In short: Human judgement is driven not simply (or at all) by logic, but rather by cognitive shortcuts and social influences

1. PS409
Psychology, Science,
& Pseudoscience
Dr Brian Hughes
School of Psychology
brian.hughes@nuigalway.ie @b_m_hughes

2. Evidentiary reasoning:
Why do people believe weird
things?

3. Question:
If not on mathematical probability, then
what do we base our judgments of
likelihood on?

4. Life (Survival Rate)
75%
Now Year 1 Year 2 Year 3 Year 4 Year 5
Surgery 90 68 51 40 35 34
Radiation 100 77 44 28 23 22
A numerical representation of the choice between surgery and radiation for
hypothetical sixty-year-olds with lung cancer (from McNeil et al., 1982)

5. 58% Death (Mortality Rate)
Now Year 1 Year 2 Year 3 Year 4 Year 5
Surgery 10 32 49 60 65 66
Radiation 0 23 56 72 77 78
A numerical representation of the choice between surgery and radiation for
hypothetical sixty-year-olds with lung cancer (from McNeil et al., 1982)

6. Language factors
58% Death (Mortality Rate) & Life (Survival Rate) 75%
Now Year 1 Year 2 Year 3 Year 4 Year 5
Surgery 10 32 49 60 65 66
90 68 51 40 35 34
Radiation 0 23 56 72 77 78
100 77 44 28 23 22
A numerical representation of the choice between surgery and radiation for
hypothetical sixty-year-olds with lung cancer (from McNeil et al., 1982)

7. Confirmation bias
Imagine I have a pack of cards, each of which has a letter printed
on one side and a number printed on the other. Imagine I show you
these four cards from the pack:
EK4 7
Question: Which card or cards must you turn over in order to
decide whether or not the following RULE is true? Turn over as few
cards as necessary.
RULE: If a card has a vowel on one side, then it must
have an even number on the other.

8. Confirmation bias
Imagine I have a pack of cards, each of which has a letter printed
on one side and a number printed on the other. Imagine I show you
these four cards from the pack:
EK4 7
×
Question: Which card or cards must you turn over in order to
75%
decide whether or not the following RULE is true? Turn over as few
cards as necessary.
RULE: If a card has a vowel on one side, then it must
have an even number on the other.

9. Confirmation bias
Imagine I have a pack of cards, each of which has a letter printed
on one side and a number printed on the other. Imagine I show you
these four cards from the pack:
EK4 7

Question: Which card or cards must you turn over in order to
10%
decide whether or not the following RULE is true? Turn over as few
cards as necessary.
RULE: If a card has a vowel on one side, then it must
have an even number on the other.

10. Confirmation bias
Tendency to focus on confirmatory information
Willingness to base interpretations on partial data
Tendency to fail to consider counter-examples
Failure to control for base-rates

11. Heuristics
Linda is a former student activist, very intelligent, single, good with Conjunction fallacy
numbers, and a philosophy graduate. (Tversky & Kahneman,
1983)
Which of the following is most likely:
(a) Linda is a bank teller
(b) Linda is a feminist
(c) Linda is a feminist bank teller
Steve is very shy and withdrawn, invariably helpful, but with little interest Representativeness
in people…A meek and tidy soul, he has a need for order and structure heuristic
and a passion for detail. (Tversky & Kahnemen,
1974)
Which of the following is most likely:
(a) Steve is a farmer
(b) Steve is a pilot
(c) Steve is a doctor
(d) Steve is a librarian
If a word of three letters or more, is it more likely that the word starts Availability heuristic
with “r” or has “r” as its third letter? (Tversky & Kahneman,
1974)
What is the probability that you will die next year? Support theory
(Tversky & Koehler,
What is the probability that you will die on your next summer holiday 1994)
from a disease, a sudden heart attack, an earthquake, terrorist activity, a
civil war, a car accident, a plane crash, or from any other cause?

12. Heuristics
Where mental shortcuts systematically undermine our
judgement of probabilities
May explain why rare events are seen as common,
common events seen as rare, etc.

13. Social factors

14. Motivation
The Lake Wobegon effect
(Gilovich, 1991)
where “the women are
strong, the men are good-
looking, and all the children
are above average”
Interpretation of situations
in self-serving ways
Personal qualities/fortunes
Causal attributions
e.g. athletes, students, lecturers,
researchers

15. Motivation
Common illusions Benefits
Unrealistically positive self-regard Happiness or contentment
Illusions of control Ability to care for others
Unrealistic optimism Capacity for creativity/productivity

16. PS409
Psychology, Science,
& Pseudoscience
Dr Brian Hughes
School of Psychology
brian.hughes@nuigalway.ie @b_m_hughes