<p>It's a bit more complicated. Treating open-loop strategies as closed-loop strategies- that happen to play a given action in each period irrespective of the history- will not give you a subgame-perfect equilibrium. So there is no given closed-loop equilibrium that you can test using the one-deviation principle for being a subgame perfect equilibrium.</p>
<p>Rather, the claim is that the outcome path of an open-loop equilibrium will also be the outcome path of a closed-loop subgame perfect equilibrium with a continuum of individually insignificant players. The textbook doesn't quite spell out the technical details, which can be found in [Fudenberg, Drew, and David K. Levine. &quot;Open-loop and closed-loop equilibria in dynamic games with many players.&quot; <em>Journal of Economic Theory</em> 44.1 (1988): 1-18.].</p>
<p>The idea is that most subgames can actually not be reached by individual deviations, so how play would proceed there becomes strategically irrelevant. If players ignore deviations of measure-zero sets of agents- which they can do since payoffs will not depend on those agents- the strategic situation will not change and it will still be optimal to follow the given plan. Individual players cannot get the play into subgames in which a positive measure set of players play differently. But in order to get a subgame perfect equilibrium giving rise to the outcome, one would need to specify strategies appropriately for such hard-to-reach subgames too.</p>

CC BY-SA 4.0
2022-06-29T21:33:38.500Z
2022-06-29T21:33:38.500Z
Michael Greinecker
<p>This question is motivated by an example in <em>Section 4.7.3. Open-Loop and Closed-Loop Equilibria in Games with many players</em> in <em><strong>Game Theory by Fudenberg and Tirole</strong></em>.</p>
<p>The definitions of closed-loop and open-loop in the book are given below.</p>
<blockquote>
<p><strong>Closed-loop and Open-loop strategies(Section 4.7.1 in FT):</strong> Our definition of a multi-stage game with observed actions corresponds to the closed-loop information structure, where players condition their play at time <span class="math-container">$t$</span> on the history of play until that date. The corresponding strategies are called closed-loop strategies, while open-loop strategies are functions of calendar time alone.</p>
</blockquote>
<p>The following example is given to illustrate that an open-loop equilibrium can also be subgame-perfect equilibrium.</p>
<blockquote>
<p><strong>Section 4.7.3 in FT:</strong> Consider a game has a continuum of nonatomic individuals of each player type--a continuum of player 1s, a continuum of player 2s, and so on. (Let the set of individuals be copies of the unit interval endowed with Lebesgue measure for concreteness). Suppose further that each player <span class="math-container">$i$</span>'s payoff is independent of the actions of any subset of opponents with measure <span class="math-container">$0$</span>. Then if one individual player <span class="math-container">$j$</span> deviates, and all players <span class="math-container">$k\not=i,j$</span> ignore <span class="math-container">$j$</span>'s deviation, it is clearly optimal for player <span class="math-container">$i$</span> to ignore the deviation as well. Thus the outcome of an open-loop equilibrium is subgame perfect.</p>
</blockquote>
<p>My question is that, in the argument above, why do we need the part that it's optimal for player <span class="math-container">$i$</span> to ignore <span class="math-container">$j$</span>'s deviation given others doing so?</p>
<p>To my understanding, to prove an open-loop equilibrium to be subgame perfect, we need to apply the one-shot deviation princinple. Therefore suppose player <span class="math-container">$j$</span> deviates, then because of the assumption and the fact that the measure of player <span class="math-container">$j$</span> is <span class="math-container">$0$</span>, this will not affect the payoff of any player. Hence if a strategy profile constitutes an open-loop equilibrium, it's also subgame perfect.</p>
<p>However, the argument in the book seems to indicate that we not only need to check whether player <span class="math-container">$j$</span>'s deviation is profitable, we also need to check whether there is a profitable deviation for player <span class="math-container">$i$</span> in the subgame in which <span class="math-container">$j$</span> already deviates. I am a little confused about how to apply the one-shot deviation principle.</p>
<p>To make my confusion more specific, consider an infinitely repeated version of the two-person prisoners' dilemma, and consider the following strategy profile: Both players will choose <span class="math-container">$C$</span>, and if one player chooses <span class="math-container">$D$</span> at some <span class="math-container">$t$</span>, both players will play <span class="math-container">$D$</span> for all <span class="math-container">$\tau&gt;t$</span>.</p>
<p>Suppose we want to check whether the above strategy profile constitutes a subgame perfect equilibrium, we apply the one-shot deviation principle by the following steps.</p>
<p>Step 1:We assume player <span class="math-container">$i$</span> deviates to <span class="math-container">$D$</span> at some time <span class="math-container">$t$</span>, and no deviation has occurred until <span class="math-container">$t$</span>, we check whether this deviation is profitable.</p>
<p>Step 2:Next we assume player <span class="math-container">$i$</span> deviates to <span class="math-container">$C$</span> in some subgame in which a deviation has already occurred and check whether such deviation is profitable.</p>
<p>Step 3:If the answer is no in both steps, we say that the above strategy profile constitutes a subgame-perfect equilibrium.</p>
<p>In this particular setting, my question is that, in step 1, do we also need to check whether there is a profitable deviation for player <span class="math-container">$-i$</span> after the deviation of <span class="math-container">$i$</span> from <span class="math-container">$C$</span> to <span class="math-container">$D$</span>? How to distinguish between deviations in all subgames and one-shot deviation?</p>
<p>Thanks in advance for your help!</p>

