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Competition in persuasion
Matthew Gentzkow
Emir Kamenica
University of Chicago
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Motivation
Does competition among persuaders increase information?
Long tradition in political and legal thought says: Yes
Media ownership regulation
First Amendment law
Adversarial judicial system
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Motivation
Does competition among persuaders increase information?
Long tradition in political and legal thought says: Yes
Media ownership regulation
First Amendment law
Adversarial judicial system
![Page 4: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/4.jpg)
Motivation
Does competition among persuaders increase information?
Long tradition in political and legal thought says: Yes
Media ownership regulation
First Amendment law
Adversarial judicial system
![Page 5: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/5.jpg)
Example
Two pharmaceutical companies, j = 1, 2
ωij ∈ l , h is the quality of drug j for consumer i
Qualities are independent and Pr (ωij = h) = 15
Unit mass of consumers, all prefer high to low quality1
2always buy the drug with higher expected quality
1
2buy the drug with higher expected quality if Pr (ωi = h) > 1
2
Each rm j chooses one of two signals:
null : an uninformative signal
revealj : fully reveals quality of own drug for all i
null reveal2null 1
4 ,14
25 ,
15
reveal115 ,
25
1750 ,
1750
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Example
Two pharmaceutical companies, j = 1, 2
ωij ∈ l , h is the quality of drug j for consumer i
Qualities are independent and Pr (ωij = h) = 15
Unit mass of consumers, all prefer high to low quality1
2always buy the drug with higher expected quality
1
2buy the drug with higher expected quality if Pr (ωi = h) > 1
2
Each rm j chooses one of two signals:
null : an uninformative signal
revealj : fully reveals quality of own drug for all i
null reveal2null 1
4 ,14
25 ,
15
reveal115 ,
25
1750 ,
1750
![Page 7: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/7.jpg)
Example
Two pharmaceutical companies, j = 1, 2
ωij ∈ l , h is the quality of drug j for consumer i
Qualities are independent and Pr (ωij = h) = 15
Unit mass of consumers, all prefer high to low quality1
2always buy the drug with higher expected quality
1
2buy the drug with higher expected quality if Pr (ωi = h) > 1
2
Each rm j chooses one of two signals:
null : an uninformative signal
revealj : fully reveals quality of own drug for all i
null reveal2null 1
4 ,14
25 ,
15
reveal115 ,
25
1750 ,
1750
![Page 8: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/8.jpg)
Example
Two pharmaceutical companies, j = 1, 2
ωij ∈ l , h is the quality of drug j for consumer i
Qualities are independent and Pr (ωij = h) = 15
Unit mass of consumers, all prefer high to low quality1
2always buy the drug with higher expected quality
1
2buy the drug with higher expected quality if Pr (ωi = h) > 1
2
Each rm j chooses one of two signals:
null : an uninformative signal
revealj : fully reveals quality of own drug for all i
null reveal2null 1
4 ,14
25 ,
15
reveal115 ,
25
1750 ,
1750
![Page 9: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/9.jpg)
Prisoners' dilemma
null reveal2null 1
4 ,14
25 ,
15
reveal115 ,
25
1750 ,
1750
Prisoners' dilemma
revealing information benecial for the rm's joint prots
revealing information unilaterally unattractive
null is a dominant strategy
Unique equilibrium: (null , null)
Unique collusive outcome: (reveal1, reveal2)
Enhancing competition (blocking a merger) leads to less information
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Framework and goal
Symmetric information
Number of senders simultaneously choose signals about the state
Each sender has arbitrary preferences over the information revealed
Competition can reduce information
Competition can increase information
Information environment species information available to each sender
Goal: Find a condition on the information environment such that
Theorem
Competition unambiguously increases information if and only if this
condition is satised.
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Framework and goal
Symmetric information
Number of senders simultaneously choose signals about the state
Each sender has arbitrary preferences over the information revealed
Competition can reduce information
Competition can increase information
Information environment species information available to each sender
Goal: Find a condition on the information environment such that
Theorem
Competition unambiguously increases information if and only if this
condition is satised.
