Prof.
Bryan Caplan
bcaplan@gmu.edu
http://www.bcaplan.com
Econ
812
Weeks 34:
Intro to Game Theory
I.
The Hard Case: When Strategy Matters
A.
You can go surprisingly far with general equilibrium theory, but
ultimately many people find it unsatisfying. In the real world, people frequently
stand in between the oneagent and the nearinfiniteagent poles.
B.
Even when people start out in the nearinfiniteagent case, they often
ex post end up interacting with a few people.
1.
Ex: Marriage market
C.
Game theory tries to analyze situations where strategy does
matter. It generally ends up with
less determinate answers than GE, but is often arguably more realistic. ("I'd rather be vaguely right than
clearly wrong.")
II.
Extensive and Normal Forms
A.
Standard consumer choice provides the basic building blocks: game
theory retains the standard assumption that people maximize utility
functions. Slight change: Game
theorists often talk about "payoffs" instead of utility. The concept is the same: Given a choice
of payoffs, agents pick the largest.
1.
Payoffs are usually interpreted as von NeumannMorgenstern utilities to
sidestep issues of risk aversion.
B.
Any game can be represented in two different ways: extensive
form and normal form.
C.
Extensive forms display every possible course of game events, turn by
turn. They show how behavior
branches out from "choice nodes," showing payoffs at the end of each
branch as it ends. For this reason,
extensive forms are often called "decision trees."
D.
Simple example: Your career game tree. At each node you can keep going to
school, or get a job and get your payout.
E.
More interesting example: The French Connection subway
game. Criminal decides whether to
get on or off the subway; then Popeye decides whether to get on or off. From the first node, the tree spreads
out into two branches; then each of those branches spreads out to two further
branches; then the game ends.
Payoffs for {Criminal, Popeye}: (on, on)=(0,10); (on, off)=(10,0); (off,
on)=(10,0); (off,off)=(0,10).
F.
Complications:
1.
Nature as a random player.
2.
Information sets: simultaneous moves are equivalent to sequential moves
with uncertainty.
3.
If you learn something before you decide, node representing what is
learned must precede node where decision is taken.
G.
Normal forms (aka "strategic forms"), in contrast, display a
complete grid of strategy profiles and payoffs. The grid has one dimension per player.
1.
Important: Strategy profiles often contain irrelevant information about
what you would have done in situations that did not in fact arise.
H.
Normal form of your 1player career game:
Drop out before H.S. 
Finish H.S., stop 
Finish B.A., stop 
Finish Ph.D., stop 
Finish 2 Ph.D.s, stop 
10 
15 
20 
30 
0 
I.
Normal form of the French Connection Game:

Popeye 

Criminal 

On 
Off 
On 
0,10 
10,0 

Off 
10,0 
0,10 
J.
Example from Kreps: Player 1 chooses A or D. If D, game ends. If A, then player 2 chooses a
or d. If d,
game ends. If a,
player 1 chooses a or d, and either way, the game ends.
K.
Normal form:

a 
d 
Aa 
3,1 
4,3 
Ad 
2,3 
4,3 
Da 
1,2 
1,2 
Dd 
1,2 
1,2 
L.
Challenge: Write down the extensive form.
III.
Strictly and Weakly Dominant Strategies
A.
So what does game theory claim people do? It begins with some relatively weak
assumptions, then gradually strengthens them until a plausible answer emerges.
B.
Weakest assumption: People do not play strictly dominated
strategies. If there is a strategy
that is strictly worse for you no matter what your opponent does, you do
not play it. If elimination of
strictly dominated strategies leaves you with a single equilibrium, the game is
dominance solvable.
C.
Classic example: Prisoners' Dilemma.
D.
If all players think this way, you can extend this idea to successive
strict dominance. If your
opponent would never play a strategy, you can cross out that row or
column. This may in turn imply that
some more of your strategies are strictly dominated, and so on.
1.
Fun fact: Order of iteration does not matter.
E.
A dominance solvable normal form from Kreps:

t1 
t2 
t3 
s1 
4,3 
2,7 
0,4 
s2 
5,5 
5,1 
4,2 
F.
Further refinement: If probabilistic combination of strategies strictly
dominates another for any probability distribution, that too may be
eliminated. Then this normal form
from Kreps becomes dominance solvable:

