Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

192 RAYMOND S. NICKERSON
situations are. In situations characterized by
interactions among numerous variables and in
which the cause-effect relationships are obscure,
data tend to be open to many interpretations.
When that is the case, the confirmation
bias can have a great effect, and people should
not be surprised to see knowledgeable, wellintentioned
people draw support for diametrically
opposed views from the same set of data.
Medicine
The importance of testing theories, hypotheses,
speculations, or conjectures about the
world and the way it works, by understanding
their implications vis-a-vis observable phenomena
and then making the observations necessary
to check out those implications, was a common
theme in the writings of the individuals who
gave science its direction in the 16th, 17th, and
18th centuries. This spirit of empirical criticism
had not dominated thinking in the preceding
centuries but was a new attitude. Theretofore
observation, and especially controlled experimentation,
had played second fiddle to logic and
tradition.
Consider, for example, the stagnated status of
medical knowledge: "For 1500 years the main
source of European physicians' knowledge
about the human body was not the body itself
... [but] the works of an ancient Greek
physician [Galen]. 'Knowledge' was the barrier
to knowledge. The classic source became a
revered obstacle." Boorstin (1985, p. 344), who
wrote these words, noted the irony of the fact
that Galen himself was an experimentalist and
urged others who wished to understand anatomy
or medicine to become hands-on investigators
and not to rely only on reading what others had
said. But Galen's readers found it easier to rely
on the knowledge he passed on to them than to
follow his advice regarding how to extend it.
Although some physicians did "experiment"
with various approaches to the treatment of
different diseases and ailments, as Thomas
(1979) pointed out, until quite recently, this
experimentation left something to be desired as
a scientific enterprise:
Virtually anything that could be thought up for the
treatment of disease was tried out at one time or
another, and, once tried, lasted decades or even
centuries before being given up. It was, in retrospect,
the most frivolous and irresponsible kind of human
experimentation, based on nothing but trial and error,
and usually resulting in precisely that sequence.
Bleeding, purging, cupping, the administration of
infusions of every known plant, solutions of every
known metal, every conceivable diet including total
fasting, most of these based on the weirdest imaginings
about the cause of disease, concocted out of nothing but
thin air—this was the heritage of medicine up until a
little over a century ago. It is astounding that the
profession survived so long, and got away with so
much with so little outcry. Almost everyone seems to
have been taken in. (p. 159)
How is it that ineffective measures could be
continued for decades or centuries without their
ineffectiveness being discovered? Sometimes
people got better when they were treated;
sometimes they did not. And sometimes they got
better when they were not treated at all. It
appears that people's beliefs about the efficacy
of specific treatments were influenced more
strongly by those instances in which treatment
was followed by recovery than in either those in
which it was not or those in which it occurred
spontaneously. A prevailing tendency to focus
exclusively or primarily on positive cases—
cases in which treatment was followed by
recovery—would go a long way toward accounting
for the fact that the discoveries that certain
diseases have a natural history and people often
recover from them with or without treatment
was not made until the 19th century.
Fortunately such a tendency is no longer
characteristic of medical science as a whole; but
one would be hard pressed to argue that it no
longer influences the beliefs of many people
about the efficacy of various treatments of
medical problems. Every practitioner of a form
of pseudomedicine can point to a cadre of
patients who will testify, in all sincerity, to
having benefited from the treatment. More
generally, people engage in specific behaviors
(take a pill, rest, exercise, think positively) for
the express purpose of bringing about a specific
health-related result. If the desired result occurs,
the natural tendency seems to be to attribute it to
what was done for the purpose of causing it;
considering seriously the possibility that the
result might have been obtained in the absence
of the associated "cause" appears not to come
naturally to us but to have to be learned.
Medical diagnosis, as practiced today, has
been the subject of some research. In looking for
causes of illness, diagnosticians tend to generate
one or a small set of hypotheses very early in the