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Framework and goal
Symmetric information
Number of senders simultaneously choose signals about the state
Each sender has arbitrary preferences over the information revealed
Competition can reduce information
Competition can increase information
Information environment species information available to each sender
Goal: Find a condition on the information environment such that
Theorem
Competition unambiguously increases information if and only if this
condition is satised.
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Framework and goal
Symmetric information
Number of senders simultaneously choose signals about the state
Each sender has arbitrary preferences over the information revealed
Competition can reduce information
Competition can increase information
Information environment species information available to each sender
Goal: Find a condition on the information environment such that
Theorem
Competition unambiguously increases information if and only if this
condition is satised.
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Model
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Model
Finite state space Ω; typical state ω
n senders with a common prior
A signal is a random variable (potentially) correlated with ω
A set of signals P induces a distribution of posteriors 〈P〉Simultaneous move game:
sender i chooses signal πi ∈ Πi
strategy prole π = (π1, ..., πn)sender i 's payo: vi (〈π〉)
Focus on pure strategy equilibria
Assumption:
π ∈ Πi ∀i : 〈P ∪ π〉 = 〈P〉 ∀P
Terminology:
τ is feasible if ∃π ∈ Π s.t. τ = 〈π〉
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Model
Finite state space Ω; typical state ω
n senders with a common prior
A signal is a random variable (potentially) correlated with ω
A set of signals P induces a distribution of posteriors 〈P〉
Simultaneous move game:
sender i chooses signal πi ∈ Πi
strategy prole π = (π1, ..., πn)sender i 's payo: vi (〈π〉)
Focus on pure strategy equilibria
Assumption:
π ∈ Πi ∀i : 〈P ∪ π〉 = 〈P〉 ∀P
Terminology:
τ is feasible if ∃π ∈ Π s.t. τ = 〈π〉
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Model
Finite state space Ω; typical state ω
n senders with a common prior
A signal is a random variable (potentially) correlated with ω
A set of signals P induces a distribution of posteriors 〈P〉Simultaneous move game:
sender i chooses signal πi ∈ Πi
strategy prole π = (π1, ..., πn)sender i 's payo: vi (〈π〉)
Focus on pure strategy equilibria
Assumption:
π ∈ Πi ∀i : 〈P ∪ π〉 = 〈P〉 ∀P
Terminology:
τ is feasible if ∃π ∈ Π s.t. τ = 〈π〉
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Model
Finite state space Ω; typical state ω
n senders with a common prior
A signal is a random variable (potentially) correlated with ω
A set of signals P induces a distribution of posteriors 〈P〉Simultaneous move game:
sender i chooses signal πi ∈ Πi
strategy prole π = (π1, ..., πn)sender i 's payo: vi (〈π〉)
Focus on pure strategy equilibria
Assumption:
π ∈ Πi ∀i : 〈P ∪ π〉 = 〈P〉 ∀P
Terminology:
τ is feasible if ∃π ∈ Π s.t. τ = 〈π〉
![Page 19: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/19.jpg)
Model
Finite state space Ω; typical state ω
n senders with a common prior
A signal is a random variable (potentially) correlated with ω
A set of signals P induces a distribution of posteriors 〈P〉Simultaneous move game:
sender i chooses signal πi ∈ Πi
strategy prole π = (π1, ..., πn)sender i 's payo: vi (〈π〉)
Focus on pure strategy equilibria
Assumption:
π ∈ Πi ∀i : 〈P ∪ π〉 = 〈P〉 ∀P
Terminology:
τ is feasible if ∃π ∈ Π s.t. τ = 〈π〉
![Page 20: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/20.jpg)
Model
Finite state space Ω; typical state ω
n senders with a common prior
A signal is a random variable (potentially) correlated with ω
A set of signals P induces a distribution of posteriors 〈P〉Simultaneous move game:
sender i chooses signal πi ∈ Πi
strategy prole π = (π1, ..., πn)sender i 's payo: vi (〈π〉)
Focus on pure strategy equilibria
Assumption:
π ∈ Πi ∀i : 〈P ∪ π〉 = 〈P〉 ∀P
Terminology:
τ is feasible if ∃π ∈ Π s.t. τ = 〈π〉
![Page 21: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/21.jpg)
The Blackwell order
Blackwell order on the set of outcomes
Partial order
τ τ ′ → τ is more informative than τ ′
τ ′ τ → τ is no less informative than τ ′
