Another limitation of IO models (shared by SAM models) is that they are predicated upon an assumption of entirely passive supply: they are completely demand-driven. In circumstances where there are important supply constraints in the host region, the supply side of the economy has to be modelled explicitly. In principle, computable general equilibrium models (CGEs) permit a complete, theory-consistent model of the demand and supply sides of all markets. In CGEs prices are endogenous, and typically adjust to equate demand and supply in each market. However, for demand-side disturbances they replicate the comparable IO systems where supply is passive (e.g. where there is significant unemployment and spare capacity). These studies have modelled, for example, the macroeconomic impact of increased demand connected with marine expenditures, the potential legacy effects of such expenditures, and the creation of new export opportunities