Evolutionary Stable Strategy (ESS)

A strategy resistant to invasion by sufficiently rare alternatives under a specified population game.

An evolutionarily stable strategy resists invasion by a sufficiently small share of an alternative strategy in a specified population game. The comparison depends on expected payoffs against the mostly resident population, including encounters involving the rare alternative. In biology these payoffs represent consequences for reproductive success; other applications must define their own transmission mechanism.

ESS is stronger than merely being a symmetric Nash equilibrium. If an alternative ties against the resident, further comparisons matter. An ESS need not be socially best, unique, or reached from every starting state, and some games have none. In a standard hawk–dove model, the stable result depends on the value of the resource relative to the cost of conflict; a fixed mixed ratio is not universal.

When to use it

Useful in evolutionary game theory and carefully specified analogies about persistent conventions.

How it can help

Analyze rare alternatives using payoffs, matching, and a defined process of strategy transmission.