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The Blackwell order
Blackwell order on the set of outcomes
Partial order
τ τ ′ → τ is more informative than τ ′
τ ′ τ → τ is no less informative than τ ′
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Key features of the set-up
Information generated directly observed
Senders have no private information when they choose their signals
All available signals are equally costly
Arbitrary Πi 's allow some signals to be prohibitively costly
Allow for comparative advantage
No sender can down out information provided by others:
P ′ ⊂ P =⇒ 〈P〉 〈P ′〉
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Key features of the set-up
Information generated directly observed
Senders have no private information when they choose their signals
All available signals are equally costly
Arbitrary Πi 's allow some signals to be prohibitively costly
Allow for comparative advantage
No sender can down out information provided by others:
P ′ ⊂ P =⇒ 〈P〉 〈P ′〉
![Page 25: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/25.jpg)
Key features of the set-up
Information generated directly observed
Senders have no private information when they choose their signals
All available signals are equally costly
Arbitrary Πi 's allow some signals to be prohibitively costly
Allow for comparative advantage
No sender can down out information provided by others:
P ′ ⊂ P =⇒ 〈P〉 〈P ′〉
![Page 26: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/26.jpg)
Key features of the set-up
Information generated directly observed
Senders have no private information when they choose their signals
All available signals are equally costly
Arbitrary Πi 's allow some signals to be prohibitively costly
Allow for comparative advantage
No sender can down out information provided by others:
P ′ ⊂ P =⇒ 〈P〉 〈P ′〉
![Page 27: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/27.jpg)
Basic intuition
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The sum game
Each player i chooses qi ∈ NOutcome of the game is τ =
∑i qi
Player i 's payo is vi (τ)
Unique collusive outcome that maximizes∑
i vi (τ)
Proposition
Any pure strategy equilibrium outcome is weakly greater than the collusive
outcome.
suppose τ c > τ∗
for at least one player vi (τ c) > vi (τ∗)
player i can protably deviate to qi = q∗i + (τ c − τ∗)
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The sum game
Each player i chooses qi ∈ NOutcome of the game is τ =
∑i qi
Player i 's payo is vi (τ)
Unique collusive outcome that maximizes∑
i vi (τ)
Proposition
Any pure strategy equilibrium outcome is weakly greater than the collusive
outcome.
suppose τ c > τ∗
for at least one player vi (τ c) > vi (τ∗)
player i can protably deviate to qi = q∗i + (τ c − τ∗)
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The sum game
Each player i chooses qi ∈ NOutcome of the game is τ =
∑i qi
Player i 's payo is vi (τ)
Unique collusive outcome that maximizes∑
i vi (τ)
Proposition
Any pure strategy equilibrium outcome is weakly greater than the collusive
outcome.
suppose τ c > τ∗
for at least one player vi (τ c) > vi (τ∗)
player i can protably deviate to qi = q∗i + (τ c − τ∗)
![Page 31: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/31.jpg)
The sum game
Each player i chooses qi ∈ NOutcome of the game is τ =
∑i qi
Player i 's payo is vi (τ)
Unique collusive outcome that maximizes∑
i vi (τ)
Proposition
Any pure strategy equilibrium outcome is weakly greater than the collusive
outcome.
suppose τ c > τ∗
for at least one player vi (τ c) > vi (τ∗)
player i can protably deviate to qi = q∗i + (τ c − τ∗)
![Page 32: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/32.jpg)
The sum game: key properties
No downward deviation feasible: qi ≥ 0
Equilibria with excessively high outcomes possible
Information also has this feature
Every upward deviation feasible: every i can deviate to any τ ≥∑
qi
Equilibria with excessively low outcomes not possible
Information does not always have this feature
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The information environment
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Information environment
Refer to Π ≡ ×iΠi as the information environment
Denition
Π is Blackwell-connected if ∀i , ∀π ∈ Π, π′ ∈ Π−i s.t. 〈π〉 〈π′〉,∃πi ∈ Πi s.t. 〈π〉 = 〈π′ ∪ πi 〉.
