The US dollar because of it's status of the worlds reserve currency and what that implies makes a paradox in terms of a positive trade account. It is called the "Triffin" dilemma named after the Yale economist who it is named after.




 In October 1959, a Yale professor sat in front of Congress' Joint Economic Committee and calmly announced that the Bretton Woods system was doomed. The dollar could not survive as the world's reserve currency without requiring the United States to run ever-growing deficits. This dismal scientist was Belgium-born Robert Triffin, and he was right. The Bretton Woods system collapsed in 1971, and today the dollar's role as the reserve currency has the United States running the largest current account deficit in the world.

TUTORIAL: The Federal Reserve: Introduction
For much of the 20th century, the U.S. dollar was the currency of choice. Central banks and investors alike bought dollars to hold as foreign exchange reserves, and with good reason. The U.S. had a stable political climate, did not experience the ravages of world wars like Europe had and had a steadily growing economy that was large enough to absorb shocks. 

By "agreeing" to have its currency used as a reserve currency, a country pins its hands behind its back. In order to keep the global economy chugging along, it may have to inject large amounts of currency into circulation, driving up inflation at home. The more popular the reserve currency is relative to other currencies, the higher its exchange rate and the less competitive domestic exporting industries become. This causes a trade deficit for the currency-issuing country, but makes the world happy. If the reserve currency country instead decides to focus on domestic monetary policy by not issuing more currency then the world is unhappy. (To know more about the relationship between trade and currency, read: Global Trade And The Currency Market. )

Reserve Currency Paradox
Becoming a reserve currency presents countries with a paradox. They want the "interest-free" loan generated by selling currency to foreign governments, and the ability to raise capital quickly, because of high demand for reserve currency-denominated bonds. At the same time they want to be able to use capital and monetary policy to ensure that domestic industries are competitive in the world market, and to make sure that the domestic economy is healthy and not running large trade deficits. Unfortunately, both of these ideas – cheap sources of capital and positive trade balances – can't really happen at the same time. 

This is the Triffin dilemma, named after Robert Triffin, an economist who wrote of the impending doom of the Bretton Woods system in his 1960 book "Gold and the Dollar Crisis: The Future of Convertibility." He pointed out that the years of pumping dollars into the world economy through post-war programs, such as the Marshall Plan, was making it increasingly difficult to stick to the gold standard. In order to maintain the standard, the country had to both instill international confidence by having a current account surplus while also having a current account deficit by providing immediate access to gold. 

Issuing a reserve currency means that monetary policy is no longer a domestic-only issue – it's international. Governments have to balance the desire to keep unemployment low and economic growth steady with its responsibility to make monetary decisions that will benefit other countries. The reserve currency status is, thus, a threat to national sovereignty.