t1 
t2 
t3 
s1 
4,10 
3,0 
1,3 
s2 
0,0 
2,10 
10,3 
G.
It may happen that one strategy is sometimes strictly worse and never
strictly better than another. Using
the criterion of weak dominance, such strategies may also be
eliminated. Unfortunately, with
weak dominance, order of iteration may matter.
IV.
Backwards Induction
A.
In any game perfect information, each node marks the beginning of what
can be seen as another game of perfect information.
B.
Question: What happens if we apply the procedure of "backwards
induction," i.e., repeatedly apply strict dominance to these
"subgames"?
C.
Intuition: Systematically reason "If we get to this point in the
game, no one would even do suchandsuch, so we can erase that part of the
tree."
D.
Modest Answer: We can eliminate more possibilities than before.
1.
Consider extensive and normal forms from Kreps (Figure 12.5).
E.
Immodest Answer: Any finite game of complete and perfect information
without ties becomes dominance solvable.
1.
Chess example
F.
Ex: The Centipede game (Figure 12.6)
V.
Pure Strategy Nash Equilibrium
A.
You can only get so far with strict dominancetype reasoning. Backwards induction seems impressive at
first, but it only works for finite games of perfect and complete
information. Very few interesting
situations fit that description.
B.
This leads us to a very different equilibrium concept, the pure
strategy Nash equilibrium. A
set of player strategies is a PSNE if and only if NO player could do strictly
better by changing strategies, holding all other players' strategies fixed.
1.
Imagine asking players onebyone if they would like to do
something different. If ALL of them
answer no, you have a PSNE.
2.
From the definition, it should be obvious that a game can have multiple
PSNE or zero PSNE.
C.
Example #1. Find the
PSNE. How does this differ from
strict dominance?


Player 2 

Player 1 

Left 
Right 
Up 
15,10 
8,15 

Down 
10,7 
6,8 
D.
Example #2: Find the
PSNE. How does this differ from
strict dominance?


Player 2 

Player 1 

Left 
Right 
Up 
10,10 
0,15 

Down 
15,0 
5,5 
E.
Example #3: Note the absence of any PSNE.


Player 2 

Player 1 

Left 
Right 
Up 
10,0 
0,10 

Down 
0,10 
10,0 
F.
The PSNE concept is probably the most used in game theory and modern
economics generally. It is somewhat
paradoxical, however, because it seems to assume away strategic interaction,
precisely what game theory was intended to address! A more strategic player might think
"I'm not going to switch just because I would be better off holding my
opponent's action constant. Maybe
he'll respond in a way that makes me wish I hadn't changed in the first
place."
VI.
Mixed Strategy Nash Equilibrium
A.
Talking about "pure strategy" NE strongly suggests a
contrasting concept of "mixed strategy" NE. Instead of just asking whether any
player has an incentive to change strategies, you could ask whether any player
has an incentive to change his probability of playing various
strategies.
B.
How do you solve for MSNE?
Each player has to play a mixture that leaves all other players
indifferent.
C.
Ex: Return to the game where:


