| The evolutionarily stable strategy (or ESS; also evolutionary stable strategy) is a central concept in game theory introduced by John Maynard Smith and George R. Price in
1973 (a full account is given by Maynard Smith, 1982). It is based on a concept of a population of organisms playing a certain
strategy, that a mutant allele that causes organisms to adopt a different strategy
cannot invade the population, but will instead be selected out by natural selection.
The concept was based on W.D. Hamilton's (1967) unbeatable strategy; the
difference is that an unbeatable strategy is resistant to large migrations of different strategies. Through Hamilton's work on
sex ratios the concept can be traced back through Ronald Fisher (1930)
and Charles Darwin (1859) (see Edwards, 1998).
An ESS depends on the idea of invasion, where a population of strategy-X players is visited by a strategy-Y player. The new
player is said to invade if, following strategy Y, he scores better than the average strategy-X player. Assuming players
are able to choose and switch strategies, this would induce the indigenous population to start switching to strategy Y. In many
cases there are diminishing returns for the later adopters, and what follows is an equilibrium ratio of strategy-X players to
strategy-Y players.
A strategy X is evolutionarily stable if there is no strategy Y that can invade it. That is, anybody bringing a new strategy
into a population of strategy-X players will fare no better on average than the X players are already doing. (See the
closely-related Nash equilibrium) ESS is stable in respect to
randomly and occasionally occurring invading strategies, thus it is not stable in respect to mass counts of invaders. Similarly,
the concept of an ESS only exists under the assumption of infinitely large populations. In finite populations, stochastic effects
of genetic drift may force the ESS to become unstable.
For example, consider a large population of people who, in terms of the prisoner's dilemma, always play Tit_for_Tat in
transactions with each other. (Since almost any transaction requires trust, most transactions can be modelled with the
prisoner's dilemma.) If the entire population plays the Tit-for-Tat strategy, and a group of newcomers enter the
population who prefer the Always Defect strategy (i.e. they try to cheat everyone they meet), the Tit-for-Tat
strategy will prove more successful, and the defectors will be converted or lose out. Tit_for_Tat is therefore an
ESS. On the other hand, an island of Always Defect players will be stable against the invasion of a few Tit-for-Tat
players, but not against a large number of them. The (relatively simple) math behind this can be found in Robert Axelrod's The Evolution of Cooperation, or more briefly [|
here (http://www.urticator.net/essay/2/217.html)].
The recent, controversial sciences of sociobiology and now evolutionary psychology attempt to explain animal and human
behavior and social structures, largely in terms of evolutionarily stable strategies. For example, in one well-known 1995 paper (http://www.bbsonline.org/Preprints/OldArchive/bbs.mealey.html) by Linda Mealey, sociopathy (chronic antisocial/criminal behavior) is explained as a combination of two
such strategies.
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