The Rules of Contagion

governments and other organisations want to address health issues
that are contagious, they’ll need to know more about the actual extent
of social contagion, and what impact different policies might have. If
one person in a friendship group becomes overweight, exactly how
much influence will it have on others? If you become happier, how
much will your community’s happiness increase? Christakis and
Fowler have acknowledged that it’s tricky to estimate the precise
extent of social contagion. What’s more, addressing such questions
often means using imperfect data and methods. But as new datasets
become available, they point out others will be able to build on their
analysis, moving towards an accurate measurement of contagion.
By studying potentially contagious behaviour, researchers are also
uncovering some crucial differences between biological and social
outbreaks. In the 1970s, sociologist Mark Granovetter suggested that
information could spread further through acquaintances than through
close friends. This was because friends would often have multiple
links in common, making most transmission redundant. ‘If one tells a
rumor to all his close friends, and they do likewise, many will hear the
rumor a second and third time, since those linked by strong ties tend
to share friends.’ He referred to the importance of acquaintances as
the ‘strength of weak ties’: if you want access to new information, you
may be more likely to get it through a casual contact than a close
friend.[46]
These long distance links have become a central part of network
science. As we’ve seen, ‘small-world’ connections can help biological
and financial contagion jump from one part of a network to another. In
some cases, these links may also save lives. There is a long-standing
paradox in medicine: people who have a heart attack or stroke while
surrounded by relatives take longer to get medical care. This may
well be down to the structure of social networks. There’s evidence
that close-knit groups of relatives tend to prefer a wait-and-see
approach after witnessing a mild stroke, with nobody willing to
contradict the dominant view. In contrast, ‘weak ties’ – like co-workers
or non-relatives – can bring a more diverse set of perspectives, so
flag up symptoms faster and call for help sooner.[47]