evidence from an experiment: if people stopped smoking, it should
reduce their chances of cancer.
In some cases, Bradford Hill said it’s possible to relate the level of
exposure to the risk of disease. For instance, the more cigarettes a
person smokes, the more likely they are to die from them. What’s
more, it may be possible to draw an analogy with a similar cause and
effect, such as another chemical that causes cancer. Finally, Bradford
Hill suggested it’s worth checking to see whether the cause is
biologically plausible and fits with what’s already known to scientists.
Bradford Hill emphasised that these viewpoints were not a
checklist to ‘prove’ something beyond dispute. Rather, the aim was to
help answer a crucial question: is there any better explanation for
what we are seeing than simple cause and effect? As well as
providing evidence that smoking caused cancer, these kinds of
methods have helped researchers uncover the source of other
diseases. During the 1950s and 1960s, epidemiologist Alice Stewart
gathered evidence that low-dose radiation could cause leukaemia.
[41] At the time, new X-ray technology was regularly being used on
pregnant women; there were even X-rays in shoe shops, so people
could see their feet inside the shoes. After a long battle by Stewart,
these hazards were removed. More recently, researchers at the US
CDC used the Bradford Hill viewpoints to argue that infections with
Zika were causing birth defects.[42]
Establishing such causes and effects is inherently difficult. Often
there will be an intense debate about what is responsible and what
should be done. Still, Stewart believed that, faced with troubling
evidence, people should act despite the inevitable uncertainty
involved. ‘The trick is to get the best guess of the thickness of the ice
when crossing a lake,’ she once said. ‘The art of the game is to get
the correct judgment of the weight of the evidence, knowing that your
judgment is subject to change under the pressure of new
observations.’[43]
W originally set out to study social
contagion, they’d planned to do it from scratch. The idea was to
recruit 1,000 people, get each of them to name five contacts, and