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An Introduction to Quantum Models of Cognition and
Decision-making
Jennifer S. TruebloodUniversity of California, Irvine
Thursday, September 5, 13
Motivation
• Judgments often deviate from classical probability theory
• Conjunction & disjunction fallacies, base rate fallacies, subadditivity, etc.
• Heuristics and biases approach lacks a coherent theoretical explanation for judgment effects
• Quantum probability theory has the potential to explain judgments from a theoretical standpoint
Thursday, September 5, 13
Probability Theories
• Classic (Kolmogorov 1933) Theory
• Boolean logic: follows the extension rule A ⊆ B, p(A) < p(B)
• Quantum (von Neumann, 1932) Theory
• A generalization of classical theory
• Drops unicity (a single sample space)
• Can violate the extension rule
Thursday, September 5, 13
Six Reasons for a Quantum Approach to Cognition and Decision
1. Judgments are based on indefinite states
2. Judgments create rather than record
3. Judgments disturb each other, introducing uncertainty
4. Judgments do not always obey classic logic
5. Judgments do not obey the principle of unicity
6. Cognitive phenomena may not be decomposable
Thursday, September 5, 13
Reason 1: Judgments are based on indefinite states
• Stochastic model: a particle producing a definite sample path through a state space
• Quantum model: a wave moving across time over the state space
• superposition state: an indefinite state capturing ambiguity and/or confusion
Thursday, September 5, 13
Reason 2: Judgments create rather than record
• Classic model: answers are “read outs” from stored states
• Quantum model: answers are constructed from the interaction of an indefinite state and the question we ask
• Example: ambiguity of emotional state
• “Are you excited?”
• “Are you sad?”
Thursday, September 5, 13
Reason 3: Judgments disturb each other
• Classical models cannot capture the effects of measurement disturbances
• Quantum theory can allow for one question to disturb the answer to another.
Thursday, September 5, 13
Reason 4: Judgments do not always obey classic logic
• Defendant is guilty or innocent; defendant is good or bad
• Classical logic obeys the distributive axiom:
• Guilty ⋀ (Good ⋁ Bad) = (Guilty ⋀ Good) ⋁ (Guilty ⋀ Bad)
• The law of total probability in classical probability theory is derived from the distributive axiom
• p(Guilty) = p(Good)p(Guilty | Good) + p(Bad)p(Guilty | Bad)
• Quantum logic does not always obey the distributive axiom and thus can violate the law of total probability
Thursday, September 5, 13
Reason 5: Judgments do not obey the principle of unicity
• Classical probability theory assumes a single sample space - a complete and exhaustive description of all events
• Quantum probability allows for multiple sample spaces that are pasted together in a coherent way
Future events
Thursday, September 5, 13
Reason 6: Cognitive phenomena may not be decomposable
• Two seemingly distinct and separated systems behave as one: quantum correlated
• Example, words in a memory experiment
• Two theories:
1.Spreading activation model: words are discrete nodes that can be highly connected
2.Study word and its associates behave as one: a word’s associative network arises in synchrony with the word being studied
Thursday, September 5, 13
Examples from Cognition and Decision
• Examples of paradoxical findings from cognition and decision that can be modeled with quantum probability
1. Interference effects in perception
2. Interference of categorization on decision-making
3. The disjunction effect
4. Violations of dynamic consistency
5. Survey question order effects
6. Conceptual combinations
Thursday, September 5, 13
Interference Effects in Perception
• Consider two different perceptual judgment tasks:
• Task A: binary forced choice response
• Task B: confidence rating on a 7 point scale
• Participants are randomly assigned to two groups:
• Group 1: Task B
• Group 2: Task A followed by Task B
Thursday, September 5, 13
Response Probabilities
Thursday, September 5, 13
Response Probabilities
Task B
.
.
.
1
2
7
S
pB(R
B=1)
pB(R
B=7)
Task A Task B
.
.
.