i.e., given any strategy prole, any sender can unilaterally deviate to
any feasible outcome that is more informative
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Information environment
Refer to Π ≡ ×iΠi as the information environment
Denition
Π is Blackwell-connected if ∀i , ∀π ∈ Π, π′ ∈ Π−i s.t. 〈π〉 〈π′〉,∃πi ∈ Πi s.t. 〈π〉 = 〈π′ ∪ πi 〉.
i.e., given any strategy prole, any sender can unilaterally deviate to
any feasible outcome that is more informative
![Page 36: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/36.jpg)
Information environment
Refer to Π ≡ ×iΠi as the information environment
Denition
Π is Blackwell-connected if ∀i , ∀π ∈ Π, π′ ∈ Π−i s.t. 〈π〉 〈π′〉,∃πi ∈ Πi s.t. 〈π〉 = 〈π′ ∪ πi 〉.
i.e., given any strategy prole, any sender can unilaterally deviate to
any feasible outcome that is more informative
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Examples of environments
Number of draws: given π, each sender chooses the number of
independent draws
Precisions: sender i generates an independent signal N(ω, σ2i
)Partitions: each sender chooses a partition of Ω
Facts: each fact in set F generates an i .i .d . signal; each i chooses
Fi ⊂ F
All-or-nothing: each sender can say nothing or fully reveal everything
Rich: each sender conducts any experiment, potentially correlated
with others
All of these information environments are Blackwell-connected
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Examples of environments
Number of draws: given π, each sender chooses the number of
independent draws
Precisions: sender i generates an independent signal N(ω, σ2i
)Partitions: each sender chooses a partition of Ω
Facts: each fact in set F generates an i .i .d . signal; each i chooses
Fi ⊂ F
All-or-nothing: each sender can say nothing or fully reveal everything
Rich: each sender conducts any experiment, potentially correlated
with others
All of these information environments are Blackwell-connected
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Individual vs. aggregate feasibility
Key implication of environment being Blackwell-connected:
Claim
Suppose Π is Blackwell-connected. Then, 〈π〉|π ∈ Πi = 〈π〉|π ∈ Π∀i .
Each sender can provide as much information as many senders can
provide together
Necessary but not sucient for environment to be Blackwell-connected
Π can also be too `coarse'
e.g., each sender chooses ni ∈ 0, 2, 3, ... independent draws
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Individual vs. aggregate feasibility
Key implication of environment being Blackwell-connected:
Claim
Suppose Π is Blackwell-connected. Then, 〈π〉|π ∈ Πi = 〈π〉|π ∈ Π∀i .
Each sender can provide as much information as many senders can
provide together
Necessary but not sucient for environment to be Blackwell-connected
Π can also be too `coarse'
e.g., each sender chooses ni ∈ 0, 2, 3, ... independent draws
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Individual vs. aggregate feasibility
Key implication of environment being Blackwell-connected:
Claim
Suppose Π is Blackwell-connected. Then, 〈π〉|π ∈ Πi = 〈π〉|π ∈ Π∀i .
Each sender can provide as much information as many senders can
provide together
Necessary but not sucient for environment to be Blackwell-connected
Π can also be too `coarse'
e.g., each sender chooses ni ∈ 0, 2, 3, ... independent draws
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Main result
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Competition vs. collusion
An outcome τ c is collusive if it maximizes∑
i vi (τ)
results generalize to all monotone social welfare functions
Expositional ease: assume collusive outcome is unique
generically satised
Compare equilibrium outcomes with the collusive outcome
![Page 44: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/44.jpg)
Competition vs. collusion
An outcome τ c is collusive if it maximizes∑
i vi (τ)
results generalize to all monotone social welfare functions
Expositional ease: assume collusive outcome is unique
generically satised
Compare equilibrium outcomes with the collusive outcome
![Page 45: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/45.jpg)
Competition vs. collusion
An outcome τ c is collusive if it maximizes∑
i vi (τ)
results generalize to all monotone social welfare functions
Expositional ease: assume collusive outcome is unique
generically satised
Compare equilibrium outcomes with the collusive outcome
![Page 46: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/46.jpg)
Competition vs. collusion
An outcome τ c is collusive if it maximizes∑
i vi (τ)
results generalize to all monotone social welfare functions
Expositional ease: assume collusive outcome is unique
generically satised
Compare equilibrium outcomes with the collusive outcome
![Page 47: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/47.jpg)
Competition vs. collusion
Proposition
Every equilibrium outcome is no less informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected.
