Brian Bloch
反向投資的基礎是完全可靠的。當所有人都像瘋了一樣買入，價格就會非常高，因此那時將成為最好的賣出時機，反之亦然。有大量關於從重風險和買入最好的時機是在別人賣出時等的理論。然而，像投資行業其他可靠方法一樣，這個方法也有缺點。
反向投資的本質
基本理念是大多數市場參與者通常是錯誤的。大衆會在繁榮時忘乎所以並一旦有麻煩的迹象出現時開始恐慌。因此，他們可能在錯誤的時間買入或者賣出。因此這個吸引人的邏輯就是通過簡單和大衆進行反向操作，可以獲得買入和賣出的正確時間。
方法
根據德國專家Uwe Lang2009年對反向投資人的定期記錄顯示，市場越是強勁和頻繁地展現出買入信號，反向投資人越容易賣出，反之亦然。基本邏輯是行情通報展現主流和主導的市場情緒，已經被當前價格預測出來了。所以最好的做法是與之相反。
缺陷和問題
Uwe Lang指出在20世紀80年代，這種方法十分流行並且人們十分相信它。然而，想以此為基礎持續成功也是衆所週知不可能的。
一個根本問題是如果在所有的行情通報中使用此方法，那麼他們必須與自己的建議相反。這將導致一個惡性循環的混亂和困惑。此外，這種情況是一個典型的合成謬誤，意味著在這種情況下如果一個行情通報是相反的，那麼可以成功，但是如果所有都是相反的，那麼程序將會崩塌。並且在現實的角度看來，怎樣知道何時謬論開始起作用和怎樣知道多少經紀人正在進行這個反向遊戲呢？
何時和為什麼反向邏輯失敗
Uwe Lang說在過去10年中，大衆情緒不再傳達任何可靠的信號，例如在1998年8月14日，在最大經濟危機損失即將發生時，幾乎沒有市場情緒是樂觀的。占主導地位的悲觀情緒給出了一個強烈的買入信號，但是事實上只有一個下降趨勢。
在2000年發生了相同的事件，當時的行情通報和大衆情緒又非常消極，反向投資人希望市場迅速恢複，然而在金融領域卻是呈現出一邊倒的熊市形勢。
為了使事情更複雜，Uwe Lang懷疑2000三月的行情通報和市場情緒是否真的如此悲觀。問題的關鍵是如何確定市場情緒是悲觀還是樂觀？大多數行情通報都小心翼翼地做出一系列陳述，傾向於至少使有利和不利至少在一部分上平衡。
在2007年中期，行情通報宣佈股票市場將不會進一步下跌，恰恰是因為太多投資者感到緊張和銀行不景氣。然而事實上。股票市場的繁榮通常在人們歡欣鼓舞的階段結束。所以再一次，市場情緒是在誤導。
在2007年尾，這個過程又被重複了，因為在8月出現巨大的悲觀情緒。根據反向投資人的觀點，價格因此將會上升，但是七月的高點在年底都沒有達到。200年，金融危機和混亂隨之而來，所以對反向投資人也沒有特別有用的消息。
啟示
在過度興奮的市場中賣出和在過度絕望的市場中買入都不是問題。但是往往是在特殊的情況下反向法才有用。另一方面，常規的週期性起伏不可靠的反映在行情通告表達的市場情緒中。
市場情緒通常還是能為投資決定提供有用的信息，但是它並非靈丹妙藥，不可能讓你以此發家。Uwe Lang說可以學到的基本教訓是心理工具是一個可靠的市場指標。
市場心理在投資管理中是一個非常重要的元素。但是嘗試衡量市場心理並僅僅用它進行基本趨勢預測則是非常愚蠢的。著名的“市場行情”確實存在，投資者可以判斷它是否可靠準確，但是不能僅僅使用大衆心理。
總結
盡管有瑕疵，一定程度的反向思維是非常重要的。市場處於歷史最高點的情況是非常危險的，相反也是如此。在上升市場買進的確是欠考慮的，在下跌時賣出也同樣如此。
然而，特別是在中間時期，大衆心理不是一個指導走向的很好指標。過度地依賴此方法是非常危險的。定期量化大衆心裡作為投資策略的單一基礎是非常不明智的。
Why Contrarian Investing Is Not Very Smart
By Brian Bloch
The basis of contrarian investment is entirely sound. When everyone is buying like mad, prices are too high, so it is in fact best to sell then, and vice versa. There is a huge amount of literature on the dangers of going with the herd, and why the right time to buy is when there is "blood in the streets" and so on. However, like most allegedly sure-fire methods in the investment industry, this one has its flaws too. (For more, see Averaging Down: Good Idea Or Big Mistake?)
