Reem Heakal
货币数量理论（QTM）的概念起始于16世纪。当时黄金和白银从美洲流入欧洲并被制造成硬币，通胀开始上升。这导致经济学家Henry Thornton在1082年认为更多的货币等于更大的通货膨胀，也就是更多的货币供应并不意味着经济产出的增加。我们在这里将讨论这个假设和潜在QTM的计算以及它与货币主义理论的关系和理论受到的挑战。
QTM概括
货币数量理论（QTM）指出经济体中的货币数量与货物或服务价格有直接的关系。货币数量理论认为如果一个经济体中的货币数量加倍，价格也会加倍，引起通货膨胀。因此消费者将需要为之前相同的产品和服务支付双倍的价钱。
理解这一理论的另一方式是把货币当成其他任意一种商品，增加其供应将会降低其边际价值（一个单位货币的购买力）。因此货币的增加将引起价格（通货膨胀）上涨，因为减少的边际价值需要得到弥补。
该理论的计算
以最简单的方式该理论被表示为：
MV = PT（费雪方程式）
每个变量意义如下：
M：货币供应
V：货币流通速度（货币转手的次数）
P：平均价格水平
T：交易的商品和服务数量
理论被17世纪正统经济学家所接受并被20世纪经济学家费雪和米尔顿•弗里德曼检查并制定上述方程式。
上面的方程式基础是下面交易方程式：
资金数量x流通速度=总消费
也就是说如果一个经济体有3美元，并且这3美元在一个月内被使用5次，那么这个月的总消费就是15美元。
QTM假设
QTM为交易方程式添加假设。在其最基本的形式中，该理论假设V（流通速度）和T（交易数量）在短期内是不变的。然而这些假设受到了大量指责，尤其是假设V不变。争论的焦点是流通速度取决于消费者和企业的消费冲动，不可能保持不变。
该理论还假设由外部因素决定的货币数量是一个社会经济活动的主要影响。货币供应的变化导致价格水平和产品和服务供应的变化。根本上，货币储量的变化会引起消费支出的变化。而流通速度不取决于可用的货币数量或者当前的价格水平，而是价格水平的变化。
最后，交易次数（T）是由劳动力、资本、自然资源、知识和组织决定的。这个理论假设的是经济平衡和充分就业下的情况。
从本质上来说，这个理论的假设表明货币的价值取决于经济体中可用货币的数量。货币供应增加导致货币价值下降是因为货币供应增加导致通货膨胀的上升。随着通货膨胀上升，购买力或者货币价值就会下降。因此需要花费更多的资金购买相同数量的商品或者服务。
货币供应、通货膨胀和货币主义
正是因为QTM认为货币数量决定货币价值，它成为了货币主义的基石。
货币主义认为货币供应的快速增加导致通货膨胀的急剧增加。货币增加速度超过经济产出增长速度导致通货膨胀，因为货币将远远多余商品和服务的产量。为了抑制通货膨胀，货币增长必须低于经济产出的增长。
这个前提决定货币政策是如何被管理的。货币主义者认为货币供应应该被控制在一个可接受的范围内，这样可以控制通胀水平。因此大多数货币主义者认为短期内货币供应的增加可以推动急速增长经济体对产量增加的要求。但是货币政策这种影响的长期效果还不明显。
相反，不那么正统的货币主义者认为货币供应增加不会对实际经济活动产生影响。但是大多数货币主义者都认为任何抵制通货膨胀的政策都应从减少货币供应入手。大多数货币主义者认为政府与其不断调整经济政策还不如让非通胀政策引导经济体进行充分就业。
John Maynard Keynes（凯恩斯）在20世纪30年对货币质量理论提出质疑，他认为增加货币供应将导致流通次数和实际收入的减少，而货币将流入生产环节。因此货币流通将会随着货币供应变化。许多后世经济学家认为他的观点是正确的。
因为其植根于货币主义，QTM在20世纪80年代受到许多主要经济体的欢迎，例如在里根执政时期的美国和撒切尔执政时期的英国。当时，这些领导人试图利用这一理论的原理设定货币增长目标。然而随着时间的推移，许多人开始发现严格的货币控制未必是治疗经济衰退的灵丹妙药。
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.
本文翻译由兄弟财经提供
文章来源：http://www.investopedia.com/articles/05/010705.asp