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♪ [music] ♪
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- [Mary Clare] Today, we're going
to learn more about Game Theory
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by using it to solve
a simple problem.
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Bob and Al are two
prestigious rival magicians
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who have developed a new trick
that is quite popular.
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They've then agreed
to limit performances
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so they can charge more.
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If both magicians perform
only one show a week,
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each will earn $10,000.
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However, if one magician
breaks the agreement
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and performs five times a week
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while the other continues
to perform once a week --
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that double-crosser
will make $15,000
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while the other magician
will make only $1,000.
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And if both magicians
break the agreement
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and perform five times a week,
each will earn $6,000.
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So, what is the Nash equilibrium
of how many shows
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they will each perform?
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The Nash equilibrium means
that no person has an incentive
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to change their behavior
or strategy
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unless someone else changes
their behavior or strategy.
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In order to find
the Nash equilibrium
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of Bob and Al's performances,
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we have to first analyze
Bob's behavior
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based on Al's behavior
and vice versa.
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It will be easier
to track everything
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if we fill out a 2-by-2 matrix.
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There are two individuals
with two options.
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In each box of the matrix
we'll list each person's path
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given the state of the world.
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So we'll list Bob's path first
and Al's second.
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So let's first look
at Bob's best strategy
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based on Al's behavior.
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Al will either keep her promise
to perform once a week,
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or she'll break her promise
and perform five shows.
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If she cooperates
and performs one show,
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what is Bob's best strategy?
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Again, if we just look
at what he stands to gain,
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then his best option
would be to cheat
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and perform five times a week
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and make $15,000 versus
performing once a week
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and making $10,000.
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Now, what if Al backstabs Bob
and performs five shows?
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Bob's best strategy here is also
to perform five shows a week
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and make $6,000 versus
performing once a week
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and making only $1,000.
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Given that Bob's best strategy
is to cheat and perform five shows
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regardless of what Al does,
cheating is his dominant strategy.
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Now, let's look at it
from Al's perspective.
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I bet you can see
where this is going.
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If Bob keeps his promise
and performs one show per week,
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then Al's best option
is to perform five shows.
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She'll earn $15,000
instead of $10,000.
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And, if Bob decides
to break his promise
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and perform five shows,
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Al's best option is also to cheat
and perform five shows
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because she'll earn $6,000
instead of $1,000.
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Given that Al's best strategy
is to perform five times per week --
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again, regardless of what Bob does --
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this is also considered
her dominant strategy.
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So if Bob's dominant strategy
is to cheat as well,
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then the Nash equilibrium
in this game
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is for both of them
to break their promises.
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They'll each perform five shows
and earn $6,000.
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Notice that this isn't
an optimal outcome.
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It would be so much better
for them to each perform
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only one show per week.
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They'd earn a lot more money,
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and they'd also have a lot more
free time on their hands.
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But if we're just
evaluating what to do
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from the payoffs
listed in our matrix,
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it is in both Bob's best interest
and Al's best interest to cheat.
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Thus, it's the Nash equilibrium.
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Of course, there's a real world
outside of the matrix.
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The world is much more
complicated than this.
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People care about keeping promises,
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and they think about the long run,
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rather than just week to week.
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So think of this example
as just a simple
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but powerful starting point
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to better understand
human decision-making.
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As always, let us know
what you think.
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And, if you'd like more practice,
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check out our additional
challenge questions
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at the end of this video.
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♪ [music] ♪