Player 2 

Player 1 

Left 
Right 
Up 
8,10 
1,15 

Down 
12,0 
9,5 
D.
When is player 2 indifferent between playing Left and playing
Right? Let player 1's probability
of playing Up be s, and Down be (1s). Then player 2 is indifferent so long as:_{}, which simplifies to: s=.5.
E.
When is player 1 indifferent between playing Up and playing Down? Let player 2's probability of playing
Left be j, and Right be (1j). Then player 1 is indifferent so long as:
_{}, which simplifies to j=5/7.
F.
So there is a MSNE of (s,j)=(.5,
5/7). When player 1 plays Up with
probability .5, and player 2 players Left with probability 5/7, neither could
do better by changing their mix.
(They wouldn't do worse either, admittedly!).
G.
Many people find the MSNE bizarre, but I maintain the opposite. The MSNE concept brilliantly
accommodates the strategic complexity of realworld smallnumbers
interaction. Think of it this way:
You make your opponents indifferent in order to eliminate behavioral
patterns they could exploit.
1.
Ex: Sports. You don't do
the same thing all of the time because opponents will notice the pattern and
play the most effective response. A
predictable player is easy to beat. In racquetball, for example, you play a
mix of hard and soft serves, aiming at different locations on the court.
2.
Ex: Strategy games. If you
always attack the same place, your opponent will put all of his defensive
strength there. In Diplomacy, for
example, you randomize your attacks because a fully anticipated attack is easy
to repel.
3.
Ex: Rock, Paper, Scissors.
You randomize to avoid being a sucker. Of course, if you play against someone
who doesn't randomize, you don't want to randomize either; but maybe they are
just tricking you into thinking they don't randomize!
4.
Ex: Bargaining. If you are
a hard bargainer, you get better but fewer deals. If you are a soft bargainer, you get
worse but more deals. Which
strategy works better? Neither!
H.
MSNE cuts the Gordian knot of unlimited secondguessing,
thirdguessing, etc. All of these
layers of thought can be reinterpreted as a randomizing device.
I.
Solve the French Connection game. (Note the parallels to the Austrians'
Sherlock Holmes example).
VII.
Subgame Perfection
A.
Suppose I threaten to fail any student who leaves early from any
class. If you believe my threat,
you will not leave early, and I will never have to impose my threat. This sounds like a Nash equilibrium 
since I get what I want at no cost to me, and you prefer sitting in class to
failing, neither wants to change.
B.
But this sounds like an implausible prediction, because I probably
would not want to carry out that threat.
There would be a big fight, I would have to explain myself to the
chairman, the dean, etc. How can a
threat I would never carry out change your behavior?
C.
In general terms, this is known as the problem of "out of
equilibrium" play. I can
optimally choose bizarre behavior in situations that I know will never
happen. But knowing what I would
do in situations that will never happen can affect your actual behavior in
situations that routinely happen!
D.
This gives rise to the Nash refinement of subgame perfection. Subgame perfection, in essence, requires
Nash play in every subgame of a game.
E.
To check for subgame perfection, you apply backwards induction as far
as you are able. Thus in games of
perfect and complete information, the result you get from backwards induction
is always subgame perfect.
F.
Standard example: Entry game.
The two PSNE are (In, Accommodate) and (Out, Fight). But only the first is subgame perfect.
G.
In games of imperfect information, though, you have to switch from
strict dominance to Nash.
VIII.
Prisoners' Dilemma
A.
Surely the most analyzed game in economics is the Prisoners'
Dilemma. Standard representation:


Player 2 

Player 1 

Coop 
Don't 
Coop 
5,5 
0,6 

Don't 
6,0 
1,1 
B.
Natural solution concept: Strict dominance. Player 1 is better off not cooperating
no matter what Player 2 does.
Player 2 is better off not cooperating no matter what Player 1
does. So neither cooperates.
C.
The Prisoners' Dilemma has many applications: public goods and
externalities, collusion, voting, revolution... Others?
D.
There is a lot of experimental literature on the PD. The extreme prediction is rarely borne
out (people will cooperate even when defection is strictly dominant). But people do "leave money on the
table," and there are a number of standard ways to reduce cooperation
levels.
E.
Moreover, no experiment that I know of has people play for, say, a
year. I would strongly expect
largeN, longterm play to closely match the game theoretic prediction.
IX.
Coordination Games
A.
Another game with a high profile in both theoretical and policy
discussions is the Coordination game.
Standard representation:


Player 2 

Player 1 

Left 
Right 
Left 
3,3 
0,0 

Right 
0,0 
5,5 
B.
Natural solution concept: PSNE.
If Player 1 plays Left, Player 2 is better off playing Left. If Player 1 plays Right, Player 2 is
better off playing Right. And vice
versa.
C.
Coordination games underlie the whole pathdependence literature. Main idea: It is possible for
people to be "lockedin" to Pareto inferior equilibria. (Of course, mere possibility is hardly
proof!)
D.
Problems like this naturally lead us to the notion of focal or
"Schelling" points. Some
coordination equilibrium are in some sense more obvious than others.
1.
The classic NYC meeting example.
E.
What would it take to actually get people into the Paretoinferior
NE? Most plausibly, at least a
moderate number of players and gradual information dispersion.
F.
Experimental evidence? Not
too surprising.
X.
Ultimatum Games
A.
The Ultimatum Game is another game that has received a lot of academic
attention. Standard setup: Player
1 proposes one way to divide $10 between himself and Player 2. Player 2 accepts or rejects the
division. If he accepts, they get
Player 1's proposal; if he rejects, they both get 0.


Player 2 

Player 1 

Accept 
Reject 
t 
(10t),t 
0,0 
B.
Natural solution concept: Subgame perfection. Player 2 will accept any amount greater
than 0, so Player 1 offers $.01 and takes $9.99 for himself.
C.
Experimentally, no one does this.
Even splits are common, and people often reject "ungenerous"
offers.
D.
Is this motivated purely by spite?
Parallel Dictator game proves otherwise.