凱恩斯的宏觀經濟學：概述

凱恩斯的宏觀經濟學：概述

蔣聞銘

 

如果用通俗的語言來表達，John Maynard Keynes的經濟理論其實相當直觀，普通人也很容易理解。

他觀察到，一個社會中人們的收入可以分為兩部分：一部分用於消費（CONSUMPTION），另一部分則被儲蓄（SAVING）下來。通過銀行體係、股票市場以及其他各種投資渠道，儲蓄部分會被完全返送回經濟體係之中，轉化為新的投資資本。

因此可以得到一個基本關係：

儲蓄 = 投資（SAVING = INVESTMENT）

他接下來的理論，大致分成了三部分

 

第一部分：投資乘數（investment multiplier）。

既然儲蓄等於投資，那麽用T代表總收入，

T=總收入=消費+儲蓄=(m(T)+1)⋅儲蓄=(m(T)+1)⋅投資

因此，他認為，經濟的繁榮程度（即總收入）取決於投資（INVESTMENT）的規模：投資增加，收入隨之增加；投資減少，收入也隨之下降。

他還指出，m(T) 是 T的一個遞減函數。而

k(T)=1+m(T)  

則被稱為他的投資乘數（investment multiplier）。

 

第二部分：投資的誘因（Inducement of Investments）

他理論的第二個組成部分，是關於什麽因素促使投資發生。

他認為，投資者在決策時會估計某項投資在未來可能獲得的收益率，並將其與貨幣利率（money-interest rate）進行比較。隻有當他們預期的收益率——也就是所謂的 “資本邊際效率” （the marginal efficiency of capital）——高於利率時，才會進行投資。

按照這一理論，解決失業問題有兩種途徑：一是降低利率，二是提高資本邊際效率。

利率可以由貨幣當局加以調控，但這種調控是有限度的，因為利率必須保持為正值。而資本邊際效率，在John Maynard Keynes看來，具有一定的心理性質，因此可以通過政府的赤字支出（deficit spending）來加以刺激。

在經濟極度蕭條的時期，例如Great Depression，當資本邊際效率接近於零時，往往需要同時采取這兩種手段，才能緩解失業這一嚴重問題。

 

第三部分：利率與貨幣 (Interest rate and Money)

要使上述分析成立，一個至關重要的前提是：企業家所預期的投資回報率與貨幣利率必須是相互獨立的。這便引出了該理論的第三個組成部分——利率與貨幣理論。按照John Maynard Keynes的觀點，貨幣具有三項使其獨特的性質：

(a) 貨幣並非普通商品，因為它不能通過勞動生產出來；相反，貨幣當局可以幾乎不花成本地，隨意將其投放到流通中或從流通中抽回。
(b) 貨幣沒有持有成本。
(c) 貨幣可以立即用於購買任何其他商品（即貨幣的流動性）。

凱恩斯認為，第(c)項體現了貨幣利率所代表的客觀價值；而由(b)與(c)兩項可知，貨幣利率必須為正。至於(a)項則更為微妙，它意味著利率可以被貨幣當局所操控。貨幣當局可以通過調節流通中的貨幣總量來影響利率：例如，當流通中的貨幣總量減少時，第(c)項所體現的客觀價值便會上升，反之亦然。這便是他的利率與貨幣理論。

 

An Overview:

The gist of Keynes's economic theory, if spelled in plain English, is one that a layman could easily understand.

He observed that population's income is divided into two parts: the part spent for CONSUMPTION and the part that is SAVED. Through banking system, stock market and other investment avenue, the saved part is completely recirculated back into the economic system as new capital. Hence

SAVING = INVESTMENT. 

(Investment Multiplier) The first component of his theory is on the investment multiplier. He argued that the ratio of consumption vs saving depends mainly on the size of the total income. This is to say that let m = COMSUMPTION / SAVING, and T be the total income, then m = m(T).  Because

SAVING = INVESTMENT,

we have

T = Consumption + Saving = (m(T) + 1) Saving = (m(T) + 1) Investment.

Hence, he claimed, that prosperity (total income) depends on the size of INVESTMENT. It grows and drops as INVESTMENT grows and drops. He also argued that m(T) is a decreasing function of T.  k(T) = 1 + m(T) is his investment multiplier.

(Inducement of Investments) The second component of his theory is on what induces investment. He reckoned that certain calculations are made by investors: they would estimate the rate of return they are likely to acquire in the future for their intended investment and compare it with the money-interest rate. He will invest only if he expects a return rate (which is called under the name ``the marginal efficiency of capital") that is higher than the money-interest rate.

According to this theory then, there are two ways one could address the issue of unemployment: to reduce the interest rate and to raise the marginal efficiency of capital. The interest rate can be manipulated by monetary authority, though this is not without its limit, for the interest rate will have to stay positive. The marginal efficiency of capital, according to Keynes, is partly psychological therefore can hopefully be stimulated by deficit spending of the government. In particularly bad times, like that of the great depression, in which the marginal efficiency of capital was close to zero, one would need both to alleviate the evil of unemployment.

(Interest rate and Money) For the above to make sense it is critically important that the expected return rate of the entrepreneur and the money-interest rate are independent. This then led to the third component of this theory: the theory of interest rate and money. Money, according to Keynes, has three properties that make it unique.

(a) Money is not an ordinary commodity in the sense that it could not be produced by labor. It, on the other hand, can be extracted from and put back into circulation by the monetary authority at will with virtually no cost.

(b) Money has no carrying cost.

(c) Money can be used instantly to buy any of the other commodity. (Liquidity of money).

Keynes argued that item (c) is the objective value represented by the money interest rate, and by item (b) and (c) money-interest must be positive. Item (a) is tricky, it implies that interest rate can be manipulated by monetary authority. The monetary authority can adjust the total quantity of money in circulation to manipulate the interest rate: observe that if total quantity of money in circulation is reduced, then the objective value of (c) would increase and vice versa. This is his theory on interest rate and money.