An expectancy-attribution model of the effects of placebos

Abstract

Distinguishes between primary assessments, which measure the direct effects of expectancies associated with the placebo, and secondary assessments, which measure S's inferences about underlying dispositions that are not believed to be directly affected by the placebo (inductive effects). Placebos whose alleged impact will counteract the recipient's symptoms (counteractive expectancies) are distinguished from placebos whose alleged effects will parallel the recipient's symptoms (parallel expectancies). Research findings are reviewed and classified into 1 of 4 cells produced by a combination of these 2 factors. Standard placebo effects (changes in the recipient's condition in accordance with the placebo's alleged impact) were found to be most common on primary assessments of counteractive expectancies placebos; reverse inductive effects (altered inferences about an underlying disposition, in the direction opposite to the placebo's alleged impact) were most common on secondary assessments of parallel expectancies placebos. Psychological processes that underlie these effects and implications for clinical practice are considered. (94 ref)

Full text

Psychological
Review
1981,
Vol.
88, No. 5,
408-437
Copyright
1981
by the
American Psychological Association, Inc.
0033-295X/81
/8805-0408S00.75
An
Expectancy-Attribution Model
of the
Effects
of
Placebos
Michael Ross
University
of
Waterloo
Waterloo, Ontario, Canada
James
M.
Olson
University
of
Western Ontario
London,
Ontario, Canada
A
model
is
presented
to
resolve
the
inconsistencies
in the
literature
on the
effects
of
placebos. Primary assessments, which measure
the
direct
effects
of the ex-
pectancies associated with
the
placebo,
are
distinguished
from
secondary
as-
sessments, which measure
the
subject's inferences about
underlying
dispositions
that
are not
believed
to be
directly affected
by the
placebo
(inductive
effects).
Also,
placebos whose alleged impact
will
counteract
the
recipient's symptoms
(counteractive expectancies)
are
distinguished
from
placebos whose alleged
ef-
fects
will
parallel
the
recipient's symptoms (parallel
expectancies).
The
relevant
findings are
reviewed
and
classified
into
one of the
four
cells produced
by a
combination
of
these
two
factors. Standard placebo effects (changes
in the re-
cipient's condition
in
accordance
with
the
placebo's alleged impact) were most
common
on
primary assessments
of
counteractive expectancies placebos; reverse
inductive
effects (altered inferences about
an
underlying
disposition,
in the di-
rection
opposite
to the
placebo's alleged impact) were most common
on
secondary
assessments
of
parallel expectancies placebos. Psychological processes that might
underlie
these effects,
and
implications
of the findings for
research
on
placebo
effects
and for
clinical practice
are
considered.
A
placebo
is a
substance
or
procedure
that
is
administered with suggestions
that
it
will
modify
a
symptom
or
sensation
but
which,
unknown
to its
recipient,
has no
specific
pharmacological impact
on the
reaction
in
question.
A
standard placebo effect occurs
when
the
administration
of the
placebo
al-
ters
the
recipient's
state
in
accordance
with
its
presumed impact.
Physicians have employed
placebos
for
centuries
to
alleviate human suffering.
In-
deed,
until
the
beginning
of
this century,
most medicines
had no
specific pharmaco-
logical
properties
that
could assist healing
(Honigfeld,
1964au
Houston,
1938;
Shapiro,
This article
was
partially prepared
while
the first au-
thor
was a
Visiting Scholar
at the
Department
of
Psy-
chology,
Stanford University.
The
writing
of the
article
was
supported
by a
Canada Council Leave Fellowship
and
a
Social Sciences
and
Humanities Research Council
research grant
to the first
author.
We are
grateful
to
Richard Bootzin, Garth Fletcher,
Tory Higgins, John Holmes, Nick
Kuiper,
Kevin
McCaul,
Donald
Meichenbaum,
Clive
Seligman, Neil
Vidmar,
Mark Zanna,
and an
anonymous reviewer
for
their
comments
on an
earlier version
of the
manuscript.
Requests
for
reprints
can be
sent
to
Michael Ross,
Department
of
Psychology, University
of
Waterloo,
Waterloo, Ontario, Canada
N2L
3G1.
1971;
Shapiro
&
Morris, 1978).
The as-
sortment
of
therapies used
by
healers
was
astonishingly diverse, including such deli-
cacies
as
"lizard's
blood, crocodile dung,
the
teeth
of
swine,
the
hoof
of an
ass,
putrid
meat,
and fly
specks"
(Honigfeld,
1964a,
p.
145).
As an
example
of the
medical treatment
lavished
on our
forefathers, consider
the
fol-
lowing
attempts
to
cure
George
Washington
of
what
was
probably
a
throat infection com-
pounded
by
pneumonia:
Because
he
could
afford
the
best cure available,
he was
given
a
mixture
of
molasses,
vinegar,
and
butter,
and
then made
to
vomit
and
have
diarrhea.
But
he
lapsed.
In
desperation,
his
physicians
ap-
plied
irritating poultices
to
blister
his
feet
and his
throat,
while
draining several pints
of
blood. Then
he
died. (Power, 1978,
p. 24)
Remedies
of
true value were introduced only
rarely,
as
when fresh fruits were
used
to
cure
scurvy.
Astute
observers questioned
the
efficacy
of
medical
treatments; note,
for
example,
the
disdain with which
the
French playwright
Moliere
treated physicians. Nevertheless,
healers
attained high standing
in all
cultures.
408