11
2
1
1
2
7
S
pAB
(RB=1|R
A=1)
pAB
(RB=7|R
A=2)
pAB
(RA=2)
pAB
(RA=1)
Thursday, September 5, 13
Total Probability and Interference Term
• The law of total probability gives:
TP (RB = k) =2X
j=1
pAB(RA = j) · pAB(RB = k | RA = j)
• The interference effect for level k of the response to task B:
IntB(k) = pB(RB = k)� TP (RB = k)
Thursday, September 5, 13
Conte et al. (2009)
• Experiments on perceptual judgment tasks with pairs of ambiguous figures
Thursday, September 5, 13
Conte et al. (2009) Experiment 1• Task was to decide whether the objects (circles or horizontal lines) were
equal or not
Figure A Figure B
Thursday, September 5, 13
Experiment 1 Results
• The inference effect for level B+ (horizontal lines are equal):
IntB(+) = pB(RB = +)� TP (RB = +)
= 0.6667� 0.5000 = 0.1667
Total Probability for A followed by B
Thursday, September 5, 13
Interference of Categorization on Decision-making
• Townsend et al. (2000) Task:
• Categorize faces as ‘Good guy’ or ‘Bad guy’
• Decide to act Friendly (withdraw) or Aggressive (attack)
• Narrow faces had a 0.6 probability of being ‘Bad’ and wide faces had a 0.6 probability of being ‘Good’
Thursday, September 5, 13
Townsend et al. (2000)
• Two conditions
Thursday, September 5, 13
Busemeyer et al. (2009)
• Replication of Townsend (2000)
• Results:
• Total probability of attacking after categorization = 0.59
• Probability of attacking without categorization = 0.69
Interference Effect
Thursday, September 5, 13
Savage’s Sure Thing Principle
• Suppose
• when is the state of the world, you prefer action A over B
• when is the state of the world, you also prefer action A over B
S
S̄
• Therefore you should prefer A over B even when S is unknown
Thursday, September 5, 13
The Disjunction Effect• Violations of the Sure Thing Principle (Tversky & Shafir, 1992) in
the Prisoner’s Dilemma game
You DefectYou
Cooperate
Other Defects
other: 10you: 10
other: 25you: 5
Other Cooperates
other: 5you: 25
other: 20you: 20
StudyKnown to
defectKnown to cooperate
Unknown
Shafir & Tversky (1992)
97 84 63
Croson (1999)
67 32 30
Li & Taplan (2002)
83 66 60
Busemeyer et al. (2006)
91 84 66
Observed proportion of defections
Thursday, September 5, 13
Dynamic Consistency• Dynamic consistency: Final decisions agree with planned decisions (Barkan
and Busemeyer, 2003)
• Two stage gamble
1. Forced to play stage one, but outcome remained unknown2. Made a plan and final choice about stage two
• Plan:
• If you win, do you plan to gamble on stage two?• If you lose, do you plan to gamble on stage two?
• Final decision
• After an actual win, do you gamble on stage two?• After an actual loss, do you now choose to gamble on stage two?
Thursday, September 5, 13
Barkan and Busemeyer (2003) Results
Risk averse after a win
Risk seeking after a loss
Thursday, September 5, 13
Question Order Effects• A Gallup Poll question in 1997 (N = 1002, split sample)
• Do you generally think Bill Clinton is honest and trustworthy?
• How about Al Gore?
• Do you generally think Al Gore is honest and trustworthy?
• How about Bill Clinton?
Thursday, September 5, 13
Question Order Effects: Assimilation
• Proportion of “Yes” responses
• Do you generally think Bill Clinton is honest and trustworthy? (50%)
• How about Al Gore? (60%)
• Do you generally think Al Gore is honest and trustworthy? (68%)
• How about Bill Clinton? (57%)
18%
3%
Thursday, September 5, 13
Question Order Effects: Contrast
• Proportion of “Yes” responses
• Do you generally think Newt Gingrich is honest and trustworthy? (41%)
• How about Bob Dole? (64%)
• Do you generally think Bob Dole is honest and trustworthy? (60%)
• How about Newt Gingrich? (33%)
19%
31%
Thursday, September 5, 13
Conceptual Combinations
• Hampton (1988) asked subjects to rate typicality
1. rate whether an item belonged to category A
2. rate whether it belonged to category B
3. rate whether it belonged to A or B
4. rate whether it belonged to A and B
Thursday, September 5, 13
Results• Is drinking beer a member of games (.78)
• Is drinking beer a member of hobbies (.20)
• Is drinking beer a member of games or hobbies (.58)
• Is a spider a pet (.40)
• Is a spider a farmyard animal (.33)
• Is a spider both a pet and a farmyard animal (.65)
• Is guppy a pet (.09)
• Is guppy a fish (.08)
