Bayes Nash Equilibrium and Auctions
Christopher Makler
Stanford University Department of Economics
Econ 51: Lecture 15
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Most of the really big mistakes you'll make in your life
aren't because you play the game wrong,
but because you don't know the game you're playing.
Games of
Incomplete Information
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Suppose one of these
two games is being played.
Both players know there is an equal probability of each game.
Only player 1 knows which game is being played right now.
What is player 1's strategy space?
Player 2's?
Nature
Heads
(1/2)
Tails
(1/2)
Both players know there is an equal probability of each game.
Only player 1 knows which game is being played right now.
We can model this "as if" there is a nonstrategic player called Nature who moves first, flipping a coin, and picks which game is being played based on the coin flip.
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\(A^H\)
\(B^H\)
\(A^T\)
\(B^T\)
Nature
Heads
(1/2)
Tails
(1/2)
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\(A^H\)
\(B^H\)
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\(A^T\)
\(B^T\)
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The Bayesian Normal Form representation of the game shows the expected payoffs for each of the strategies the players could play:
\(A^HA^T\)
\(A^HB^T\)
\(B^HA^T\)
\(B^HB^T\)
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Bayes Nash Equilibrium is the NE of this game. It maps private information onto (simultaneously taken) actions.
OneShot Bayesian Game
Nature reveals private information to one or more of the players:
e.g., a firm's cost, the state of demand, a person's valuation of a good
The players take simultaneous actions (e.g., submit bids, produce a good)
Payoffs are revealed
Critical feature: there is no opportunity for information to be revealed through play;
we get to that next time with Perfect Bayesian Equilibria!
Cournot with Unknown Costs
Market demand: \(p = 10  Q\)
Firm 1's costs: \(c_1(q_1) = 0\)
Firm 2's costs: \(c_2(q_2) = \begin{cases}0 & \text{ w/prob }\frac{1}{2}\\4q_2 & \text{ w/prob }\frac{1}{2}\end{cases}\)
Firm 2 knows its own costs; Firm 1 knows that firm 2's costs are 0 and 4q with equal probability.
pollev.com/chrismakler
What is a strategy for firm 1?
What is a strategy for firm 2?
Market demand: \(p = 10  Q\)
Firm 1's costs: \(c_1(q_1) = 0\)
Firm 2's costs: \(c_2(q_2) = \begin{cases}0 & \text{ w/prob }\frac{1}{2}\\4q_2 & \text{ w/prob }\frac{1}{2}\end{cases}\)
Firm 1's strategy is a single quantity (\(q_1\)), since it doesn't know firm 2's costs.
Its expected profit is based on its profit if firm 2 has low cost and produces \(q_2^L\), or high cost and produces \(q_2^H\).
Firm 2's strategy must be to choose an amount to produce if it has low costs (\(q^L\)),
and an amount to produce if it is has high costs (\(q^H\)). It will choose the amount, knowing its own cost.
(if \(MC_2 = 0\))
(if \(MC_2 = 4\))
Bayes Nash Equilibrium will specify:
\(q_1\) which is a best response to firm 2 playing \(q_2^L\) and \(q_2^H\) with equal probability;
and \(q_2^L\) and \(q_2^H\) which are each best responses to \(q_1\) in their respective states of the world.
Calculate Best Responses
Firm 2's best response to \(q_1\) if MC = 0
Firm 2's best response to \(q_1\) if MC = 4
Firm 1's best response to firm 2 playing \(q_2^L\) and \(q_2^H\) with equal probability
Solve for Nash Equilibrium
Interestingly, the firm with unknown costs produces less (and therefore makes less profits) than the firm with known costs, even when they both have no costs.
Can you figure out why?
Auctions
PrivateValue Auctions
A single object is being auctioned off. Rules of the auction:
 each player \(i\) submits bid \(b_i\)
 a rule determines who gets the object, and how much they pay,
as a function of the vector of bids \(b = (b_1, b_2, \cdots, b_n)\)  examples of rules:

who gets the object?
 today: always the highest bidder

sealedbid vs. open bid: are bids public information?
 today: sealed bid, so the game is a simultaneous game

firstprice vs. secondprice: what does the winner pay, based on the bids?
 firstprice: the winner pays their own bid
 secondprice: the winner pays the secondhighest bid

