then get each of their contacts to name five more contacts. In total,
they would have had to track the behaviour of 31,000 people in detail
for multiple years. A study that large would have cost around $30m.
[44]
While exploring options, the pair got in touch with the team running
the Framingham Heart Study, because it would be easier to recruit
those initial 1,000 people from an existing project. When Christakis
visited Marian Bellwood, the project co-ordinator, she mentioned they
kept forms in the basement with details of each participant. To avoid
losing contact with participants, they’d got people to list their relatives,
friends and co-workers on the forms. It turned out that many of these
contacts were also in the study, which meant their health information
was being recorded too.
Christakis was astonished. Rather than recruiting a completely
new set of social contacts, they could instead piece together the
social network among Framingham participants. ‘I called James from
the parking lot and said, “you won’t believe this!”,’ he recalled. There
was just one catch: they’d have to go through twelve thousand names
and fifty thousand addresses to identify the existing links. ‘We had to
decipher everyone’s handwriting,’ Christakis said. ‘It took two years to
computerise it.’
The pair had initially thought about analysing the spread of
smoking, but decided obesity was a better starting point. Smoking
depended on what participants reported, whereas obesity could be
observed directly. ‘Because we were doing something so novel, we
wanted to start with something that could be objectively measured,’
Christakis said.
The next step was to estimate whether obesity was being
transmitted through the network. This meant tackling the reflection
problem, separating potential contagion from homophily or
environmental factors. To try and rule out the birds-of-a-feather effect
of homophily, the pair included a time lag in the analysis; if obesity
really spread from one person to their friend, the friend couldn’t have
become obese first. Environmental factors were trickier to exclude,
but Christakis and Fowler tried to tackle the issue by looking at the
direction of friendship. Suppose I list you as a friend in a survey, but