Impossible trinity

The Impossible trinity (also known as the Trilemma, or Unholy Trinity) is a concept in international economics which states that it is impossible to have all three of the following at the same time:

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.

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:

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. A central bank has to forgo one of the three objectives. Therefore, a central bank has three policy combination options.

In terms of the diagram above (Oxelheim, 1990), the options are:

Currently, Eurozone members have chosen the first option (a) while most other countries have opted for the second one (b). By contrast, Harvard economist Dani Rodrik advocates the use of the third option (c) in his book The Globalization Paradox, emphasising that world GDP grew fastest during the Bretton Woods era when capital controls were accepted in mainstream economics. Rodrik also argues that the expansion of financial globalization and the free movement of capital flows are the reason why economic crises have become more frequent in both developing and advanced economies alike. Rodrik has also developed the "political trilemma of the world economy", where "democracy, national sovereignty and global economic integration are mutually incompatible: we can combine any two of the three, but never have all three simultaneously and in full.".

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