One of the approaches to determine or predict curency exchange rates is that involving the analysis of interest rate differentials (the Interest Rate Approach). This involves a number of different principles and we shall go through them briefly and in turn. The first principle involves the basic interest rate parity theory, which is that:

An exchange rate’s forward % premium/discount = its interest rate differential

Thus, for instance, the traditional forward discount on the dollar–yen exchange rate should equal the interest rate differential between the two currencies. This is seen as the equilibrium reflecting the relationship between the exchange rate and interest rates. Because forwards are a traded instrument and thus subject to supply and demand, the forward premium or discount can vary briefly from this equilibrium, but should always revert to norm. After all, if for argument’s sake the forward premium/discount for some reason did not equal the interest rate differential between the two currencies an arbitrageur could in theory make risk-free profits by borrowing in one currency, investing in the securities of the other currency and simultaneously opening a forward contract in the exchange rate for the same period as the initial loan. This is called covered interest rate arbitrage.

The theory of interest rate parity is a guiding principle for several economic and financial models. Under this theory, it is assumed that the expected (interest rate) returns of a currency should be equalized through speculation in another country once converted back to the first currency. This may sound like gibberish, but basically this is an interest rate version of PPP— and like PPP its results are decidedly mixed. Indeed, there can be significant violations of the interest rate parity theory for substantial periods of time without the immediate reversal that covered interest rate arbitrage might suggest. Not too surprisingly, this is a dismal predictor of exchange rates.

Indeed, before we go further into the theory, it is important to point out a practical flaw in the theory involving incentive, which is undoubtedly a key contributing factor to its poor predictive track record—the theory supposes an automatically causal relationship between interest rates and the exchange rate, yet in practice most currency market practitioners trade currencies with directional rather than interest rate considerations in mind. Even this statement is a generalization. On a simple numerical basis, the majority of currency market practitioners are made up of interbank dealers, thus it is important and necessary to look at their motivation for trading. Spot traders for the most part care not one whit about a currency’s interest rate, in part because they hold positions for too short a time for it to matter, in part because they are seeking to predict direction—and thus make capital gains on their position, not primarily to make interest income. Forward traders are a different breed entirely and more akin to money market or interest rate traders. Indeed, theway they hedge out their forward exposure frequently involves an array of interest rate-related instruments. Eventually, interest rate parity violations will be reversed, but there is little incentive to do so in the immediate term if you don’t care about the interest rate in the first place.

Returning to the theory for now, interest rate parity theory states that the difference between a spot and forward exchange rate expressed as a percentage should equal the interest rate differential between the two currencies. Yet, we know from the PPP principle that exchange rates and inflation rates are linked. Can we not link these also with interest rates? Indeed we can, thanks to the seminal work of the economist Irving Fisher. Thus, according to what has become known as the “Fisher effect:”

The difference in interest rates = the difference in expected inflation rates

Thus, we have gone from the difference between the spot and the forward exchange rate equating to the interest rate differential through the interest rate parity theory, which in turn equates to the difference in expected inflation rates through the Fisher effect. Yet, PPP tells us that absolute or relative price growth levels can be used to forecast future exchange rates.

Thus, through PPP we can extrapolate this one stage further to suggest that:

The difference in expected inflation rates = the expected exchange rate change

Bringing all these together, we get: (1) The difference in spot and forward rates = the difference in interest rates (Interest rate parity theory)

(2) The difference in interest rates = the difference in expected inflation rates (Fisher effect)

(3) The difference in expected inflation rates = the expected change in spot exchange rate (Purchasing Power Parity)

Logically from this, one may conclude that the difference between the spot and forward rates expressed as a percentage should equal the expected change in the spot exchange rate. This is known as the expectations theory of exchange rates.

Finally, there is the theory that: (4) The difference in interest rates = the expected change in the spot exchange rate (International Fisher effect)

On the face of it, the ideas presented above seem logical and follow a clear and persuasive train of thought. There is only one small problem—this clear train of thought rarely works in practice. More specifically, the difference in interest rates or expected inflation rates may be equal to the theoretical construct of the “expected change in the spot exchange rate”, but in practice it is of the future exchange rate. In line with this, the forward rate is also a very poor predictor of the future exchange rate, a fact that economists have labelled “forward rate bias” or the “forward premium puzzle”. As Bansal and Dahlquist confirmed in their exhaustive study, in contrast to the theory, empirical evidence suggests that in fact current interest rate differentials and future spot exchange rates are frequently negatively correlated. This is particularly the case within the developed economies, though the picture is somewhat more mixed within emerging market economies.

Over the long term, the interest rate parity theory is seen to work as enough market participants can be found to “discover” the opportunities available for covered interest rate arbitrage between currencies and interest rates, thus in the process eliminating such disparities. However, there are much longer lags than the theory might suggest is possible. Here again, the issue of incentive must be a focus. As noted earlier, it should behove the theorists to know that the majority of currency market practitioners are currency interbank dealers and moreover that the main incentive for these to trade is directional gain rather than interest income. Currency markets do focus on interest rate differentials for extended periods of time, but equally they focus on other factors, in many cases completely disregarding interest rates.

Real Interest Rate Differentials and Exchange Rates

Currency strategists do however use models comparing the real interest rate differential with either the nominal or the real exchange rate between two countries. The logic behind this relates to both the international Fisher effect and to PPP, where on the one hand the difference in interest rates should, if not be exactly equal to an expected change in the spot exchange rate, at least be an important driver of it, and on the other hand where nominal interest rate differentials are adjusted for inflation (i.e. domestic price growth) and thus relate to the exchange rate through the law of one price.

The link or correlation between real interest rate differentials and the exchange rate appears to have grown exactly in line with the gradual move since the end of the Bretton Woods exchange rate system to liberalize capital flows globally. As barriers to capital movement have fallen, so the overall importance of capital flow has grown exponentially relative to that of trade flow. Exchange rate models that focused solely on the current account no longer seemed appropriate in such a world, those that focused on capital flows seemed increasingly so. As capital flows have gained in importance, so their importance within overall currency market flows has grown and thus the correlation between the two increased. Thus, currency strategists across the market continue to track this relationship between real interest rate differentials and nominal exchange rates as one of many useful and important indicators of currency over- or undervaluation. Figure 1 compares the Euro–dollar exchange rate against the 10–year bond yield differential from 1996 through to the end of January, 2002. babelt010