Countries have a vested interest in the exchange rate of their currency to their trading partner's currency because it affects trade flows. When the domestic currency has a high value, its exports are expensive. This leads to a trade deficit, decreased production, and unemployment. If the currency's value is low, imports can be too expensive though exports are expected to rise.
Purchasing Power Parity
Purchasing power parity is a way of determining the value of a product after adjusting for price differences and the exchange rate. Indeed, it does not make sense to say that a book costs $20 in the US and £15 in England: the comparison is not equivalent. If we know that the exchange rate is £2/$, the book in England is selling for $30, so the book is actually more expensive in England
If goods can be freely traded across borders with no transportation costs, the Law of One Price posits that exchange rates will adjust until the value of the goods are the same in both countries. Of course, not all products can be traded internationally (e.g. haircuts), and there are transportation costs so the law does not always hold.
The concept of purchasing power parity is important for understanding the two models of equilibrium exchange rates below.
Balance of Payments Model
The balance of payments model holds that foreign exchange rates are at an equilibrium level if they produce a stable current account balance. A nation with a trade deficit will experience a reduction in its foreign exchange reserves, which ultimately lowers, or depreciates, the value of its currency. If a currency is undervalued, its nation's exports become more affordable in the global market while making imports more expensive. After an intermediate period, imports will be forced down and exports will rise, thus stabilizing the trade balance and bringing the currency towards equilibrium.
Asset Market Model
Like purchasing power parity, the balance of payments model focuses largely on tangible goods and services, ignoring the increasing role of global capital flows . In other words, money is not only chasing goods and services, but to a larger extent, financial assets such as stocks and bonds. The flows from transactions involving financial assets go into the capital account item of the balance of payments, thus balancing the deficit in the current account. The increase in capital flows has given rise to the asset market model.
Share of Stock
The key difference between the balance of payments and asset market models is that the former includes financial assets, such as stock, in its calculation.
The asset market model views currencies as an important element in finding the equilibrium exchange rate. Asset prices are influenced mostly by people's willingness to hold the existing quantities of assets, which in turn depends on their expectations on the future worth of the assets. The asset market model of exchange rate determination states that the exchange rate between two currencies represents the price that just balances the relative supplies of, and demand for, assets denominated in those currencies. These assets are not limited to consumables, such as groceries or cars. They include investments, such as shares of stock that is denominated in the currency, and debt denominated in the currency.