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Psychological Review
VOLUME
90
NUMBER
4
OCTOBER 1983
Extensional Versus Intuitive Reasoning:
The
Conjunction Fallacy
in
Probability Judgment
Amos Tversky Daniel
Kahneman
Stanford University University
of
British
Columbia,
Vancouver,
British
Columbia,
Canada
Perhaps
the
simplest
and the
most
basic
qualitative
law of
probability
is the
con-
junction
rule:
The
probability
of a
conjunction,
P(A&B),
cannot
exceed
the
prob-
abilities
of its
constituents,
P(A)
and
.P(B),
because
the
extension
(or the
possibility
set)
of the
conjunction
is
included
in the
extension
of its
constituents.
Judgments
under
uncertainty,
however,
are
often
mediated
by
intuitive
heuristics
that
are not
bound
by the
conjunction
rule.
A
conjunction
can be
more
representative
than
one
of its
constituents,
and
instances
of a
specific
category
can be
easier
to
imagine
or to
retrieve
than
instances
of a
more
inclusive
category.
The
representativeness
and
availability
heuristics
therefore
can
make
a
conjunction
appear
more
probable
than
one of its
constituents.
This
phenomenon
is
demonstrated
in a
variety
of
contexts
including
estimation
of
word frequency,
personality
judgment,
medical
prognosis,
decision
under risk,
suspicion
of
criminal
acts,
and
political forecasting.
Systematic
violations
of the
conjunction rule
are
observed
in
judgments
of lay
people
and of
experts
in
both
between-subjects
and
within-subjects
comparisons.
Alternative interpretations
of the
conjunction fallacy
are
discussed
and
attempts
to
combat
it are
explored.
Uncertainty
is an
unavoidable aspect
of the the
last decade (see, e.g., Einhorn
&
Hogarth,
human
condition. Many significant choices 1981; Kahneman, Slovic,
&
Tversky, 1982;
must
be
based
on
beliefs
about
the
likelihood Nisbett
&
Ross, 1980). Much
of
this research
of
such uncertain events
as the
guilt
of a de- has
compared intuitive inferences
and
prob-
fendant,
the
result
of an
election,
the
future
ability judgments
to the
rules
of
statistics
and
value
of the
dollar,
the
outcome
of a
medical
the
laws
of-probability.
The
student
of
judg-
operation,
or the
response
of a
friend.
Because ment uses
the
probability calculus
as a
stan-
we
normally
do not
have adequate
formal
dard
of
comparison much
as a
student
of
per-
models
for
computing
the
probabilities
of
such ception might compare
the
perceived sizes
of
events,
intuitive judgment
is
often
the
only objects
to
their physical sizes. Unlike
the
cor-
practical
method
for
assessing uncertainty. rect size
of
objects, however,
the
"correct"
The
question
of how lay
people
and
experts probability
of
events
is not
easily
defined.
Be-
evaluate
the
probabilities
of
uncertain events cause individuals
who
have
different
knowl-
has
attracted considerable research interest
in
edge
or who
hold
different
beliefs must
be
al-
lowed
to
assign
different
probabilities
to the
This
research
was
supported
by
Grant
NR197-058
from
same
event>
no
sing!e
value
can
be
correct
for
the
U.S.
Office
of
Naval
Research.
We
are
grateful
to
friends
all
people.
Furthermore,
a
correct
probability
and
colleagues,
too
numerous
to
list
by
name,
for
their
cannot always
be
determined even
for a
single
useful
comments
and
suggestions
on an
earlier
draft
of
person
Outside
the
domain
of
random
sam-
'Xuets
for
reprints
should
be
sent
to
Amos
Tversky,
PlinS>
Probability theory does
not
determine
Department
of
Psychology,
Jordan Hall, Building
420,
the
probabilities
of
uncertain
events—it
merely
Stanford
University, Stanford, California 94305.
imposes constraints
on the
relations among
Copyright 1983
by the
American Psychological Association, Inc.
293
