The Unholy Trinity is an international economic principle that the policymakers of a country may pursue only two out of three policy directions. The three policy directions are the free movement of capital, an independent monetary policy, and a fixed or pegged exchange rate policy.
Out of the three policy directions, states would most likely wish to pursue an independent monetary policy and exchange rate stability. However, as most governments now know, capital controls are hard to enforce as economic actors are easily able to find ways to evade such restrictions. Furthermore, capital controls discourage foreign economic actors from investing their capital in a country since they may not be able to freely remove their capital in the future. As a result, most governments simply cannot impose workable capital controls, which all but ensures that the free movement of capital is one of the two policy directions that governments will choose.
This servers as the basis for Milton Friedman’s declaration that flexible exchange rates are necessary for free trade, as foreign and domestic investors must be permitted to freely move capital across international borders to satisfy balance of payments and for governments to retain independent monetary policies, especially as an anti-recessionary measure.
Benjamin Cohen argues that although international coordination may solve the problem of the Unholy Trinity with coordinated state action in regard to exchange rates, states are induced by ever-changing incentives to cheat or cooperate on such arrangements in order to further their own short-term policy priorities. Such uncertainty thus makes exchange rate coordination untenable. 
The Unholy Trinity in Practice
By applying the above assumption of free capital mobility to hypothetical models, one can see how the Unholy Trinity puts constraints on policy makers. Specifically, the policy contraints inherent in the Unholy Trinity affect how policy makers employ macroeconomic stabilization policies to manage national income, unemployment, and inflation. The following generalized hypothetical examples illustrate the policy trade-offs and constraints created by the Unholy Trinity. It should be noted that in differentiating between the effects of the following two regime-type decisions, one must note whom each decision constrains and empowers.
Fixed Exchange Rates with Capital Mobility
In this monetary regime type, as Milton Friedman also points out, monetary authorities lose policy autonomy, and thus, the efficacy of monetary policy. In other words, any change in interest rates through the manipulation of the money supply made by monetary authorities would be offset by capital movements in response. But, in order to maintain the country's fixed exchange rate, central bankers would be forced again to manipulate the money supply, moving interest rates in the opposite direction, and thus negating their original action.
On the other hand, fiscal authorities see their policy efficacy improve under a fixed exchange rate regime with free movement of capital. For example, if fiscal authorities decided to use expansionary fiscal policy to increase national income, this would increase interest rates, which would then induce large capital inflows to the country and create a surplus of foreign currency. In order to maintain the fixed exchange rate, central bankers would be forced to use the national currency to buy foreign reserves, increasing the money supply and decreasing interest rates, which would further increase national income. In other words, during fiscal expansion, a fixed exchange rate regime reduces crowding out by the government on investment through decreasing interest rates.
In summary, a fixed exchange rate regime with capital mobility gives monetary authorities (central bankers) less policy autonomy, while increasing the efficacy of fiscal policy.
Flexible Exchange Rates with Capital Mobility
It logically follows that, under a flexible exchange rate regime, fiscal policy loses efficacy while monetary policy gains efficacy (and independence).
- Friedman, M. (1953). The Case for Flexible Exchange Rates. In M. Friedman, Essays in Positive Economics (pp. 157-203). Chicago: University of Chicago Press.
- Cohen, B. (2000). The Triad and the Unholy Trinity: Problems of International Monetary Cooperation. In J. Frieden, International Political Economy: Perspectives on Global Power and Wealth (pp. 245-256). London: Routledge.
- Grieco, Joseph M., and G. John Ikenberry. State Power and World Markets: The International Political Economy. New York: W.W. Norton & Co., 2003. p 88.
- Grieco, Joseph M., and G. John Ikenberry. State Power and World Markets: The International Political Economy. New York: W.W. Norton & Co., 2003. pp 86-87.
- Grieco, Joseph M., and G. John Ikenberry. State Power and World Markets: The International Political Economy. New York: W.W. Norton & Co., 2003. pp 88-89.