Public lecture, Mary Immaculate College, University of Limerick, 9 April 2013

1. The ‘Delusion’ Delusion:
The persistence of counter-evidentiary
reasoning
Brian Hughes
National University of Ireland, Galway

2. Number of tickets sold for the last Millionaire
Raffle = 180,000
Once-off chance of success = 1 in 180,000
But, four Millionaire Raffles each year, giving
you four chances of winning
Corrected chance of being attacked by a bear = 1 in
270,000
Corrected number of bear attacks in Ireland per
annum = 17
Therefore…
Chance of being attacked by a bear = 1 in 1,080,000
Number of bear attacks in Ireland = 4
Pr (millionaire) = 6 x Pr (bear attack)
HT locum-motion on boards.ie

3. Extinct in Ireland

8. http://www.fark.com/comments/7535504/Not-to-alarm-anyone-but-Google-may-
have-run-over-a-donkey-in-Botswana

12. http://www.neowin.net/news/google-maps-car-hits-a-deer-records-it-all

13. Mail on Sunday, 10 February 2013

14. Private Eye, 22 February 2013

16. http://www.gallup.com/poll/19558/Paranormal-Beliefs-Come-SuperNaturally-Some.aspx

17. http://www.gallup.com/poll/19558/Paranormal-Beliefs-Come-SuperNaturally-Some.aspx

19. http://goo.gl/SReC0

21. “Delusions bad”?

22. Linda is 31 years old, single, outspoken, and very bright. As a
student, she was deeply concerned with issues of discrimination
and social justice.
Which of the following is most likely:
(a) Linda is a bank teller
(b) Linda is a bank teller and is active in the feminist movement

23. Linda is 31 years old, single, outspoken, and very bright. As a
student, she was deeply concerned with issues of discrimination
and social justice.
Which of the following is most likely:
(a) Linda is a bank teller
(b) Linda is a bank teller and is active in the feminist movement

25. Conjunction fallacy
IF Pr (Linda is a bank teller) = 0.10
AND Pr (Linda is a feminist) = 0.95…
THEN Pr (Linda is a bank teller and
a feminist)
= 0.10 × 0.95
= or 0.095
i.e., lower than Pr (Linda is a bank teller)
Based on http://paradigmsanddemographics.blogspot.com/2011/09/logical-fallacy-of-week-conjunction.html

26. A 55-year-old woman had pulmonary embolism documented
angiographically 10 days after cholecystectomy.
Rate the likelihood of the following subsequent after-effects:
(a) The woman experienced pleuritic chest pain
(b) The woman experienced dyspnea and hemiparesis
(c) The woman experienced hemiparesis
(d) The woman experienced hemoptysis
(e) etc.

27. 1
2
3
4
5
Dyspnea AND
Hemiparesis
Hemiparesis
More
probable
Less
probable
37 medical postgrads at Harvard University
plus 66 internists at New England Medical Center
32 physicians at Stanford University
Tversky & Kahneman (1983)

28. Tossing one coin, what is the
chance of it landing on “Heads”?
A:0.50
(or, in other words, a fifty-fifty chance)
Throwing one dice, what is the
chance of it landing on “5”?
A: 0.17
(or, in other words, a one-in-six chance)
Difficulties with probabilistic reasoning

29. Tossing two coins, what is the
chance of getting two “Heads”?
A: 0.25
(or, in other words, a 1-in-4 chance)
Throwing two dice, what is the
chance of getting two “5”s?
A: 0.028
(or, in other words, a 1-in-36 chance)
Difficulties with probabilistic reasoning

30. Difficulties with probabilistic reasoning
If a test to detect a disease whose prevalence is 1/1000 has
a false positive rate of 5%, what is the chance that a person
found to have a positive result actually has the disease,
assuming you know nothing about the person’s symptoms
or signs?
Cited by Pinker (1997)
•Among Staff and Students of Harvard Medical School (n = 60):
Most popular answer
= 0.95 (i.e., a 19 out of 20 chance)
Average of all answers
= 0.56 (c., a fifty-fifty chance)

