不可能三角保匯率

1. 蒙代爾不可能三角

在現代金融理論中有一個非常著名的定理，即蒙代爾不可能三角(Impossible triangle)。一個國家不可能同時實現資本流動自由，貨幣政策的獨立性和匯率的穩定性。也就是說，一個國家只能擁有其中兩項，而不能同時擁有三項。如果一個國家想允許資本流動，又要求擁有獨立的貨幣政策，那麼就難以保持匯率穩定。如果要求匯率穩定和資本流動，就必須放棄獨立的貨幣政策。

在這三個目標之間究竟哪一個更為重要？如果放棄資本流動，將退回到閉關鎖國的封閉經濟體系，不利於經濟增長。如果放棄匯率穩定性，對於任何國家來說都是一場災難。「兩害相較取其輕」，只有放棄獨立的貨幣政策比較可行。放棄獨立的貨幣政策也就是向單一的區域貨幣或世界貨幣過渡。於是，人們在貿易全球化的同時開始越來越多地考慮金融全球化，如果貨幣統一了，就不存在固定匯率、浮動匯率和資本管制的各種弊病了。

斯蒂格里茨曾經作過一個比喻：「小型開放經濟就如同在狂風大浪中的小船一樣，不論駕駛船的技術怎麼樣，雖然我們不知道船在什麼時候被傾覆，但是，毫無疑問，船最終會被大浪打翻。」在貨幣市場上可以非常明顯地觀察到經濟規模對穩定性的重要影響。經濟規模越大，投機炒作的風險就越小。統一貨幣的最大的好處，就是讓度部分貨幣主權，換取金融體制的穩定，能夠防範和化解金融危機。

某個經濟地區實現了單一貨幣，還可以提高貨幣體系的透明性。由於統一貨幣區內各國都放棄了獨立的貨幣政策，不存在某個國家突然增發貨幣的可能性，從而大大提高了民眾對貨幣政策的信任度。統一貨幣能夠進一步發揮市場機制的作用，節約信息成本和交易成本，促進商品、資本、人員的流動，使得資源配置更加合理化。統一貨幣可以減少內部磨擦，促進投資，提高國際競爭力。

2. 蒙代爾不可能三角和特里芬之謎的關系

美聯儲若不大量發行美元，就無法彌補美國龐大的貿易赤字和對外投資；但如果大量發行，各國就會失去對這種貨幣的信任。這就是所謂的「特里芬之謎」。
而布雷頓森林體系的缺陷在於：
1、美元的清償能力和對美元的信心構成矛盾，表現為美元的國際貨幣儲備地位和國際清償力的矛盾、儲備貨幣發行國與非儲備貨幣發行國之間政策協調的不對稱性以及固定匯率制下內外部目標之間的兩難選擇等；
2、匯率體制僵硬，無法通過匯率浮動自動實現國際收支平衡，調節國際收支失衡的責任主要落在非儲備貨幣發行國一方，犧牲了它們的經濟發展目標。
綜上我們其實可以得出，布雷頓森林體系的確是在盡其所能實現不可能三角，但是並不是因為這樣才出現了特里芬之謎，而是因為特里芬之謎——這樣一個矛盾的存在使布雷頓森林體系的推進困難重重，最後導致崩潰，因此其因果關系應該是特里芬之謎導致布雷頓森林體系的崩潰，但是特里芬之謎並不是由布雷頓森林體系產生的。
如果還有疑問可以閱讀以下相關材料：
http://ke..com/view/1648635.htm?fr=ala0_1
http://ke..com/view/2050241.htm
http://hi..com/alvin_lian/blog/item/b1fe57906a163d8ba877a47f.html