• Is guppy a pet fish (.39)Thursday, September 5, 13
Two Dimensional Example
Thursday, September 5, 13
Uncertainty Principle• Given True is observed
• State changes to True (S T)
• Prob True now equals 1.0
• Prob of Good equals .50
• Certain about True implies uncertain about Good and visa versa
Thursday, September 5, 13
Interference Principle
• If initially asked True vs False, and True is observed, state changes to True (becoming uncertain about Good)
• If next asked about Good vs Bad and Good is observed, state changes to Good (becoming uncertain about True)
Thursday, September 5, 13
Distributive Rule• Classic Theory
G = G⋀(T⋁F)
= (G⋀T)⋁(G⋀F)
• Quantum Theory
G = G⋀(T⋁F)
≠ (G⋀T)⋁(G⋀F)
(G⋀T) doesn’t exist
(G⋀F) doesn’t exist
Thursday, September 5, 13
Unicity (closure)• Classic Theory
If G, T are events then (G⋀T) is an event
• Quantum Theory
If G, T are compatible, then (G⋀T) is an event
If G, T are incompatible, then (G⋀T) doesn’t exist
Thursday, September 5, 13
Compatible vs Incompatible Representations
• If T,F are compatible with G, B then
Require at least a 4 dimensional space
S = ↵TG|TGi+ ↵TB |TBi+ ↵FG|FGi+ ↵FB |FBi
S = ↵T |T i+ ↵F |F i = �G|Gi+ �B |Bi
• If T, F are incompatible with G,B then
Require at least a 2 dimensional space
Thursday, September 5, 13
Three Dimensional Example
• Voting Event:
1. democrat (outcome D)
2. republican (outcome R)
3. independent (outcome I)
• Ideology Event:
1. liberal (outcome L)
2. conservative (outcome C)
3. moderate (outcome M)
Thursday, September 5, 13
Classic Set Representation
L C M
D D ∩ L D ∩ C D ∩ M
R R ∩ L R ∩ C R ∩ M
I I ∩ L I ∩ C I ∩ M
Vote
Ideology
Thursday, September 5, 13
Classic Probability Function
p(L) p(C) p(M)
p(D) p(D ∩ L) p(D ∩ C) p(D ∩ M)
p(R) p(R ∩ L) p(R ∩ C) p(R ∩ M)
p(I) p(I ∩ L) p(I ∩ C) p(I ∩ M)
Probabilities sum to 1 for
nine joint outcomes
Thursday, September 5, 13
Compatibility
• Compatible events
• Two events can be realized simultaneously
• Order of events does not matter
• Incompatible events
• Two events cannot be realized simultaneously
• Order of events does matter
} }QuantumProbability
ClassicProbability
Thursday, September 5, 13
Compatible Events in Quantum Probability
• State vector within nine dimensional space
↵i\j
same as classical probability
q(i \ j) = ||Pij | i||2 = ||↵i\j ||2 = p(i \ j)
Projector (9x9 matrix with all zeros except a 1 for i∩j)
| i =
2
6666666666664
↵D\L
↵D\C
↵D\M
↵R\L
↵R\C
↵R\M
↵I\L
↵I\C
↵I\M
3
7777777777775
• Probability amplitude for voting outcome i and ideology outcome j:
• Quantum probability for voting outcome i and ideology outcome j
Thursday, September 5, 13
Vector Space For Incompatible Events
• Represented by two basis for the same 3 dimensional vector spaceD
R
I
C
M
L • Ideology Basis:
L = liberal
C = conservative
M = moderate
• Voting Basis:
D = democrat
R = republican
I = independent
• Ideology Basis is a unitary transformation of the Voting Basis:
Id = {U |Di, U |Ri, U |Ii}
V = {|Di, |Ri, |Ii} Id = {|Li, |Ci, |Mi}
Thursday, September 5, 13
Calculating Quantum Probabilities
• Belief State:D
R
I
C
M
L| i = S
| i = �L|Li+ �C |Ci+ �M |Mi
= U�L|Di+ U�C |Ri+ U�M |Ii
| i = ↵D|Di+ ↵R|Ri+ ↵I |Ii
↵ = U� � = U †↵
q(D) = ||PD| i||2 = ||↵D||2
q(C) = ||PC | i||2 = ||�C ||2
• Unitary Transformations relate the probability amplitudes:
• Calculating probabilities
Thursday, September 5, 13
Mixed vs Superposed States
• Suppose voter is NOT independent
• Mixed State:
• 0.5 probability state = D
• 0.5 probability state = R
• Superposition:
S =1p2(|Di+ |Ri)
Thursday, September 5, 13
Mixed vs Superposed States• For both mixed and superposed states
• Equal probability of voting democrat or republican
• If democrat, probability moderate equals 0.5
• If republican, probability moderate equals 0.25• In a classical mixed state
• Either democrat or republican exactly
• Total probability moderate = P(D)P(M|D) + P(R)P(M|R) = (.5)x(.5) +(.5)x(.25) = .375
S =1p2(|Di+ |Ri)• In a quantum superposed state
• Neither a democrat or republican exactly
• The probability of being a moderate is zero because S is orthogonal to the moderate event
Thursday, September 5, 13
Thank You• What’s coming next...
• Quantum models of human judgments
• Dynamic quantum decision models
Thursday, September 5, 13