Impossible trinity

The impossible trinity (also known as the impossible trilemma or the Unholy Trinity) is a concept in international economics and international political economy which states that it is impossible to have all three of the following at the same time: a fixed foreign exchange rate free capital movement (absence of capital controls) an independent monetary policy It is both a hypothesis based on the uncovered interest rate parity condition, and a finding from empirical studies where governments that have tried to simultaneously pursue all three goals have failed. The concept was developed independently by both John Marcus Fleming in 1962 and Robert Alexander Mundell in different articles between 1960 and 1963. Historically in advanced economies, the periods pre-1914 and 1970–2014 were characterized by stable foreign exchange rates and free capital movement, whereas monetary autonomy was limited. The periods 1914–1924 and 1950–1969 had restrictions on capital movement (e.g. capital controls), but exchange rate stability and monetary autonomy were present. According to the impossible trinity, a central bank can only pursue two of the above-mentioned three policies simultaneously. To see why, consider this example (which abstracts from risk but this is not essential to the basic point): Assume that world interest rate is at 5%. If the home central bank tries to set domestic interest rate at a rate lower than 5%, for example at 2%, there will be a depreciation pressure on the home currency, because investors would want to sell their low yielding domestic currency and buy higher yielding foreign currency. If the central bank also wants to have free capital flows, the only way the central bank could prevent depreciation of the home currency is to sell its foreign currency reserves. Since foreign currency reserves of a central bank are limited, once the reserves are depleted, the domestic currency will depreciate. Hence, all three of the policy objectives mentioned above cannot be pursued simultaneously.