Why PPP deviations matter and core explanations
Understanding persistent deviations from purchasing power parity matters because exchange rates that stray from PPP affect real incomes, trade competitiveness, inflation and investment decisions across societies and territories. Sticky-price monetary models emphasize how nominal rigidities prevent immediate goods-price adjustments when monetary shocks hit. Maurice Obstfeld at the University of California, Berkeley and Kenneth Rogoff at Harvard University developed formal frameworks showing that prices adjust slowly across countries, so exchange rates move to restore short-run asset market equilibrium while goods prices lag. This creates predictable short-run departures from PPP even when long-run parity holds.
Short-run dynamics: overshooting and risk premia
The classic overshooting model by Rudiger Dornbusch at the Massachusetts Institute of Technology explains why exchange rates can deviate sharply from PPP after unexpected monetary shocks. In this account, asset prices respond instantly while goods prices are rigid; the exchange rate temporarily "overshoots" its long-run PPP level and then converges as prices adjust. Financial market expectations and time-varying risk premia amplify these movements, a point emphasized in later work by international macroeconomists who link portfolio flows and global risk appetite to sustained PPP gaps. Emerging markets frequently exhibit larger premia and more persistent deviations because of thinner capital markets and higher perceived default risk.
Structural causes: productivity and nontraded sectors
Systematic, cross-country deviations arise from the Balassa-Samuelson effect articulated by Béla Balassa at Johns Hopkins University and Paul Samuelson at the Massachusetts Institute of Technology. High productivity growth in tradable-goods sectors raises wages and prices in nontraded services, appreciating real exchange rates relative to low-productivity economies. This mechanism explains why richer countries often have higher price levels than PPP would predict. Territorial and cultural factors matter because consumption baskets and the relative size of nontraded sectors vary with income, urbanization and social preferences, creating persistent, explainable departures from PPP.
Microstructure and behavioral approaches
Microstructure models that focus on order flow, transaction costs and dealer behavior account for high-frequency deviations that are invisible to aggregate PPP logic. Behavioral equilibrium exchange rate approaches and empirically calibrated reduced-form models combine fundamentals, such as productivity and terms of trade, with market frictions to produce more realistic valuations. Policy relevance is clear: misaligned currencies alter local welfare, influence export-dependent communities, and shape environmental outcomes through changing incentives for resource extraction.