Reem Heakal
貨幣數量理論（QTM）的概念起始於16世紀。當時黃金和白銀從美洲流入歐洲並被制造成硬幣，通脹開始上升。這導致經濟學家Henry Thornton在1082年認為更多的貨幣等於更大的通貨膨脹，也就是更多的貨幣供應並不意味著經濟產出的增加。我們在這裡將讨論這個假設和潛在QTM的計算以及它與貨幣主義理論的關系和理論受到的挑戰。
QTM概括
貨幣數量理論（QTM）指出經濟體中的貨幣數量與貨物或服務價格有直接的關系。貨幣數量理論認為如果一個經濟體中的貨幣數量加倍，價格也會加倍，引起通貨膨脹。因此消費者將需要為之前相同的產品和服務支付雙倍的價錢。
理解這一理論的另一方式是把貨幣當成其他任意一種商品，增加其供應將會降低其邊際價值（一個單位貨幣的購買力）。因此貨幣的增加將引起價格（通貨膨脹）上漲，因為減少的邊際價值需要得到彌補。
該理論的計算
以最簡單的方式該理論被表示為：
MV = PT（費雪方程式）
每個變量意義如下：
M：貨幣供應
V：貨幣流通速度（貨幣轉手的次數）
P：平均價格水平
T：交易的商品和服務數量
理論被17世紀正統經濟學家所接受並被20世紀經濟學家費雪和米爾頓•弗裡德曼檢查並制定上述方程式。
上面的方程式基礎是下面交易方程式：
資金數量x流通速度=總消費
也就是說如果一個經濟體有3美元，並且這3美元在一個月内被使用5次，那麼這個月的總消費就是15美元。
QTM假設
QTM為交易方程式添加假設。在其最基本的形式中，該理論假設V（流通速度）和T（交易數量）在短期内是不變的。然而這些假設受到了大量指責，尤其是假設V不變。争論的焦點是流通速度取決於消費者和企業的消費沖動，不可能保持不變。
該理論還假設由外部因素決定的貨幣數量是一個社會經濟活動的主要影響。貨幣供應的變化導致價格水平和產品和服務供應的變化。根本上，貨幣儲量的變化會引起消費支出的變化。而流通速度不取決於可用的貨幣數量或者當前的價格水平，而是價格水平的變化。
最後，交易次數（T）是由勞動力、資本、自然資源、知識和組織決定的。這個理論假設的是經濟平衡和充分就業下的情況。
從本質上來說，這個理論的假設表明貨幣的價值取決於經濟體中可用貨幣的數量。貨幣供應增加導致貨幣價值下降是因為貨幣供應增加導致通貨膨脹的上升。隨著通貨膨脹上升，購買力或者貨幣價值就會下降。因此需要花費更多的資金購買相同數量的商品或者服務。
貨幣供應、通貨膨脹和貨幣主義
正是因為QTM認為貨幣數量決定貨幣價值，它成為了貨幣主義的基石。
貨幣主義認為貨幣供應的快速增加導致通貨膨脹的急劇增加。貨幣增加速度超過經濟產出增長速度導致通貨膨脹，因為貨幣將遠遠多餘商品和服務的產量。為了抑制通貨膨脹，貨幣增長必須低於經濟產出的增長。
這個前提決定貨幣政策是如何被管理的。貨幣主義者認為貨幣供應應該被控制在一個可接受的範圍内，這樣可以控制通脹水平。因此大多數貨幣主義者認為短期内貨幣供應的增加可以推動急速增長經濟體對產量增加的要求。但是貨幣政策這種影響的長期效果還不明顯。
相反，不那麼正統的貨幣主義者認為貨幣供應增加不會對實際經濟活動產生影響。但是大多數貨幣主義者都認為任何抵制通貨膨脹的政策都應從減少貨幣供應入手。大多數貨幣主義者認為政府與其不斷調整經濟政策還不如讓非通脹政策引導經濟體進行充分就業。
John Maynard Keynes（凱恩斯）在20世紀30年對貨幣質量理論提出質疑，他認為增加貨幣供應將導致流通次數和實際收入的減少，而貨幣將流入生產環節。因此貨幣流通將會隨著貨幣供應變化。許多後世經濟學家認為他的觀點是正確的。
因為其植根於貨幣主義，QTM在20世紀80年代受到許多主要經濟體的歡迎，例如在裡根執政時期的美國和撒切爾執政時期的英國。當時，這些領導人試圖利用這一理論的原理設定貨幣增長目標。然而隨著時間的推移，許多人開始發現嚴格的貨幣控制未必是治療經濟衰退的靈丹妙藥。
What Is the Quantity Theory of Money?