Suppose Π is Blackwell-connected
Suppose τ c τ∗ = 〈π∗〉There is some sender i s.t. vi (τ c) > vi (〈π∗〉)Let π∗−i =
(π∗1, ..., π
∗i−1, π
∗i+1, ..., π
∗n
)∈ Π−i
We have τ c 〈π∗〉 〈π∗−i 〉Π Blackwell-connected ⇒ ∃πd
i ∈ Πi s.t. τc = 〈π∗−i ∪ πd
i 〉
only if part is constructive
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Competition vs. collusion
Proposition
Every equilibrium outcome is no less informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected.
Suppose Π is Blackwell-connected
Suppose τ c τ∗ = 〈π∗〉
There is some sender i s.t. vi (τ c) > vi (〈π∗〉)Let π∗−i =
(π∗1, ..., π
∗i−1, π
∗i+1, ..., π
∗n
)∈ Π−i
We have τ c 〈π∗〉 〈π∗−i 〉Π Blackwell-connected ⇒ ∃πd
i ∈ Πi s.t. τc = 〈π∗−i ∪ πd
i 〉
only if part is constructive
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Competition vs. collusion
Proposition
Every equilibrium outcome is no less informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected.
Suppose Π is Blackwell-connected
Suppose τ c τ∗ = 〈π∗〉There is some sender i s.t. vi (τ c) > vi (〈π∗〉)
Let π∗−i =(π∗1, ..., π
∗i−1, π
∗i+1, ..., π
∗n
)∈ Π−i
We have τ c 〈π∗〉 〈π∗−i 〉Π Blackwell-connected ⇒ ∃πd
i ∈ Πi s.t. τc = 〈π∗−i ∪ πd
i 〉
only if part is constructive
![Page 50: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/50.jpg)
Competition vs. collusion
Proposition
Every equilibrium outcome is no less informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected.
Suppose Π is Blackwell-connected
Suppose τ c τ∗ = 〈π∗〉There is some sender i s.t. vi (τ c) > vi (〈π∗〉)Let π∗−i =
(π∗1, ..., π
∗i−1, π
∗i+1, ..., π
∗n
)∈ Π−i
We have τ c 〈π∗〉 〈π∗−i 〉
Π Blackwell-connected ⇒ ∃πdi ∈ Πi s.t. τ
c = 〈π∗−i ∪ πdi 〉
only if part is constructive
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Competition vs. collusion
Proposition
Every equilibrium outcome is no less informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected.
Suppose Π is Blackwell-connected
Suppose τ c τ∗ = 〈π∗〉There is some sender i s.t. vi (τ c) > vi (〈π∗〉)Let π∗−i =
(π∗1, ..., π
∗i−1, π
∗i+1, ..., π
∗n
)∈ Π−i
We have τ c 〈π∗〉 〈π∗−i 〉Π Blackwell-connected ⇒ ∃πd
i ∈ Πi s.t. τc = 〈π∗−i ∪ πd
i 〉
only if part is constructive
![Page 52: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/52.jpg)
Competition vs. collusion
Proposition
Every equilibrium outcome is no less informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected.
Suppose Π is Blackwell-connected
Suppose τ c τ∗ = 〈π∗〉There is some sender i s.t. vi (τ c) > vi (〈π∗〉)Let π∗−i =
(π∗1, ..., π
∗i−1, π
∗i+1, ..., π
∗n
)∈ Π−i
We have τ c 〈π∗〉 〈π∗−i 〉Π Blackwell-connected ⇒ ∃πd
i ∈ Πi s.t. τc = 〈π∗−i ∪ πd
i 〉
only if part is constructive
![Page 53: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/53.jpg)
Caveats
Equilibrium outcomes might not be comparable to the collusive
outcome
Proposition
Every equilibrium outcome is more informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected and any two feasible outcomes are comparable.
