Reasoning and Decision Making
The Shortcuts of the Human Mind
(a.k.a Heuristics)
by Elan Dubrofsky and Dina Tsirlin
• Cognitive processes by which people start
with information and come to conclusions that
go beyond that information
Deductive Reasoning
• Syllogism
– Two statements called premises
– Third statement called conclusion
• Categorical Syllogism
– Describe relation between two categories
using all, no, or some
Premise 1: All computer scientists are nerds.
Premise 2: All nerds can name all six Star
Wars movies.
Conclusion: All computer scientists can
name all six Star War movies.
Deductive Reasoning
• Syllogism is valid if conclusion follows
logically from its two premises
• Aristotle’s “perfect” syllogism
– Premise 1: All A are B
– Premise 2: All B are C
– Conclusion: Therefore, All A are C
Deductive Reasoning
• If two premises of a valid syllogism are true,
the syllogism’s conclusion must be true.
– Do not confuse “validity” with “truth”
The following syllogism is valid but not true
All birds are animals
All animals have four legs
All birds have four legs
How Well Can People Judge Validity?
• Errors in evaluation
– Atmosphere effect: use of words “all”,
“some” or “no” in premises increase the
probability of a conclusion with those
Belief bias: if syllogism is true or agrees with a
person’s beliefs, more likely to be judged valid
How do people go about determining
whether a syllogism is valid/invalid?
Mental Models of Deductive Reasoning
• Specific situation that is represented in a
person’s mind that can be used to help
determine the validity of syllogisms
• Iterative process
Look for exceptions
if no exception accept model and establish
if exception modify the model until can be
Deductive Reasoning
• Conditional syllogisms
– “If p, then q.”
If I lend Emt $20, Then I won’t get it back.
I lent Emt $20. Therefore, I won’t get my $20
– Four types of conditional syllogisms
• Affirming the antecedent
• Denying the consequent
• Affirming the consequent
• Denying the antecedent
The Wason Four-Card Problem
• Effect of using real-world items in a
conditional-reasoning problem
– Determine minimum number of cards to
turn over to test: If there is a vowel on one
side, then there is an even number on the
other side.
Caption: The Wason four-card problem (Wason, 1966).
The Wason Four-Card Problem
Falsification principle: to test a rule, you must
look for situations that falsify the rule
– Most participants fail to do this
– When problem is stated in concrete
everyday terms, correct responses greatly
The Wason Four-Card Problem
• Pragmatic reasoning schema: thinking about
cause and effect in the world as part of
experiencing everyday life
– Permission schema: if A is satisfied, B can
be carried out
• Used in the concrete versions
• People are familiar with rules
Evolutionary Perspective on Cognition
• Evolutionary principles of natural selection
• Wason task governed by built-in cognitive
program for detecting cheating
Evolutionary Perspective on Cognition
• Cosmides and Tooby (1992)
– Created unfamiliar situations where
cheating could occur
– Participants did well
– Evidence against permission schema
Inductive Reasoning
• Premises are based on observation and we
generalize from these cases to more general
conclusions with varying degrees of certainty
Inductive Reasoning
• Strength of argument
– Representativeness of observations
– Number of observations
– Quality of observations
ACTIVITY: Which argument is stronger? Why?
Observation: All sushi places I’ve seen in Vancouver
charge a lot for sashimi. When I visited my family in Ottawa,
the sashimi was expensive too.
Conclusion: All sushi
places charge a lot for sashimi.
Observation: Here in Ottawa, the sun has risen every
Conclusion: The sun is going to rise in
Ottawa tomorrow.
Inductive Reasoning
• Used to make scientific discoveries
– Hypotheses and general conclusions
• Used in everyday life
– Make a prediction about what will happen
based on observation about what has
happened in the past
• Availability heuristic: events more easily
remembered are judged as being more
probable than those less easily remembered
• Is it easier to die for car accident of plane
Caption: Likely-causes-of-death experiment results. Pairs of “causes of death” are
listed below the graph, with the least likely cause on the left. The number in
parentheses on the right indicates how many more times more people were actually
killed by the cause on the right. The bars in the graph indicate the number of people
who judged the least likely alternative in each pair as causing the most deaths.
(Adapted from Lichtenstein et al., 1978).
• Illusory correlations: correlation appears to
exist, but either does not exist or is much
weaker than assumed
– Stereotypes
A little experiment...
Rate info: Among 100 people, 70 are
lawyers, 30 are engineers
A little experiment...
Description: Jack, 45 yrs old, 4 kids,
conservative, careful. Not interested in
politics, many hobbies: math puzzles &
carpentry – Lawyer or engineer?
• Representativeness Heuristic: the probability
that A comes from B can be determined by
how well A resembles properties of B
– Use base rate information if it is all that is
– Use descriptive information if available and
disregard base rate information
• Violation of Conjunction rule
• Conjunction rule: probability of two events
cannot be higher than the probability of the
single constituents
Caption: Because feminist bank tellers are a subset of bank
tellers, it is always more likely that someone is a bank teller
than a feminist bank teller.
• The Confirmation Bias: tendency to
selectively look for information that conforms
to our hypothesis and overlook information
that argues against it
• The Confirmation Bias
• Lord and coworkers (1979)
– Had those in favor of capital punishment
and those against capital punishment read
the same article
• Those in favor found the article in favor
• Those against found the article against
Decision Making
• Economic utility theory
– People are rational and if they have all
relevant information they will make a
decision which results in the maximum
expected utility
Decision Making
• Utility: outcomes that are desirable because
they are in the person’s best interest
– Maximum monetary payoff
Decision Making
• Problems for Utility Approach
– Not necessarily money, people find value
in other things
– Many decisions involve payoffs that cannot
be calculated
“Good enough” philosophy (Herb Simon,
Psychologist Nobel Prize!!!)
Caption: Behavioral results of Sanfey and coworkers’ (2003)
experiment, showing responders’ acceptance rates in response
to different offers made by human partners and computer
Decision Making
• Focusing illusion: focus on just one aspect of
situation and ignore other aspects that may
be important
– Dating and happiness
– California versus Midwest living
Decision Making
Decisions depend on how choices are
– Opt-in procedure
• active step to be organ donor
– Opt-out procedure
• Organ donor unless request not to be
Subject’s consent to research
Active Consent
Passive consent
Decision Making
• Risky decisions
– Risk-aversion strategy used when problem
is stated in terms of gains
– Risk-taking strategy when problem is
stated in terms of losses
Decision Making
• Framing effect: decisions are influenced by
how a decisions is stated
– Can highlight one aspect of situation
Decision Making
• Decision-making process includes looking for
justification so a rationale presented with
Decision Making
Tversky and Shafir (1992)
– “pass” - go on trip
– “fail” - do not
– “I don’t know yet” – wait to find out results
before making decision to go on trip or not
In Conclusion...
We're only human... therefore our thinking is very flawed.
Be careful to make sure that when you use a heuristic, it's
not leading you down a dangerous path.