September 2019, ScientificAmerican.com 57RICH PEDRONCELLI
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ful and actively try to learn the truth.
Yet they come to dangerously wrong
conclusions. How does this happen?
The contagion model is inadequate
for answering this question. Instead
we need a model that can capture cas-
es where people form beliefs on the
basis of evidence that they gather and
share. It must also capture why these
in dividuals are motivated to seek the
truth in the first place. When it comes
to health topics, there might be seri-
ous costs to acting on false beliefs. If
vac cines are safe and eff ective (which
they are) and parents do not vaccinate,
they put their kids and immuno sup-
pressed people at un necessary risk. If
vaccines are not safe, as the par ti ci-
pants in these Face book groups have
concluded, then the risks go the other
way. This means that figuring out
what is true, and acting accordingly, matters deeply.
To better understand this behavior in our research,
we drew on what is called the network epistemology
framework. It was first developed by economists 20
years ago to study the social spread of beliefs in a com-
munity. Models of this kind have two parts: a problem
and a network of individuals (or “agents”). The prob-
lem involves picking one of two choices: These could be
“vaccinate” and “don’t vaccinate” your chil dren. In the
model, the agents have beliefs about which choice is
better. Some believe vaccination is safe and effective,
and others believe it causes autism. Agent beliefs shape
their behavior—those who think vac cination is safe
choose to perform vaccinations. Their behavior, in turn,
shapes their beliefs. When agents vac ci nate and see
that nothing bad happens, they be come more con-
vinced vaccination is indeed safe.
The second part of the model is a network that rep-
resents social connections. Agents can learn not only
from their own experiences of vaccinating but also
from the experiences of their neighbors. Thus, an indi-
vidual’s community is highly important in determin-
ing what beliefs they ultimately develop.
The network epistemology framework captures
some essential features missing from contagion models:
individuals intentionally gather data, share data and
then experience consequences for bad beliefs. The find-
ings teach us some important lessons about the social
spread of knowledge. The first thing we learn is that
working together is better than working alone, because
an individual facing a problem like this is likely to pre-
maturely settle on the worse theory. For instance, he or
she might observe one child who turns out to have au-
tism after vaccination and conclude that vaccines are
not safe. In a community there tends to be some di-
versity in what people believe. Some test one action;
some test the other. This diversity means that usually
enough evidence is gathered to form good beliefs.But even this group benefit does not guarantee that
agents learn the truth. Real scientific evidence is prob-
abilistic, of course. For example, some nonsmok ers get
lung cancer, and some smokers do not get lung cancer.
This means that some studies of smokers will find no
connection to cancer. Relatedly, although there is no
actual statistical link between vaccines and autism,
some vaccinated children will be autistic. Thus, some
parents observe their children developing symptoms
of autism after receiving vaccinations. Strings of mis-
leading evidence of this kind can be enough to steer an
entire community wrong.
In the most basic version of this model, social in-
fluence means that communities end up at consensus.
They decide either that vaccinating is safe or that it is
dangerous. But this does not fit what we see in the real
world. In actual communities, we see polarization—en-
trenched disagreement about whether or not to vac-
cinate. We argue that the basic model is missing two
crucial ingredients: social trust and conformism.
Social trust matters to belief when individuals treat
some sources of evidence as more reliable than others.
This is what we see when anti-vaxxers trust evi dence
shared by others in their community more than evi-
dence produced by the Centers for Disease Control and
Prevention or other medical research groups. This mis-
trust can stem from all sorts of things, including previ-
ous negative experiences with doctors or concerns that
health care or governmental institutions do not care
about their best interests. In some cases, this distrust
may be justified, given that there is a long history of
medical researchers and clinicians ignoring legitimate
issues from patients, particularly women.
Yet the net result is that anti-vaxxers do not learn
from the very people who are collecting the best evi-
dence on the subject. In versions of the model where
individuals do not trust evidence from those who hold
very different beliefs, we find communities po lar ize,PROTESTERS
use the lan-
guage of
“choice” to
spread misin-
formation about
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