarily reacting to each other; they’re reacting to the
clouds above.[33]
It can be tough to work out which of the three explanations – social
contagion, homophily or a shared environment – is the correct one.
Do you like a certain activity because your friend does, or are you
friends because you both like that activity? Did you skip your running
session because your friend did, or did you both abandon the idea
because it was raining? Sociologists call it ‘the reflection problem’,
because one explanation can mirror another.[34] Our friendships and
behaviour will often be correlated, but it can be very difficult to show
that contagion is responsible.
What we need is a way to separate social contagion from the other
possible explanations. The most definitive way to do this would be to
spark an outbreak and watch what happens. This would mean
introducing a specific behaviour, like Aplin and her colleagues did
with birds, and measuring how it spreads. Ideally we would compare
results with a randomly selected ‘control’ group of individuals – who
aren’t exposed to the spark – to see how much effect the outbreak
has. This type of experiment is common in medicine, where it’s
known as a ‘randomised controlled trial’.
How might such an approach work in humans? Say we wanted to
run an experiment to study the spread of cigarette smoking between
friends. One option would be to introduce the behaviour we’re
interested in: pick some people at random, get them to take up
smoking, and then see whether the behaviour spreads through their
friendship groups. Although this experiment might tell us whether
social contagion occurs, it doesn’t take much to spot that there are
some big ethical problems with this approach. We can’t ask people to