Although the exchange rate stability sustained by the Bretton Woods system facilitated expanding international trade, this early success masked its underlying design flaw, wherein there existed no mechanism for increasing the supply of international reserves to support continued growth in trade. The system began experiencing insurmountable market pressures and deteriorating cohesion among its key participants in the late 1950s and early 1960s. Central banks needed more U.S. dollars to hold as reserves, but were unable to expand their money supplies if doing so meant exceeding their dollar reserves and threatening their exchange rate pegs. To accommodate these needs, the Bretton Woods system depended on the United States to run dollar deficits. As a consequence, the dollar's value began exceeding its gold backing. During the early 1960s, investors could sell gold for a greater dollar exchange rate in London than in the United States, signaling to market participants that the dollar was overvalued. Belgian-American economist Robert Triffin defined this problem now known as the Triffin dilemma, in which a country's national economic interests conflict with its international objectives as the custodian of the world's reserve currency.