APA Dictionary of Psychology

a state of equilibrium between two different adaptive strategies that allows both strategies to be maintained within a population. For example, shy animals are less likely than bold animals to behave in a way that attracts predators, but bold animals are more likely to find new food resources or secure additional mates. Within a population, therefore, there will be an optimal balance of costs and benefits for both shy and bold animals so that members of both types can survive. The evolutionarily stable strategy is a mechanism that accounts for the maintenance of diversity within a population.