N. 格里高利•曼昆（N. Gregory Mankiw）， 哈佛大学经济学教授，曾在普林斯顿大学和麻省理工学院学习经济学。他讲授过宏观经济学、微观经济学、统计学和经济学原理等课程。曼昆教授还是美国国家经济研究局（NBER）的合作研究人员，国会预算办公室、波士顿和纽约联邦储备银行的顾问，以及美国教育考试服务中心（ETS）的经济学先修课程考试研发委员会成员。2003—2005年，他曾担任美国总统经济顾问委员会的主席。
Efficient Market Hypothesis:
There's a joke on this subject: An economist walks by a twenty dollar bill on the sidewalk but decides not to p...
有效市场假说Efficient Market Hypothesis: 认为资产价格反映了关于一种资产价值的所有公开的、可获得的信息的理论。如果价格反映了所有可获得的信息，就没有一种股票是比其他任何股票更好的购买选择。你最好的选择就是购买多元化的有价证券组合。————————————————反方观点：There's a joke on this subject: An economist walks by a twenty dollar bill on the sidewalk but decides not to pick it up, because if it were really there someone would have picked it up already.Horrible jokes aside, the EMH doesn't hold up because it relies on some major assumptions:1. Perfect Information. Enron, WorldCom, and Lehman Brothers would not have been $60b+ overnight blowups if information about those organizations was transparent and efficient. Perfect information assumes both complete information and perfect ability to process all of it. Information is always incomplete, often asymmetric, and too abundant to ever be able to interpret perfectly. 2. Rational expectations. The other day I saw a sell-side trader drop a sandwich on the sidewalk. Half of it landed in a puddle. He proceeded to pick it up and blow on it, presumably to clean it, then took a bite out of it. People are not rational.3. Aligned incentives. The EMH assumes everyone is seeking the highest risk adjusted returns. In my experience there are a shockingly high number of market participants whose incentives are not aligned with this goal:"Too Big to Fail" Institutions. If they make money in the market, they keep the gains. If they lose, the taxpayers foot the bill. The incentive structure is so egregiously misaligned that they have (and will again) lead to horrible market inefficiencies.Proprietary traders. Each hotshot with a Bloomberg terminal is gambling on the house's money, not their own. If they generate great returns they get huge bonuses, but if they lose big they just work at another bank. The 'rogue' trader at UBS that lost $2.3bn had been doing it for three years. Here's another bad joke that boils down this incentive structure: what do you call a rogue trader who succeeds? A managing director.Institutional investors. Many institutional investors are pack oriented because their main incentive is to not get fired. Most are salaried and have very little opportunity for bonus. If they generate returns in line with their peers they will keep their jobs, even if returns are terrible. If they stick their neck out in order to seek above average returns they risk everything and have very little to gain.4. Same Personal Goals. Again, the EMH assumes every market participant is acting to produce the best risk adjusted return. In reality there are many different risk and return objectives. A 60 year old investor saving for retirement will have vastly different needs and risk tolerances than an endowment with an infinite time horizon. Every investor comes to the table with a different reason why they push the 'buy' and 'sell' buttons. Given this reality the market price cannot be considered 'efficient', it is merely the average of the thousands of conflicting needs of its underlying investors.5. No friction (ie: no trading costs). There are obviously trading costs, so even if the above imbalances were somehow corrected, this would prevent perfect efficiency.https://www.quora.com/What-are-the-best-arguments-against-efficient-market-theory