With mixed strategies, the environment is never Blackwell connected
mixed strategy equilibria are not unambiguously more informative than
collusive outcomes
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Caveats
Equilibrium outcomes might not be comparable to the collusive
outcome
Proposition
Every equilibrium outcome is more informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected and any two feasible outcomes are comparable.
With mixed strategies, the environment is never Blackwell connected
mixed strategy equilibria are not unambiguously more informative than
collusive outcomes
![Page 55: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/55.jpg)
Caveats
Equilibrium outcomes might not be comparable to the collusive
outcome
Proposition
Every equilibrium outcome is more informative than the collusive outcome
(regardless of preferences) if and only if the information environment is
Blackwell-connected and any two feasible outcomes are comparable.
With mixed strategies, the environment is never Blackwell connected
mixed strategy equilibria are not unambiguously more informative than
collusive outcomes
![Page 56: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/56.jpg)
Illustration of the result
Will a merger of two pharmaceuticals lead to more information?
Scenario A:
each rm commissions RCT from a third-party
each batch of subjects yields an i.i.d. signal about the two drugs
informational environment is Blackwell-connected
merger will reduce information regardless of the demand structure
Scenario B:
each rm can only generate information about its own drug
informational environment is not Blackwell-connected
impact of merger will depend on demand
for some demand structure, merger will make consumers more informed
![Page 57: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/57.jpg)
Illustration of the result
Will a merger of two pharmaceuticals lead to more information?
Scenario A:
each rm commissions RCT from a third-party
each batch of subjects yields an i.i.d. signal about the two drugs
informational environment is Blackwell-connected
merger will reduce information regardless of the demand structure
Scenario B:
each rm can only generate information about its own drug
informational environment is not Blackwell-connected
impact of merger will depend on demand
for some demand structure, merger will make consumers more informed
![Page 58: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/58.jpg)
Illustration of the result
Will a merger of two pharmaceuticals lead to more information?
Scenario A:
each rm commissions RCT from a third-party
each batch of subjects yields an i.i.d. signal about the two drugs
informational environment is Blackwell-connected
merger will reduce information regardless of the demand structure
Scenario B:
each rm can only generate information about its own drug
informational environment is not Blackwell-connected
impact of merger will depend on demand
for some demand structure, merger will make consumers more informed
![Page 59: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/59.jpg)
Illustration of the result
Will a merger of two pharmaceuticals lead to more information?
Scenario A:
each rm commissions RCT from a third-party
each batch of subjects yields an i.i.d. signal about the two drugs
informational environment is Blackwell-connected
merger will reduce information regardless of the demand structure
Scenario B:
each rm can only generate information about its own drug
informational environment is not Blackwell-connected
impact of merger will depend on demand
for some demand structure, merger will make consumers more informed
![Page 60: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/60.jpg)
Illustration of the result
Will a merger of two pharmaceuticals lead to more information?
Scenario A:
each rm commissions RCT from a third-party
each batch of subjects yields an i.i.d. signal about the two drugs
informational environment is Blackwell-connected
merger will reduce information regardless of the demand structure
Scenario B:
each rm can only generate information about its own drug
informational environment is not Blackwell-connected
impact of merger will depend on demand
for some demand structure, merger will make consumers more informed
![Page 61: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/61.jpg)
Illustration of the result
Will a merger of two pharmaceuticals lead to more information?