The Essence of Contrarian Investing
The underlying idea is that the majority of market participants are generally wrong. The "masses" get carried away in booms and then panic at the first sign of trouble. As a result, they buy and sell at the worst possible times. The appealing logic is therefore that by simply doing the opposite of the masses, one can buy and sell at the right time.
The Methodology
According to German expert Uwe Lang (2009), enthusiastic contrarians regularly note what most market letters recommend. The stronger and more frequent the recommendations to buy, the more likely the contrarians are to sell, and vice versa. The underlying logic is that market letters reflect prevailing and dominant market sentiment, which is already factored into current prices. So the best thing to do is the opposite of what the letters recommend.
The Flaws and the Problems
Uwe Lang points out that in the 1980s, this approach was popular and people really believed in it. In the meantime, however, it is known that consistent success on this basis is unlikely.
A fundamental problem is that if all market letters use this approach, they would actually have to do the opposite of what they themselves had recommended. This would lead to a vicious circle of chaos and confusion. Furthermore, such a situation is in fact a classic fallacy of composition, meaning in this case, that if one market letter is contrarian, that will work, but if most or all are, the process collapses. And in very real terms, how does one know when the fallacy starts to work, and how does one ever know how many brokers are playing this contrarian game?
When and Why Contrarian Logic Failed
For at least 10 years, says Lang, "mass sentiment" has no longer delivered any reliable signals. For example, on August 14, 1998, when the biggest losses from the Asian/Russian crisis were about to occur, very few German market letters were optimistic. The dominant pessimism would have suggested it was a great time to buy. But in fact, there was a downswing in the offering.
The same thing prevailed in March 2000 when the market letters and mass sentiment were again very negative. The associated contrarian hopes of a market recovery were dashed, however, when the real bears out there squeezed the bulls very decisively out of the financial arena.
To make things even more complicated, Lang doubts that the market letters and market sentiment were really so pessimistic in March 2000. And this is a crucial point - how does one determine whether a market letter is actually pessimistic or optimistic? Most such letters are wary of making blanket statements and tend to balance pros at least partly with cons, or the other way round.
In mid-2007, the market letters proclaimed that stock markets could not fall further, precisely because too many investors were nervous and banking on the bears. In reality, however, a stock market boom always ends in a phase of euphoria, and that was lacking in 2007. So, again, the market sentiment was misleading.
At the end of 2007, the process repeated, with considerable pessimism prevailing in August. According to the contrarian view, therefore, prices should have risen, but the highs of July were not achieved again by the end of the year. In 2008, the financial crisis struck and chaos ensued, so that there were was also no particularly useful message for the contrarians.
The Implications
The existence of excessively euphoric markets, which are best avoided, and deep despair, which is a great time to buy, is not in question. But these tend to be exceptional situations in which contrarian approaches really do work well. On the other hand, the more routine cyclical ups and downs are not reliably reflected in market sentiment as expressed in market letters.
Market sentiment generally still provides useful input to investment decisions, but it is not a panacea that leads to the proverbial pot of gold. The fundamental lesson to be learned, says Uwe Lang, is that no "psycho instrument" is a really reliable stock market indicator.
Market psychology remains an important element in managing investments. But trying to measure the psychology on an ongoing basis, and using it in isolation to make fundamental trend predictions, is a fool's paradise. The famous "state of the market" does indeed exist and one can determine it with reasonable accuracy, but not with mass psychology alone.
The Bottom Line
Despite the flaws, a degree of contrarianism is important. Situations where markets are at all-time highs are very risky and so too are the converse. Buying into euphoric markets is truly ill-advised, and so too is selling out in situations of panic when prices are plummeting.
However, particularly at in-between times, mass psychology is not a terribly good indicator of where stock markets are heading. Excessive reliance on the approach is therefore dangerous. One is going too far in regularly quantifying mass sentiment as an exclusive basis for investment strategies.
本文翻譯由兄弟財經提供
文章來源：http://www.investopedia.com/articles/trading/11/problems-with-contrarian-investing.asp