<game-theory>
One shot deviation principle in games with many players
John
# One shot deviation principle in games with many players

(Asked by: username_0 on 2022-06-29)

This question is motivated by an example in *Section 4.7.3. Open-Loop and Closed-Loop Equilibria in Games with many players* in ***Game Theory by Fudenberg and Tirole***.

The definitions of closed-loop and open-loop in the book are given below.

> **Closed-loop and Open-loop strategies(Section 4.7.1 in FT):** Our definition of a multi-stage game with observed actions corresponds to the closed-loop information structure, where players condition their play at time $t$ on the history of play until that date. The corresponding strategies are called closed-loop strategies, while open-loop strategies are functions of calendar time alone.

The following example is given to illustrate that an open-loop equilibrium can also be subgame-perfect equilibrium.

> **Section 4.7.3 in FT:** Consider a game has a continuum of nonatomic individuals of each player type--a continuum of player 1s, a continuum of player 2s, and so on. (Let the set of individuals be copies of the unit interval endowed with Lebesgue measure for concreteness). Suppose further that each player $i$'s payoff is independent of the actions of any subset of opponents with measure $0$. Then if one individual player $j$ deviates, and all players $k\not=i,j$ ignore $j$'s deviation, it is clearly optimal for player $i$ to ignore the deviation as well. Thus the outcome of an open-loop equilibrium is subgame perfect.

My question is that, in the argument above, why do we need the part that it's optimal for player $i$ to ignore $j$'s deviation given others doing so?

To my understanding, to prove an open-loop equilibrium to be subgame perfect, we need to apply the one-shot deviation princinple. Therefore suppose player $j$ deviates, then because of the assumption and the fact that the measure of player $j$ is $0$, this will not affect the payoff of any player. Hence if a strategy profile constitutes an open-loop equilibrium, it's also subgame perfect.

However, the argument in the book seems to indicate that we not only need to check whether player $j$'s deviation is profitable, we also need to check whether there is a profitable deviation for player $i$ in the subgame in which $j$ already deviates. I am a little confused about how to apply the one-shot deviation principle.

To make my confusion more specific, consider an infinitely repeated version of the two-person prisoners' dilemma, and consider the following strategy profile: Both players will choose $C$, and if one player chooses $D$ at some $t$, both players will play $D$ for all $\tau>t$.

Suppose we want to check whether the above strategy profile constitutes a subgame perfect equilibrium, we apply the one-shot deviation principle by the following steps.

Step 1:We assume player $i$ deviates to $D$ at some time $t$, and no deviation has occurred until $t$, we check whether this deviation is profitable.

Step 2:Next we assume player $i$ deviates to $C$ in some subgame in which a deviation has already occurred and check whether such deviation is profitable.

Step 3:If the answer is no in both steps, we say that the above strategy profile constitutes a subgame-perfect equilibrium.

In this particular setting, my question is that, in step 1, do we also need to check whether there is a profitable deviation for player $-i$ after the deviation of $i$ from $C$ to $D$? How to distinguish between deviations in all subgames and one-shot deviation?

Thanks in advance for your help!

**Tags:** game-theory

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## 1 answers

### Answer by username_1 on 2022-06-29. Score: 4 | Accepted answer

It's a bit more complicated. Treating open-loop strategies as closed-loop strategies- that happen to play a given action in each period irrespective of the history- will not give you a subgame-perfect equilibrium. So there is no given closed-loop equilibrium that you can test using the one-deviation principle for being a subgame perfect equilibrium.

Rather, the claim is that the outcome path of an open-loop equilibrium will also be the outcome path of a closed-loop subgame perfect equilibrium with a continuum of individually insignificant players. The textbook doesn't quite spell out the technical details, which can be found in [Fudenberg, Drew, and David K. Levine. "Open-loop and closed-loop equilibria in dynamic games with many players." *Journal of Economic Theory* 44.1 (1988): 1-18.].

The idea is that most subgames can actually not be reached by individual deviations, so how play would proceed there becomes strategically irrelevant. If players ignore deviations of measure-zero sets of agents- which they can do since payoffs will not depend on those agents- the strategic situation will not change and it will still be optimal to follow the given plan. Individual players cannot get the play into subgames in which a positive measure set of players play differently. But in order to get a subgame perfect equilibrium giving rise to the outcome, one would need to specify strategies appropriately for such hard-to-reach subgames too.


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