Two-population replicator dynamics and number of Nash equilibria in matrix games

The Abdus Salam International Centre for Theoretical Physics - Strada Costiera 11, 34014 Trieste, Italy and CNR-INFM, Trieste-SISSA Unit - V. Beirut 2-4, 34014 Trieste, Italy

Received: 9 September 2006
Accepted: 7 March 2007

Abstract

We study the connection between the evolutionary replicator dynamics and the number of Nash equilibria in large random bi-matrix games. Using techniques of disordered systems theory we compute the statistical properties of both, the fixed points of the dynamics and the Nash equilibria. Except for the special case of zero-sum games, one finds a transition as a function of the so-called co-operation pressure between a phase in which there is a unique stable fixed point of the dynamics coinciding with a unique Nash equilibrium, and an unstable phase in which there are exponentially many Nash equilibria with statistical properties different from the stationary state of the replicator equations. Our analytical results are confirmed by numerical simulations of the replicator dynamics, and by explicit enumeration of Nash equilibria.