Scenario A:
each rm commissions RCT from a third-party
each batch of subjects yields an i.i.d. signal about the two drugs
informational environment is Blackwell-connected
merger will reduce information regardless of the demand structure
Scenario B:
each rm can only generate information about its own drug
informational environment is not Blackwell-connected
impact of merger will depend on demand
for some demand structure, merger will make consumers more informed
![Page 62: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/62.jpg)
Equilibrium characterization
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Characterizing the equilibrium set
A simple equilibrium characterization if the environment is
Blackwell-connected and Πi = Π ∀i
Outcome τ is unimprovable for i if for any feasible τ ′ τ , we havevi (τ ′) ≤ vi (τ)
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Characterizing the equilibrium set
A simple equilibrium characterization if the environment is
Blackwell-connected and Πi = Π ∀i
Outcome τ is unimprovable for i if for any feasible τ ′ τ , we havevi (τ ′) ≤ vi (τ)
![Page 65: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/65.jpg)
Characterization result
Proposition
Suppose Πi = Π ∀i , Π is Blackwell-connected, and n ≥ 2. A feasible
outcome is an equilibrium outcome if and only if it is unimprovable for each
sender.
Only if follows directly from denition of Blackwell-connected
Suppose some feasible τ is unimprovable for each sender
Consider π ∈ Π s.t. 〈π〉 = τ
Strategy prole (π, ..., π) is an equilibrium
i can only deviate to τ ′ ττ unimprovable implies vi (τ ′) ≤ vi (τ)
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Characterization result
Proposition
Suppose Πi = Π ∀i , Π is Blackwell-connected, and n ≥ 2. A feasible
outcome is an equilibrium outcome if and only if it is unimprovable for each
sender.
Only if follows directly from denition of Blackwell-connected
Suppose some feasible τ is unimprovable for each sender
Consider π ∈ Π s.t. 〈π〉 = τ
Strategy prole (π, ..., π) is an equilibrium
i can only deviate to τ ′ ττ unimprovable implies vi (τ ′) ≤ vi (τ)
![Page 67: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/67.jpg)
Characterization result
Proposition
Suppose Πi = Π ∀i , Π is Blackwell-connected, and n ≥ 2. A feasible
outcome is an equilibrium outcome if and only if it is unimprovable for each
sender.
Only if follows directly from denition of Blackwell-connected
Suppose some feasible τ is unimprovable for each sender
Consider π ∈ Π s.t. 〈π〉 = τ
Strategy prole (π, ..., π) is an equilibrium
i can only deviate to τ ′ ττ unimprovable implies vi (τ ′) ≤ vi (τ)
![Page 68: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/68.jpg)
Characterization result
Proposition
Suppose Πi = Π ∀i , Π is Blackwell-connected, and n ≥ 2. A feasible
outcome is an equilibrium outcome if and only if it is unimprovable for each
sender.
Only if follows directly from denition of Blackwell-connected
Suppose some feasible τ is unimprovable for each sender
Consider π ∈ Π s.t. 〈π〉 = τ
Strategy prole (π, ..., π) is an equilibrium
i can only deviate to τ ′ ττ unimprovable implies vi (τ ′) ≤ vi (τ)
![Page 69: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/69.jpg)
Characterization result
Proposition
Suppose Πi = Π ∀i , Π is Blackwell-connected, and n ≥ 2. A feasible
outcome is an equilibrium outcome if and only if it is unimprovable for each
sender.
