Intrasexual polymorphisms have evolved in a wide range of organisms. Most of them have been interpreted as the product of conditional strategies in which the tactic an individual adopts is determined by some aspect of state (e.g., age, size, condition). However, there are a few examples that appear to represent an evolutionarily stable mixture of heritable pure strategies that are maintained by frequency-dependent selection. In the present study, we produce a model of a mating system with two morphs: a territorial morph and a sneak morph. By varying the costs and limits associated with conditional strategies, mating skew, and the proportion of matings obtained by sneaking males, we examine the conditions that favor the evolution of conditional versus pure strategies. Contrary to current thinking, our results show that as long as either costs or limits are greater than zero, conditional strategists are never able to entirely replace pure strategists, and equilibrium populations may frequently consist of a mixture of conditional and pure strategists. Our results suggest that conditional strategists will be most frequent at intermediate levels of mating skew. Polymorphisms in which conditional strategists are rare or absent are most likely to evolve when mating skew is extremely high, the costs and limits of plasticity are very high, or the benefits of being conditional are very low. The limited data available suggest that high mating skew is probably the most important factor.