3. 克魯格曼不可能三角的中國貨幣政策

美國財長斯諾訪華也傳達了同樣的觀點。他在接受采訪時說，中國要擴大對外開放，從中國自身的利益角度出發，應該實施由市場供求決定的靈活的匯率制度。政府不要干預過多，否則不可能存在比較獨立的貨幣政策。關於央行的此次緊縮銀根的行動，國內有看法認為，央行最終選擇提高准備金率來控制過快的國內信貸增長，這種選擇顯然受到我國盯住美元的匯率制度的約束，匯率穩定優先的考慮使得央行在一定程度上喪失了貨幣政策的獨立性。
事實上，目前中國貨幣政策並不具有完全的獨立性，目前我國M並不等於1。貨幣政策通過利率影響生產和消費的渠道並不暢通，沒有形成市場供求決定意義上的利率。按照「先利率，後匯率」的開放原則，利率的市場化比匯率放開更緊要。因此在中國建立一個有效的貨幣政策體系前，必須保持匯率穩定，以減少風險和波動。正如美國經濟學家羅伯特 蒙代爾所言，中國貨幣政策最大的成就就是保持了外匯穩定。外匯穩定已成為中國實施貨幣政策的重要基礎和指導方針。中國應當堅持穩定的匯率制度。如果人民幣匯率出現大幅度波動，將給中國經濟帶來許多不穩定因素，並很可能使中國重蹈20世紀七八十年代日本經濟衰退的覆轍。

4. 蒙代爾的三元悖論或不可能三角英文是什麼

三元悖論，也稱三難選擇，它是由美國經濟學家保羅·克魯格曼就開放經濟下的政策選擇問題所提出的，其含義是：本國貨幣政策的獨立性，匯率的穩定性，資本的完全流動性不能同時實現，最多隻能同時滿足兩個目標，而放棄另外一個目標。根據蒙代爾的三元悖論，一國的經濟目標有三種：1、各國貨幣政策的獨立性；2、匯率的穩定性；3、資本的完全流動性。 這三者，一國只能三選其二，而不可能三者兼得。例如，在1944年至1973年的「布雷頓森林體系」中，各國「貨幣政策的獨立性」和「匯率的穩定性」得到實現，但「資本流動」受到嚴格限制。而1973年以後，「貨幣政策獨立性」和「資本自由流動」得以實現，但「匯率穩定」不復存在。「永恆的三角形」的妙處，在於它提供了一個一目瞭然地劃分國際經濟體系各形態的方法。

根據三元悖論，在資本流動，貨幣政策的有效性和匯率制度三者之間只能進行以下三種選擇：

(1)保持本國貨幣政策的獨立性和資本的完全流動性，必須犧牲匯率的穩定性，實行浮動匯率制。這是由於在資本完全流動條件下，頻繁出入的國內外資金帶來了國際收支狀況的不穩定，如果本國的貨幣當局部進行干預，亦即保持貨幣政策的獨立性，那麼本幣匯率必然會隨著資金供求的變化而頻繁的波動。利用匯率調節將匯率調整到真實反映經濟現實的水平，可以改善進出口收支，影響國際資本流動。雖然匯率調節本身具有缺陷，但實行匯率浮動確實較好的解決了「三難選擇」。但對於發生金融危機的國家來說，特別是發展中國家，信心危機的存在會大大削弱匯率調節的作用，甚至起到惡化危機的作用。當匯率調節不能奏效時，為了穩定局勢，政府的最後選擇是實行資本管制。
(2)保持本國貨幣政策的獨立性和匯率穩定，必須犧牲資本的完全流動性，實行資本管制。在金融危機的嚴重沖擊下，在匯率貶值無效的情況下，唯一的選擇是實行資本管制，實際上是政府以犧牲資本的完全流動性來維護匯率的穩定性和貨幣政策的獨立性。大多數經濟不發達的國家，比如中國，就是實行的這種政策組合。這一方面是由於這些國家需要相對穩定的匯率制度來維護對外經濟的穩定，另一方面是由於他們的監管能力較弱，無法對自由流動的資本進行有效的管理。

(3)維持資本的完全流動性和匯率的穩定性，必須放棄本國貨幣政策的獨立性。根據蒙代爾-弗萊明模型，資本完全流動時，在固定匯率制度下，本國貨幣政策的任何變動都將被所引致的資本流動的變化而抵消其效果，本國貨幣喪失自主性。在這種情況下，本國或者參加貨幣聯盟，或者更為嚴格地實行貨幣局制度，基本上很難根據本國經濟情況來實施獨立的貨幣政策對經濟進行調整，最多是在發生投機沖擊時，短期內被動地調整本國利率以維護固定匯率。可見，為實現資本的完全流動與匯率的穩定，本國經濟將會付出放棄貨幣政策的巨大代價。

The Mundell-Fleming model is an economic model first set forth by Robert Mundell and Marcus Fleming. The model is an extension of the IS-LM model. Whereas IS-LM deals with economy under autarky, the Mundell-Fleming model tries to describe a small open economy.