who gets the object?
Auction Payoffs
Each player \(i\) knows their own valuation of the object, \(v_i\).
(We can think of this as a move by nature that occurs before the game begins.)
If you win the auction and pay some amount \(b\), your payoff is
If you lose, your payoff is zero.
We assume there is no additional emotional payoff from the fact that you won or lost.
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
What is an optimal bidding strategy?
Nature reveals private valuations \(v_i\),
uniformly distributed along [0, 100].
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
90
65
90
65
15
If you bid 90 and the highest other bid is 65, you win the object and pay 65; payoff is 80  65 = 15
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
90
65
90
65
15
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
90
75
90
65
15
75
5
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
90
85
90
65
15
75
5
85
5
If you bid 90 and the highest other bid is 85, you win the object and pay 85; payoff is 80  85 = 5
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
90
95
90
65
15
75
5
85
5
95
0
If you bid 90 and the highest other bid is 95, you don't win the object and your payoff is 0.
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
70
95
90
65
15
75
5
85
5
95
0
70
0
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
70
85
90
65
15
75
5
85
5
95
0
70
0
0
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
70
75
90
65
15
75
5
85
5
95
0
70
0
0
0
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
70
65
90
65
15
75
5
85
5
95
0
70
0
0
0
15
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
65
90
65
15
75
5
85
5
95
0
70
0
0
0
15
80
15
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
75
90
65
15
75
5
85
5
95
0
70
0
0
0
15
80
15
5
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
85
90
65
15
75
5
85
5
95
0
70
0
0
0
15
80
15
5
0
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Suppose your valuation is \(v_i = 80\). Let's make a payoff matrix based on your bid and the highest bid other than yours.
Your bid
Highest bid other than yours
80
95
90
65
15
75
5
85
5
95
0
70
0
0
0
15
80
15
5
0
0
SecondPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of the second highest bid.
Your bid
Highest bid other than yours
80
90
65
15
75
5
85
5
95
0
70
0
0
0
15
80
15
5
0
0
Bidding your true valuation is sometimes
better than underbidding, and never worse
Bidding your true valuation is sometimes better than overbidding, and never worse.
Bidding your true valuation is a
weakly dominant strategy!
FirstPrice, SealedBid Auction
Bidders simultaneously submit secret bids.
The highest bidder pays the amount of their own bid.
What is an optimal bidding strategy?
Nature reveals private valuations \(v_i\), uniformly distributed along [0, 100].
FirstPrice, SealedBid Auction
Nature reveals private valuations \(v_i\), uniformly distributed along [0, 100].
Suppose you believe player 2 is bidding some fraction \(a\) of their valuation.
What is the distribution of their bid? What is your probability of winning if you bid \(b_1\)?
Suppose you believe player 2 is bidding some fraction \(a\) of their valuation.
What is the distribution of their bid? What is your probability of winning if you bid \(b_1\)?
If the other bidder is bidding fraction \(a\) of their valuation, and their valuation is
uniformly distributed over [0, 100], what's your optimal bid if your valuation is \(v_i\)?
PAYOFF IF WIN
PROBABILITY OF WINNING
OPTIMAL TO BID HALF YOUR VALUE
Aside: Order Statistics
Two bidders: expected value of higher value is \(\frac{2}{3}\overline v\), lower value is \(\frac{1}{3}\overline v\)
Nature reveals private valuations \(v_i\), uniformly distributed along \([0, \overline v]\).
What is the expected revenue from a secondprice, sealedbid auction? From a firstprice auction?
Revenue equivalence theorem: for certain economic environments, the expected revenue and bidder profits for a broad class of auctions will be the same provided that bidders use equilibrium strategies.
Private value auction: everyone has their own personal valuation of an object.
Common value: the object has an intrinsic value, but that value is unknown
Common Value Auctions
Example: auctioning off land with an unknown amount of oil. Everyone can perform their own test (drill a hole somewhere on the land), and bids based on their private information from that test result.
Suppose I were to auction off this jar of coins.
Who would win the auction?
Suppose everyone gets a signal about the value of the coins in the jar, and that the signal is unbiased: its mean is the true value.
The winner's curse says that
in a common value auction,
then if you win the auction,
you've almost certainly overpaid.
(we won't do the math on this, it's just cool so we mention it)
Common Value Auctions
Next Time
What happens when we have dynamic games with incomplete information?
Econ 51  14  Static Games of Incomplete Information
By Chris Makler
Econ 51  14  Static Games of Incomplete Information
Uncertainty and Risk Aversion  Presentation
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