Only if follows directly from denition of Blackwell-connected
Suppose some feasible τ is unimprovable for each sender
Consider π ∈ Π s.t. 〈π〉 = τ
Strategy prole (π, ..., π) is an equilibrium
i can only deviate to τ ′ ττ unimprovable implies vi (τ ′) ≤ vi (τ)
![Page 70: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/70.jpg)
Characterization result illustrated
v1(τ)
v2(τ)
M1
M2
M
τ1 τ20 1
![Page 71: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/71.jpg)
Comparative statics illustrated
v1(τ)+v2(τ)
M1
M2
M
τc τ1 τ20 1
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Other results
Three notions of increased competition
Equilibrium outcomes vs. collusive outcomes
Presence of additional senders
Misalignment of senders' preferences
Focus on Πi = Π and minimally informative equilibria
If Π is Blackwell-connected
adding senders cannot lead to less information
more misalignment cannot lead to less information
Comparative statics on sets
![Page 73: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/73.jpg)
Other results
Three notions of increased competition
Equilibrium outcomes vs. collusive outcomes
Presence of additional senders
Misalignment of senders' preferences
Focus on Πi = Π and minimally informative equilibria
If Π is Blackwell-connected
adding senders cannot lead to less information
more misalignment cannot lead to less information
Comparative statics on sets
![Page 74: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/74.jpg)
Other results
Three notions of increased competition
Equilibrium outcomes vs. collusive outcomes
Presence of additional senders
Misalignment of senders' preferences
Focus on Πi = Π and minimally informative equilibria
If Π is Blackwell-connected
adding senders cannot lead to less information
more misalignment cannot lead to less information
Comparative statics on sets
![Page 75: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/75.jpg)
Thank you
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Minimal equilibria
Suppose the informational environment is Blackwell-connected
Suppose τ and τ ′ are two equilibrium outcomes and τ ′ τ
Then, vi (τ) ≥ vi (τ ′) for all senders i
Say τ is a minimal equilibrium outcome if there is no equilibrium
outcome τ ′ s.t. τ ′ τ
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Minimal equilibria
Suppose the informational environment is Blackwell-connected
Suppose τ and τ ′ are two equilibrium outcomes and τ ′ τThen, vi (τ) ≥ vi (τ ′) for all senders i
Say τ is a minimal equilibrium outcome if there is no equilibrium
outcome τ ′ s.t. τ ′ τ
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Minimal equilibria
Suppose the informational environment is Blackwell-connected
Suppose τ and τ ′ are two equilibrium outcomes and τ ′ τThen, vi (τ) ≥ vi (τ ′) for all senders i
Say τ is a minimal equilibrium outcome if there is no equilibrium
outcome τ ′ s.t. τ ′ τ
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Adding senders
Compare minimal equilibria when
set of senders is J
set of senders is J ′ ⊂ J
Blackwell-connectedness no longer sucient
also need Πi = Π
Proposition
Suppose Πi = Π for all i . If the information environment is
Blackwell-connected, then (regardless of preferences) any minimal
equilibrium outcome when the set of senders is some set J is no less
informative than any minimal equilibrium outcome when the set of senders
is some set J ′ ⊂ J.
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Adding senders
Compare minimal equilibria when
set of senders is J
set of senders is J ′ ⊂ J
Blackwell-connectedness no longer sucient
also need Πi = Π
Proposition
Suppose Πi = Π for all i . If the information environment is
Blackwell-connected, then (regardless of preferences) any minimal
equilibrium outcome when the set of senders is some set J is no less
informative than any minimal equilibrium outcome when the set of senders
is some set J ′ ⊂ J.
![Page 81: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/81.jpg)
Adding senders
Compare minimal equilibria when
set of senders is J
set of senders is J ′ ⊂ J
Blackwell-connectedness no longer sucient
also need Πi = Π
Proposition
Suppose Πi = Π for all i . If the information environment is
Blackwell-connected, then (regardless of preferences) any minimal
equilibrium outcome when the set of senders is some set J is no less
informative than any minimal equilibrium outcome when the set of senders
is some set J ′ ⊂ J.
![Page 82: Comp etition in persuasion · Emir Kamenica University of Chicago. Motivation Do es competition among persuaders increase information? Long tradition in political and legal thought](https://reader033.vdocuments.us/reader033/viewer/2022060922/60ae212a69003672816f6813/html5/thumbnails/82.jpg)
Preference misalignment
Suppose there are two senders j and k with
vj (τ) = f (τ) + bg (τ)
vk (τ) = f (τ)− bg (τ)
Parameter b ≥ 0 measures misalignment of preferences
Proposition
Suppose Πi = Π for all i . If the information environment is
Blackwell-connected, then any minimal equilibrium outcome when the level
of misalignment is b is no less informative than any minimal equilibrium
outcome when the level of misalignment is some b′ < b .
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Preference misalignment
Suppose there are two senders j and k with
vj (τ) = f (τ) + bg (τ)
vk (τ) = f (τ)− bg (τ)
Parameter b ≥ 0 measures misalignment of preferences
Proposition
Suppose Πi = Π for all i . If the information environment is
Blackwell-connected, then any minimal equilibrium outcome when the level
of misalignment is b is no less informative than any minimal equilibrium
outcome when the level of misalignment is some b′ < b .