Understanding Exchange Rate Theories

Exchange rates, which represent the price of one currency in terms of another, are influenced by a variety of economic factors. Economists have developed several theories to explain how exchange rates are determined and how they might change over time. These theories often differ in their assumptions and the time horizon they focus on – short-run fluctuations versus long-run trends.

Flexible Exchange Rate Systems

In a flexible exchange rate system, the value of a country's currency is determined by the forces of supply and demand in the foreign exchange market. Unlike fixed or managed systems, the central bank typically does not intervene to maintain a specific exchange rate target. This allows the exchange rate to adjust freely in response to economic conditions.

Identifying the Long-Run Prediction Theory

The question asks specifically about a theory used for making long-run predictions about exchange rates in a flexible system. Let's consider the options provided:

• Purchasing Power Parity Theory: This theory suggests that, in the long run, exchange rates should adjust so that an identical basket of goods and services costs the same in different countries when measured in the same currency. It is based on the idea that arbitrage across international goods markets should eliminate significant price differences. The core concept is the law of one price applied internationally.
• Portfolio Balance Approach: This theory views the exchange rate as the price that equilibrates the supply and demand for domestic and foreign assets (like bonds and money). It considers how investors allocate their wealth among different assets based on expected returns, risks, and preferences. While comprehensive, it's often more complex and can apply to both short and long runs but isn't singularly defined as *the* long-run price-based predictor like PPP.
• Interest Rate Approach: This typically refers to theories like Interest Rate Parity (covered or uncovered). Uncovered Interest Parity suggests that the difference in interest rates between two countries is equal to the expected change in the exchange rate. This is primarily a short-run concept focusing on financial market arbitrage and expected future spot rates, not necessarily a long-run determinant based on underlying economic fundamentals like price levels.
• Balance of Payment Theory: This theory suggests that exchange rates are determined by the overall balance of payments position of a country (the sum of the current account and capital account). While balance of payments flows certainly affect exchange rates, especially in the short to medium run, this theory doesn't provide a specific long-run equilibrium level based on fundamental economic variables like relative prices in the same way PPP does for long-run predictions. It describes the process of adjustment rather than the ultimate long-run determinant in a purely flexible system driven by purchasing power.

Comparing these theories, the Purchasing Power Parity Theory is most commonly cited as the primary framework for understanding and predicting long-run movements in exchange rates in flexible systems, based on the relationship between domestic and foreign price levels.

Purchasing Power Parity (PPP) Theory Explained

The absolute version of PPP states that the exchange rate (\(E\)) between two currencies (\(E_{currency\_A/currency\_B}\)) should equal the ratio of the price levels (\(P\)) in the two countries:

\(E = P_A / P_B\)

The relative version of PPP is more practical and states that the percentage change in the exchange rate between two countries should be approximately equal to the difference in their inflation rates.

\(\% \Delta E \approx Inflation_A - Inflation_B\)

PPP works best as a long-run predictor because it assumes that goods markets are perfectly competitive and integrated, and that transportation costs and trade barriers are negligible, allowing arbitrage to function effectively over time. In the short run, factors like capital flows, interest rate differentials, speculation, and market sentiment can cause significant deviations from PPP.

Therefore, the theory primarily used for making long-run predictions about exchange rates in a flexible exchange rate system is the Purchasing Power Parity Theory.

Revision Table: Exchange Rate Theories

Additional Information: Why PPP for the Long Run?

While deviations from Purchasing Power Parity can be substantial and persistent in reality due to factors like non-tradable goods, trade barriers, sticky prices, and imperfect information, economists still use PPP as a benchmark for the long-run equilibrium exchange rate. The idea is that over sufficiently long periods, the forces driving prices towards equality (arbitrage in goods markets) become more dominant than short-term financial market fluctuations or capital flows. Persistent deviations from PPP suggest that a currency is either undervalued or overvalued relative to its long-run fundamental value based on purchasing power.