Brian Bloch
反向投资的基础是完全可靠的。当所有人都像疯了一样买入，价格就会非常高，因此那时将成为最好的卖出时机，反之亦然。有大量关于从重风险和买入最好的时机是在别人卖出时等的理论。然而，像投资行业其他可靠方法一样，这个方法也有缺点。
反向投资的本质
基本理念是大多数市场参与者通常是错误的。大众会在繁荣时忘乎所以并一旦有麻烦的迹象出现时开始恐慌。因此，他们可能在错误的时间买入或者卖出。因此这个吸引人的逻辑就是通过简单和大众进行反向操作，可以获得买入和卖出的正确时间。
方法
根据德国专家Uwe Lang2009年对反向投资人的定期记录显示，市场越是强劲和频繁地展现出买入信号，反向投资人越容易卖出，反之亦然。基本逻辑是行情通报展现主流和主导的市场情绪，已经被当前价格预测出来了。所以最好的做法是与之相反。
缺陷和问题
Uwe Lang指出在20世纪80年代，这种方法十分流行并且人们十分相信它。然而，想以此为基础持续成功也是众所周知不可能的。
一个根本问题是如果在所有的行情通报中使用此方法，那么他们必须与自己的建议相反。这将导致一个恶性循环的混乱和困惑。此外，这种情况是一个典型的合成谬误，意味着在这种情况下如果一个行情通报是相反的，那么可以成功，但是如果所有都是相反的，那么程序将会崩塌。并且在现实的角度看来，怎样知道何时谬论开始起作用和怎样知道多少经纪人正在进行这个反向游戏呢？
何时和为什么反向逻辑失败
Uwe Lang说在过去10年中，大众情绪不再传达任何可靠的信号，例如在1998年8月14日，在最大经济危机损失即将发生时，几乎没有市场情绪是乐观的。占主导地位的悲观情绪给出了一个强烈的买入信号，但是事实上只有一个下降趋势。
在2000年发生了相同的事件，当时的行情通报和大众情绪又非常消极，反向投资人希望市场迅速恢复，然而在金融领域却是呈现出一边倒的熊市形势。
为了使事情更复杂，Uwe Lang怀疑2000三月的行情通报和市场情绪是否真的如此悲观。问题的关键是如何确定市场情绪是悲观还是乐观？大多数行情通报都小心翼翼地做出一系列陈述，倾向于至少使有利和不利至少在一部分上平衡。
在2007年中期，行情通报宣布股票市场将不会进一步下跌，恰恰是因为太多投资者感到紧张和银行不景气。然而事实上。股票市场的繁荣通常在人们欢欣鼓舞的阶段结束。所以再一次，市场情绪是在误导。
在2007年尾，这个过程又被重复了，因为在8月出现巨大的悲观情绪。根据反向投资人的观点，价格因此将会上升，但是七月的高点在年底都没有达到。200年，金融危机和混乱随之而来，所以对反向投资人也没有特别有用的消息。
启示
在过度兴奋的市场中卖出和在过度绝望的市场中买入都不是问题。但是往往是在特殊的情况下反向法才有用。另一方面，常规的周期性起伏不可靠的反映在行情通告表达的市场情绪中。
市场情绪通常还是能为投资决定提供有用的信息，但是它并非灵丹妙药，不可能让你以此发家。Uwe Lang说可以学到的基本教训是心理工具是一个可靠的市场指标。
市场心理在投资管理中是一个非常重要的元素。但是尝试衡量市场心理并仅仅用它进行基本趋势预测则是非常愚蠢的。著名的“市场行情”确实存在，投资者可以判断它是否可靠准确，但是不能仅仅使用大众心理。
总结
尽管有瑕疵，一定程度的反向思维是非常重要的。市场处于历史最高点的情况是非常危险的，相反也是如此。在上升市场买进的确是欠考虑的，在下跌时卖出也同样如此。
然而，特别是在中间时期，大众心理不是一个指导走向的很好指标。过度地依赖此方法是非常危险的。定期量化大众心里作为投资策略的单一基础是非常不明智的。
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.
本文翻译由兄弟财经提供
文章来源：http://www.investopedia.com/articles/trading/11/problems-with-contrarian-investing.asp