Typically, the Mundell-Fleming model portrays the relationship between the nominal exchange rate and the economy output (unlike the relationship between interest rate and the output in the IS-LM model) in the short run. The Mundell-Fleming model has been used to argue that an economy cannot simultaneously maintain a fixed exchange rate, free capital movement, and an independent monetary policy. This principle is frequently called "the Unholy Trinity," the "Irreconcilable Trinity," the "Inconsistent trinity" or the Mundell-Fleming "trilemma."

Under flexible exchange rate regime

We speak of a system of flexible exchange rates when governments (or central banks) allow the exchange rate to be determined by market forces alone.

Typically this means governments (or central banks) announce an interest rate target which they are prepared to buy or sell any amount of domestic currency to maintain.

[edit] Changes in money supply

An increase in money supply will shift the LM curve to the right. This directly reces the local interest rate and in turn forces the local interest rate lower than the global interest rate. This depreciates the exchange rate of local currency through capital outflow. The depreciation makes local goods cheaper compared to foreign goods and increases export and decreases import. Hence, net export is increased. Increased net export leads to the shifting of the IS curve to the right to the point where the local interest rate will equalize with the global rate. This increases the overall income in the local economy.

A decrease in money supply will cause the exact opposite of the process.

[edit] Changes in government spending

An increase in government expenditure shifts the IS curve to the right. The shift will cause the local interest rate to go above the global rate. The increase in local interest will cause capital inflow and the inflow will make the local currency stronger compared to foreign currencies. Strong exchange rate also makes foreign goods cheaper compared to local goods. This encourages greater import and discourages export and hence, lower net export. As a result, the IS will return to its original location where the local interest rate is equal to the global interest rate. The level of income of the local economy stays the same. The LM curve is not at all affected.

A decrease in government expenditure will reverse the process.

[edit] Changes in global interest rate

An increase in the global interest rate will cause an upward pressure on the local interest rate. The pressure will subside as the local rate closes in on the global rate. When a positive differential between the global and the local rate occurs, holding the LMB curve constant, capital will flow out of the local economy. This depreciates the local currency and helps boost net export. Increasing net export shifts the IS to the right. This shift will continue to the right until the local interest rate becomes as high as the global rate.

A decrease in global interest rate will cause the reverse to occur.

[edit] Under fixed exchange rate regime

We speak of a system of fixed exchange rates when governments (or central banks) announce an exchange rate (the parity rate) at which they are prepared to buy or sell any amount of domestic currency.

[edit] Changes in money supply

Under the fixed exchange rate system, the local central bank or any monetary authority will only change the money supply in order to maintain a level of exchange rate. If there is a pressure to appreciate the exchange rate, the local authority will increase its foreign reserve by purchasing foreign currencies with the local currency. This will return the exchange rate to its previous level. When there is a depreciation pressure, the authority will purchase its own currency in the market with its foreign reserve to return the currency to its pre-pressure state.

A revaluation occurs when there is a permanent increase in exchange rate and hence, decrease in money supply. Devaluation is the exact opposite of revaluation.

[edit] Changes in government expenditure

Increased government expenditure shifts the IS curve to the right. The shift encourages the interest rate to go up and hence, appreciation of the exchange rate. However, the exchange rate is controlled by the local monetary authority in the framework of a fixed system. In order to maintain the exchange and alleviate the pressure on the exchange rate, the monetary authority will purchase foreign currencies with local currencies until the pressure is gone i.e. back to the original level. Such action shifts the LM curve in tandem with the direction of the IS shift. This action increases the local currency supply in the market and lowers the exchange rate — or rather, return the rate back to its original state. In the end, the exchange rate stays the same but the general income in the economy increases.

The reverse is true when government expenditure decreases.

[edit] Changes in global interest rate

To maintain the fixed exchange rate, the central bank must offset the capital flows (in or out) which are caused by the change of the global interest rate to the domestic rate. The central bank must restore the situation where the real domestic interest rate is equal to the real global interest rate to stop net capital flows from changing the exchange rate.

If the global interest rate increases above the domestic rate, capital will flow out to take advantage of this opportunity. This would depreciate the home currency, so the central bank must buy the home currency and sell foreign currency reserves to offset this outflow. This decrease in the money supply shifts the LM curve to the left until the domestic interest rate is the global interest rate.

