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Choose an integer between 0 and 100.
The winner is the guess closest to 70% of the class average as of 11:35am.
Dominant and Dominated Strategies
Best Responses
Rationalizability and
Iterated Deletion of Dominated Strategies
One strategy strictly dominates another strategy
if it always yields a strictly higher payoff
no matter what the other players do.
A pure strategy is dominated for a player if
there’s some other strategy that that player could choose
which would give them a higher payoff
no matter what the other players are doing.
One strategy weakly dominates another strategy
if it never yields a strictly lower payoff
no matter what the other players do,
and sometimes yields a strictly higher payoff.
Right weakly dominates Left.
Top strictly dominates Bottom.
1
2
Top
Bottom
Left
Right
2
5
,
1
0
,
4
1
,
5
5
,
pollev.com/chrismakler
Which strategies are dominated?
pollev.com/chrismakler
Which strategies are dominated?
pollev.com/chrismakler
Which strategies are dominated?
1
2
0
0
,
1
1
,
1
1
,
4
4
,
U
M
L
R
D
4
4
,
0
0
,
How to search for a dominated strategy:
1. Look to see if it's dominated by another pure strategy
2. Look for candidate mixed strategies, especially different strategies with alternating large payoffs
3. Remember: you only need to find one strategy that dominates a strategy for strategy to be dominated.
pollev.com/chrismakler
Suppose you were playing this (symmetric) game as either player. What would you choose?
1
2
C
D
C
D
2
2
,
3
0
,
1
1
,
0
3
,
1
2
Cooperate
Defect
Cooperate
Defect
If you believe the other person will defect,
what is your best response?
If you believe the other person will cooperate, what is your best response?
Defect
Defect
2
2
,
3
0
,
1
1
,
0
3
,
1
2
Cooperate
Defect
Cooperate
Defect
If you believe the other person will defect,
what is your best response?
If you believe the other person will cooperate, what is your best response?
Defect
Defect
Because Defect aways results in a
strictly higher payoff than Cooperate, we say that
Defect strictly dominates Cooperate.
2
2
,
3
0
,
1
1
,
0
3
,
1
2
Cooperate
Defect
Cooperate
Defect
2
2
,
3
0
,
1
1
,
0
3
,
(C,C) pareto dominates (D,D)
(D,D) is a dominant strategy equilibrium
The First Strategic Dilemma:
Everyone doing what's best for themselves can lead to a group loss.
The First Strategic Tension:
Everyone doing what's best for themselves can lead to a group loss.
Dominated strategy: some other strategy is better
no matter what you believe the other players will do
Best response: the best strategy (or strategies) to play
given specific beliefs about what the other players will do
In plain English: given my beliefs about what the other player(s) are doing, a strategy is my "best response"
if there is no other strategy available to me
that would give me a higher payoff.
1
2
1
2
,
4
3
,
1
4
,
1
1
,
Top
Middle
Left
Center
Bottom
Right
3
0
,
2
1
,
3
2
,
8
0
,
8
0
,
How should player 1 best respond to a belief that player 2 will play Left? What about Center or Right?
Believe Left => play Middle
Believe Center => play Bottom
Believe Right => play Top or Bottom
1
2
1
2
,
4
3
,
1
4
,
1
1
,
Top
Middle
Left
Center
Bottom
Right
3
0
,
2
1
,
3
2
,
8
0
,
8
0
,
How should player 1 best respond to a belief that player 2 will play
Left with probability 1/2, and Center and Right with Probability 1/4 each?
Player 1's expected payoffs given \(\theta = ({1 \over 2}, {1 \over 4}, {1 \over 4})\)
What is player 1's best response if they believe player 2 will play L?
What is player 1's best response if they believe player 2 will play R?
What is player 1's best response if they believe player 2 will play
L or R with equal probability?
1
2
0
0
,
1
1
,
1
1
,
4
4
,
U
M
L
R
D
4
4
,
0
0
,
What is player 1's best response if they believe player 2 will play
L with probability q,
and R with probability 1 - q?
1
2
0
0
,
1
1
,
1
1
,
4
4
,
U
M
L
R
D
4
4
,
0
0
,
1
2
0
0
,
1
1
,
1
1
,
4
4
,
U
M
L
R
D
4
4
,
0
0
,
We can remove from the game any strictly dominated strategies
(by either a pure or mixed strategy).
Removal may create new dominated strategies, remove them too…
The set of strategies that’s left at the end of that process
is the set of rationalizable strategies.
turns out to be independent of the order
in which dominated strategies are removed
Which strategy or strategies is strictly dominated for a player?
1
2
1
2
,
4
3
,
1
4
,
1
1
,
Top
Middle
Left
Center
Bottom
Right
3
0
,
2
1
,
3
2
,
8
0
,
8
0
,
Center strictly dominates Right.
If we know that player 2 will never play Right, is any strategy now dominated for player 1?
Bottom strictly dominates Top.
And with that off the board...
Bottom strictly dominates Middle.
Can we eliminate anything else?
Center strictly dominates Left.
Everyone chooses an integer between 0 and 100.
The closest person to 70% of the average guess wins.
0
100
Proposed by philosopher Jean-Jacques Rousseau in his Discourse on Inequality (1755)
Two hunters independently choose whether to hunt a stag or a hare.
A player who chooses to hunt a hare always gets a payoff of 4 regardless of the other player's choice.
A stag hunt only succeeds if both players do it.
In that case, the players divide the stag, giving a payoff of 5 to each.
But if only one player hunts a stag, he fails and gets a payoff of zero.
1
2
Stag
Hare
Stag
Hare
5
5
,
4
0
,
4
4
,
0
4
,
1
2
Stag
Hare
Stag
Hare
5
5
,
4
0
,
4
4
,
0
4
,
pollev.com/chrismakler
Suppose your payoff is the expected value if you were to play against a random person in the class. Which would you choose?
So far we've mainly talked about what people won't do,
and we have a good predictor of what they will do --
but only if they have a single dominant strategy.
Next time: look for Nash Equilibrium
in which everyone is best responding to everyone else.