I consider two theories of the determination of political institutions. One of these theories stresses effects of changes in the balance of military power between the ruler and subjects on the distribution of property rights which the political system enforces. The other theory emphasizes the effect of changing informational constraints which require institutional changes to be made in order to maintain efficiency. I examine how each of these theories would apply to explaining the development of parliamentary government in thirteenth-century England. My general conclusion is that both theories are required to understand fully the process by which liberal political institutions emerge.