By Reem Heakal
The concept of the quantity theory of money (QTM) began in the 16th century. As gold and silver inflows from the Americas into Europe were being minted into coins, there was a resulting rise in inflation. This led economist Henry Thornton in 1802 to assume that more money equals more inflation and that an increase in money supply does not necessarily mean an increase in economic output. Here we look at the assumptions and calculations underlying the QTM, as well as its relationship to monetarism and ways the theory has been challenged.
QTM in a Nutshell
The quantity theory of money states that there is a direct relationship between the quantity of money in an economy and the level of prices of goods and services sold. According to QTM, if the amount of money in an economy doubles, price levels also double, causing inflation (the percentage rate at which the level of prices is rising in an economy). The consumer therefore pays twice as much for the same amount of the good or service.
Another way to understand this theory is to recognize that money is like any other commodity: increases in its supply decrease marginal value (the buying capacity of one unit of currency). So an increase in money supply causes prices to rise (inflation) as they compensate for the decrease in money's marginal value.
The Theory's Calculations
In its simplest form, the theory is expressed as:
MV = PT (the Fisher Equation)
Each variable denotes the following:
M = Money Supply
V = Velocity of Circulation (the number of times money changes hands)
P = Average Price Level
T = Volume of Transactions of Goods and Services
The original theory was considered orthodox among 17th century classical economists and was overhauled by 20th-century economists Irving Fisher, who formulated the above equation, and Milton Friedman.
It is built on the principle of "equation of exchange":
Amount of Money x Velocity of Circulation = Total Spending
Thus if an economy has US$3, and those $3 were spent five times in a month, total spending for the month would be $15.
QTM Assumptions
QTM adds assumptions to the logic of the equation of exchange. In its most basic form, the theory assumes that V (velocity of circulation) and T (volume of transactions) are constant in the short term. These assumptions, however, have been criticized, particularly the assumption that V is constant. The arguments point out that the velocity of circulation depends on consumer and business spending impulses, which cannot be constant.
The theory also assumes that the quantity of money, which is determined by outside forces, is the main influence of economic activity in a society. A change in money supply results in changes in price levels and/or a change in supply of goods and services. It is primarily these changes in money stock that cause a change in spending. And the velocity of circulation depends not on the amount of money available or on the current price level but on changes in price levels.
Finally, the number of transactions (T) is determined by labor, capital, natural resources (i.e. the factors of production), knowledge and organization. The theory assumes an economy in equilibrium and at full employment.
Essentially, the theory's assumptions imply that the value of money is determined by the amount of money available in an economy. An increase in money supply results in a decrease in the value of money because an increase in money supply causes a rise in inflation. As inflation rises, the purchasing power, or the value of money, decreases. It therefore will cost more to buy the same quantity of goods or services.
Money Supply, Inflation and Monetarism
As QTM says that quantity of money determines the value of money, it forms the cornerstone of monetarism.
Monetarists say that a rapid increase in money supply leads to a rapid increase in inflation. Money growth that surpasses the growth of economic output results in inflation as there is too much money behind too little production of goods and services. In order to curb inflation, money growth must fall below growth in economic output.
This premise leads to how monetary policy is administered. Monetarists believe that money supply should be kept within an acceptable bandwidth so that levels of inflation can be controlled. Thus, for the near term, most monetarists agree that an increase in money supply can offer a quick-fix boost to a staggering economy in need of increased production. In the long term, however, the effects of monetary policy are still blurry.
Less orthodox monetarists, on the other hand, hold that an expanded money supply will not have any effect on real economic activity (production, employment levels, spending and so forth). But for most monetarists any anti-inflationary policy will stem from the basic concept that there should be a gradual reduction in the money supply. Monetarists believe that instead of governments continually adjusting economic policies (i.e. government spending and taxes), it is better to let non-inflationary policies (i.e. gradual reduction of money supply) lead an economy to full employment.
QTM Re-Experienced
John Maynard Keynes challenged the theory in the 1930s, saying that increases in money supply lead to a decrease in the velocity of circulation and that real income, the flow of money to the factors of production, increased. Therefore, velocity could change in response to changes in money supply. It was conceded by many economists after him that Keynes' idea was accurate.
QTM, as it is rooted in monetarism, was very popular in the 1980s among some major economies such as the United States and Great Britain under Ronald Reagan and Margaret Thatcher respectively. At the time, leaders tried to apply the principles of the theory to economies where money growth targets were set. However, as time went on, many accepted that strict adherence to a controlled money supply was not necessarily the cure-all for economic malaise.
本文翻譯由兄弟財經提供
文章來源：http://www.investopedia.com/articles/05/010705.asp