Confirmation Bias: A Ubiquitous Phenomenon in Many Guises

190 RAYMOND S. NICKERSON
in many experimental situations. What makes it
especially important to understand is that it can
have significant consequences in many nonlaboratory
contexts. The point is illustrated in what
follows by a few examples.
Number Mysticism
Pythagoras discovered, by experimentation,
how the pitch of a sound emitted by a vibrating
string depends on the length of the string and
was able to state this dependency in terms of
simple numerical ratios: (a) two strings of the
same material under the same tension differ in
pitch by an octave when one of the strings is
twice as long as the other, and (b) two strings the
lengths of which are in the ratio 2 to 3 will
produce a note and its fifth, and so on.
Observation was not the new thing that
Pythagoras did in his study of harmonic
relationships; people had been observing the
heavens and recording what they saw for a long
time. What he did that was new was manipulate
what he was observing and take notice of the
effects of those manipulations. He has been
called the first experimentalist. Ironically, instead
of establishing experimentation as an
especially fruitful way to investigate the properties
of the physical world, Pythagoras's discovery
helped to usher in what Bell called "the
golden age of number mysticism" and to delay
the acceptance of experimentation as the primary
method of science for 2000 years (Bell,
1946/1991). Pythagoras's intellectual heirs were
so convinced of the validity of his pronouncement,
"everything is number," that many of the
most able thinkers over the next 2 millenia
devoted much of their cognitive energy to the
pursuit of numerology and the confirmation of
its basic assumptions. It took a Galileo to kindle
an interest in experimentation that would not
again sputter and die.
Work done as recently as the mid-19th
century involving the great pyramid of Egypt,
which has extraordinary fascination for modern
observers of artifacts of ancient cultures, illustrates
the relevance of the confirmation bias to
Pythagorean numerological pursuits. Much of
this fascination is due to certain mathematical
relationships discussed first by John Taylor
(1859) and shortly later by Charles Smyth
(1864/1890). Taylor noted, among other facts,
that the ratio of twice the pyramid's base to its
height was roughly the same as the ratio of the
diameter of a circle to its circumference, which
is to say, IT. Smyth was inspired by Taylor's
observations and set himself the task of
discovering other mathematical relationships of
interest that the monument might hide.
Smyth discovered that the ratio of the
pyramid's base to the width of a casing stone
was 365, the number of days in a year, and that
the pyramid's height multiplied by 10 9 was
approximately equal to the distance from the
earth to the sun. By comparing pyramid length
measurements in various ways, he was able to
find numbers that correspond to many quantitative
properties of the world that were presumably
unknown when the pyramid was built.
These include the earth's mean density, the
period of precession of the earth's axis, and the
mean temperature of the earth's surface. Von
Daniken (1969) used the existence of such
relationships as the basis for arguing that the
earth had been visited by intelligent extraterrestrials
in the past.
Gardner (1957) referred to Smyth's book as a
classic of its kind illustrating beautifully "the
ease with which an intelligent man, passionately
convinced of a theory, can manipulate his
subject matter in such a way as to make it
conform to precisely held opinions" (p. 176). He
pointed out that a complicated structure like the
pyramid provides one with a great assortment of
possible length measurements, and that anyone
with the patience to juggle them is quite sure to
find a variety of numbers that will coincide with
some dates or figures that are of interest for
historical or scientific reasons. One simply
makes an enormous number of observations,
tries every manipulation on measurements and
measurement relationships that one can imagine,
and then selects from the results those few
that coincide with numbers of interest in other
contexts. He wrote, "Since you are bound by no
rules, it would be odd indeed if this search for
Pyramid 'Truths' failed to meet with considerable
success" (p. 177). The search for pyramid
truths is a striking illustration of how a bias to
confirm is expressed by selectivity in the search
for and interpretation of information.
Witch Hunting
From a modern vantage point, the convictions
and executions of tens of thousands of individu-