31. 999/1000 x .05
Difficulties with probabilistic reasoning
Cited by Pinker (1997)
[Base-rate] x [Test sensitivity] / [Rate of positive results]
[1/1000]
Prevalence of
disease per 1000
[1/1]
Proportion of sick who
test positive
Number of positive
results per 1000
1/1000 + ([ )]
“False positives”
i.e., well persons who
test ‘positive’
Actual sick
persons testing
‘positive’
If a test to detect a disease whose prevalence is 1/1000 has
a false positive rate of 5%, what is the chance that a person
found to have a positive result actually has the disease,
assuming you know nothing about the person’s symptoms
or signs?

32. )]+ (0.001 0.049950.001 1.0
Difficulties with probabilistic reasoning
Cited by Pinker (1997)
[Base-rate] x [Test sensitivity] / [Rate of positive results]
= 0.019627
[x /=
= 0.001 / 0.05095
≈ 0.02
If a test to detect a disease whose prevalence is 1/1000 has
a false positive rate of 5%, what is the chance that a person
found to have a positive result actually has the disease,
assuming you know nothing about the person’s symptoms
or signs?

33. •Among Staff and Students of Harvard Medical School (n = 60):
Most popular answer
= 0.95 (i.e., a 19 out of 20 chance)
Average of all answers
= 0.56 (c., a fifty-fifty chance)
Difficulties with probabilistic reasoning
Cited by Pinker (1997)
≈ 0.02 (or, in other words, a 1-in-50 chance)
If a test to detect a disease whose prevalence is 1/1000 has
a false positive rate of 5%, what is the chance that a person
found to have a positive result actually has the disease,
assuming you know nothing about the person’s symptoms
or signs?

34. Difficulties with probabilistic reasoning
Gøtzsche PC, Nielsen M. Screening for breast cancer with mammography. Cochrane Database of
Systematic Reviews 2006, Issue 4. Art. No.: CD001877. DOI: 10.1002/14651858.CD001877.pub2
…For every 2000 women invited for screening
throughout 10 years, one will have her life
prolonged. In addition, 10 healthy women, who
would not have been diagnosed if there had not
been screening, will be [erroneously] diagnosed
and treated unnecessarily…

36. http://goo.gl/0D4JC

39. Coincidences
•Lucia de Berk (Wikipedia)
–Dutch nurse sentenced to life
imprisonment in 2003
–Found guilty of four murders and
three attempted murders, largely
on statistical evidence
–“one in 342 million against”
–Problems:
• Multiplied p-values
• Did not compare against base-rate
–Case re-opened in 2008
–Exonerated in April 2010

40. “Delusions bad”?

41. “Delusions good”?

42. www.gallup.com/poll/27847/Majority-Republicans-Doubt-Theory-Evolution.aspx

43. Tendency toward non-empirical
beliefs may be physiologically in-
built

44. Non-empirical beliefs may promote
inclusive fitness
Sosis, R. (2000). Religion and intragroup cooperation: Preliminary results of a comparative analysis of Utopian communities.
Cross-Cultural Research, 34, 70–87.

45. Non-empirical thinking may serve as
a “costly signal” of individual fitness
Sosis, R., & Bressler, E. (2003). Cooperation and commune longevity: A test of the costly signaling theory of religion. Cross-
Cultural Research, 37, 211–239.

46. Faulty beliefs may boost one’s self-
esteem
Above
average
70%
Average
28%
Below
average
2%
Self-rated "Leadership ability"
Data from 1,000,000 studentsCited by Gilovich (1991)

47. Faulty beliefs may boost one’s self-
esteem
Data from 1,000,000 studentsCited by Gilovich (1991)
Self-rated "Ability to get along with others"
Above
average
100%

48. Faulty beliefs may boost one’s self-
esteem
Data from 1,000,000 studentsCited by Gilovich (1991)
“Top 1%”
25%
“Also in top
quarter”
35%

49. Faulty beliefs may boost one’s self-
esteem
Above
average
94%
Below
average
6%
Competence at one’s job
Data from university lecturersCited by Gilovich (1991)

