Quantal response equilibrium

When analyzing data from the play of actual games, particularly from laboratory experiments, particularly from experiments with the matching pennies game, Nash equilibrium can be unforgiving. Any non-equilibrium move can appear equally "wrong", but realistically should not be used to reject a theory. QRE allows every strategy to be played with non-zero probability, and so any data is possible (though not necessarily reasonable).

The most common specification for QRE is logit equilibrium (LQRE). In a logit equilibrium, player's strategies are chosen according to the probability distribution:

${\displaystyle P_{ij}={\frac {\exp(\lambda EU_{ij}(P_{-i}))}{\sum _{k}{\exp(\lambda EU_{ik}(P_{-i}))}}}}$

${\displaystyle P_{ij}}$ is the probability of player ${\displaystyle i}$ choosing strategy ${\displaystyle j}$. ${\displaystyle EU_{ij}(P_{-i}))}$ is the expected utility to player ${\displaystyle i}$ of choosing strategy ${\displaystyle j}$ under the belief that other players are playing according to the probability distribution ${\displaystyle P_{-i}}$. Note that the "belief" density in the expected payoff on the right side must match the choice density on the left side. Thus computing expectations of observable quantities such as payoff, demand, output, etc., requires finding fixed points as in mean field theory.[3]

For dynamic (extensive form) games, McKelvey and Palfrey defined agent quantal response equilibrium (AQRE). AQRE is somewhat analogous to subgame perfection. In an AQRE, each player plays with some error as in QRE. At a given decision node, the player determines the expected payoff of each action by treating their future self as an independent player with a known probability distribution over actions. As in QRE, in an AQRE every strategy is used with nonzero probability.

The quantal response equilibrium approach has been applied in various settings. For example, Goeree et al. (2002) study overbidding in private-value auctions,[4] Yi (2005) explores behavior in ultimatum games,[5] Hoppe and Schmitz (2013) study the role of social preferences in principal-agent problems,[6] and Kawagoe et al. (2018) investigate step-level public goods games with binary decisions.[7]

• Bounded rationality
• Behavioral game theory