A Note on the computational hardness of evolutionary stable strategies

We present a very simple reduction that when given a graph G and an integer k produces a game that has an evolutionary stable strategy if and only if the maximum clique size of G is not exactly k. Formally this shows that existence of evolutionary stable strategies is hard for a complexity class called co − D, slightly strengthening (and greatly simplifying) the known NP-hardness and co-NP-hardness. En route we show that even recognizing an evolutionary stable strategy is co-NP complete.