50. Faulty beliefs may boost one’s self-
esteem
Everyone
All-round greatness
“Me!”

51. Common illusions
Unrealistically positive self-regard
Illusions of control
Unrealistic optimism
Benefits
Happiness or contentment
Ability to care for others
Capacity for creativity/productivity
Faulty beliefs may be the brain’s
default state

52. Depression, confidence, and accuracy
Szu-Ting Fu, T., Koutstaal, W., Poon, L., & Cleare, A. J. (2012). Confidence judgment in depression and
dysphoria: The depressive realism vs. negativity hypothesis. Journal of Behavior Therapy and
Experimental Psychiatry, 43, 699-704.

53. Szu-Ting Fu, T., Koutstaal, W., Poon, L., & Cleare, A. J. (2012). Confidence judgment in depression and
dysphoria: The depressive realism vs. negativity hypothesis. Journal of Behavior Therapy and
Experimental Psychiatry, 43, 699-704.
0
0.02
0.04
0.06
0.08
0.1
0.12
Healthy Depressed Chronic Fatigue
Syndrome
Over-confidence (i.e., estimate – reality)
during task

54. Szu-Ting Fu, T., Koutstaal, W., Poon, L., & Cleare, A. J. (2012). Confidence judgment in depression and
dysphoria: The depressive realism vs. negativity hypothesis. Journal of Behavior Therapy and
Experimental Psychiatry, 43, 699-704.

55. Impact on recall of depressive states
Ben-Zeev, D., & Young, M. A. (2010). Accuracy of hospitalized depressed patients’ and healthy controls’
retrospective symptom reports. Journal of Nervous and Mental Disease, 198, 280-285.
Practical Emotional

56. Too much of a bad thing
Soderstrom, N. C., Davalos, D. B., & Vázquez, S. M. (2011). Metacognition and depressive realism:
Evidence for the level-of-depression account. Cognitive Neuropsychiatry, 16, 461-472.

57. Too much of a bad thing
Soderstrom, N. C., Davalos, D. B., & Vázquez, S. M. (2011). Metacognition and depressive realism:
Evidence for the level-of-depression account. Cognitive Neuropsychiatry, 16, 461-472.

58. http://www.practiceofmadness.com/2010/03/psychiatric-drugs-a-history-in-ads/

59. http://www.practiceofmadness.com/2010/03/psychiatric-drugs-a-history-in-ads/

60. http://www.pharmacytechs.net/blog/old-school-medicine-ads

61. http://www.practiceofmadness.com/2010/03/psychiatric-drugs-a-history-in-ads/

62. http://www.practiceofmadness.com/2010/03/psychiatric-drugs-a-history-in-ads/

63. The “Third-Person Effect”
Davison, W. P. (1983). The Third-Person Effect in communication. Public Opinion Quarterly, 47, 1-15.
Self influenced by
advertising
Others influenced
by advertising
Quite a lot 32% 68%
Some 24% 28%
Not very much 20% 4%
Not at all 16% 0%
N/A 8% 0%

64. Impact of depression on Third Person Effect,
for anti-depressant advertisements
Taylor, L. D., Bell, R. A., & Kravitz, R. L. (2011). Third-person effects and direct-to-consumer
advertisements for antidepressants. Depression and Anxiety, 28, 160-165.

65. Human irrationality
is often ‘explained
away’ in a self-
serving fashion
This informs the way
we try to deal with
delusional beliefs in
the public sphere
Delusions may in
fact serve protective
functions and so be
difficult to shift…
…but just because
delusions are
‘natural’ does not
make them ‘good’!
brian.hughes@nuigalway.ie
http://thesciencebit.net

66. Human irrationality
is often ‘explained
away’ in a self-
serving fashion
This informs the way
we try to deal with
delusional beliefs in
the public sphere
Delusions may in
fact serve protective
functions and so be
difficult to shift…
…but just because
delusions are
‘natural’ does not
make them ‘good’!
brian.hughes@nuigalway.ie
http://thesciencebit.net