A cornerstone of liberal-democratic regimes is the right of free speech, granted even to nonliberals who manifestly oppose it. Communism and political Islamism are two primary examples of ideologies which are tolerated in spite of calls for the limits on the right of expression. Not surprisingly, it is often argued that a tolerant society needs laws preventing non-tolerant beliefs from attacking tolerance. Yet, does intolerance necessarily prosper in a tolerant society, or is deemed to decay? To address the question, I build an evolutionary model of competing (political and/or religious) beliefs. In the model, individuals are assumed to gain from having beliefs. The gain may increase with intolerance of the belief (premium). High intolerance, however, makes strong believers fragile in a society of tolerant people. Having examined evolutionarily stable states in two specifications, I demonstrate that (for any positive premium) heterogeneity cannot prevent intolerant beliefs from spreading out. A sufficiently small increase in intolerance, when premium exceeds losses from fragility, allows intolerance to spread. Intolerance is vulnerable only as long as the premium is non-positive. This finding can also be interpreted as follows: unless fundamentalist confessions are proved to be vital for individual human existence (positive premium), a tolerant society needs no intervention to preserve tolerance.