If the global interest rate declines below the domestic rate, the opposite occurs. Money flows in, the home currency would appreciate, so the central bank must offset this by increasing the money supply (sell domestic currency, buy foreign), the LM curve shifts right, and the domestic interest rate becomes the global interest rate.

[edit] Differences from IS-LM

It is worth noting that some of the result from this model differs from the IS-LM because of the open economy assumption. Result for large open economy on the other hand falls within the result predicted by the IS-LM and the Mundell-Fleming models. The reason for such result is because a large open economy has both the characteristics of an autarky and a small open economy.

In the IS-LM, interest rate will be the key component in making both the money market and the good market in equilibrium. Under the Mundell-Fleming framework of small economy, interest rate is fixed and equilibrium in both market can only be achieved by a change of nominal exchange rate.

[edit] Example

A much simplified version of the Mundell-Fleming model can be illustrated by a small open economy, in which the domestic interest rate is exogenously predetermined by the world interest rate (r=r*).

Consider an exogenous increase in government expenditure, the IS curve will shift upward, with LM curve intact, causing the interest rate and the output to rise (partial crowding out effect) under the IS-LM model.

Nevertheless, as interest rate is predetermined in a small open economy, the LM* curve (of exchange rate and output) is vertical, which means there is exactly one output that can make the money market in the equilibrium under that interest rate. Even though the IS* curve still shift up, it will result in a higher exchange rate and same level of output (complete crowding out effect, which is different in the IS-LM model).

The example above makes an implicit assumption of flexible exchange rate. The Mundell-Fleming model can have completely different implications under different exchange rate regimes. For instance, under a fixed exchange rate system, with perfect capital mobility, monetary policy becomes ineffective. An expansionary monetary policy resulting in an outward shift of the LM curve would in turn make capital flow out of the economy. The central bank under a fixed exchange rate system would have to intervene by selling foreign money in exchange for domestic money to depreciate the foreign currency and appreciate the domestic currency. Selling foreign money and receiving domestic money would rece real balances in the economy, until the LM curve shifts back to the left, and the interest rates come back to the world rate of interest i*.

5. 請問：國際經濟學的世界貨幣體系中的「不可能三角」指什麼有沒有「不可能三角」不成立的時候

克魯格曼不可能三角克魯格曼不可能三角
關於匯率政策與貨幣政策的協調問題，在20世紀60年代初，弗萊明和蒙代爾認為：資本的自由流動、貨幣完全獨立、匯率穩定三項目標中，一國政府最多隻能同時實現兩項。這一結論被後人稱為「蒙代爾三角」。克魯格曼在199B年初發表的一篇討論亞洲金融危機的文章中，也談到「蒙代爾三角」的問題，他稱之為「The eternal triangle」(永恆的三角形)；國內學者易綱總結為「蒙代爾一克魯格曼不可能三角形」，並且提出X+Y+M=2定理。
美國財長斯諾訪華也傳達了同樣的觀點。他在接受采訪時說，中國要擴大對外開放，從中國自身的利益角度出發，應該實施由市場供求決定的靈活的匯率制度。政府不要干預過多，否則不可能存在比較獨立的貨幣政策。關於央行的此次緊縮銀根的行動，國內有看法認為，央行最終選擇提高准備金率來控制過快的國內信貸增長，這種選擇顯然受到我國盯住美元的匯率制度的約束，匯率穩定優先的考慮使得央行在一定程度上喪失了貨幣政策的獨立性。
事實上，目前我國貨幣政策並不具有完全的獨立性，目前我國M並不等於1。貨幣政策通過利率影響生產和消費的渠道並不暢通，沒有形成市場供求決定意義上的利率。按照「先利率，後匯率」的開放原則，利率的市場化比匯率放開更緊要。因此在我國建立一個有效的貨幣政策體系前，必須保持匯率穩定，以減少風險和波動。正如美國經濟學家羅伯特 蒙代爾所言，中國貨幣政策最大的成就就是保持了外匯穩定。外匯穩定已成為中國實施貨幣政策的重要基礎和指導方針。中國應當堅持穩定的匯率制度。如果人民幣匯率出現大幅度波動，將給中國經濟帶來許多不穩定因素，並很可能使中國重蹈20世紀七八十年代日本經濟衰退的覆轍。