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Economic bubbles: Causes and conditions
经济泡沫：成因与条件
Economic bubbles occur when, for any number of reasons, excessive investment in commodities (such as oil), securities (such as stocks and bonds), real estate, or collectibles drives up prices well beyond the item's intrinsic value. The inevitable result of this boom in price is a crash or bust. The price falls sharply once it becomes clear that it has grown far beyond the purchasing power of potential customers.
不管因为何种原因，一旦人们对于商品（如石油）、证券（如股票、债券）、房地产或收藏品过度投资从而推高其价格，使其远远超过商品的内在价值，经济泡沫就会产生。而这种价格暴涨不可避免的结果就是经济的崩溃或破灭。一旦价格大大超过潜在消费者购买力的趋势明了，价格就会急剧下跌。
Speculators risk money in such investments because they hope that the price of an asset they purchased will quickly increase. Since most speculators are nervous about where they invest their money, bubbles are by no means the norm. After all, every investment entails the risk that it is overpriced. They also know that rising prices will encourage either greater production of a commodity or greater willingness of current owners to sell. Either of these conditions can serve as a "negative feedback" mechanism that adjusts prices downward. As an analogy, think of negative economic feedback like your eyes. As the light gets brighter, your pupils get smaller and let in less light. But what if, instead, your eyes worked as a "positive feedback" mechanism? In sunlight, your pupils would open wide and damage your eyes.
投机者们因为希望购买的资产价格能够急剧上涨才进行这样的风险投资。由于大多数的投机者对资金的投向都有所顾虑，因此泡沫的产生绝非常态。毕竟每笔投资都包含估价过高的风险。他们也知道价格上涨要么会推动商品产量的进一步扩大，要么促使现有的持有者更愿意卖出。不管哪种情况都会有助于形成促使价格下行的“负面反馈”机制。打个比方，把负面经济反馈比作人的眼睛，光线越强，瞳孔越小，摄入的光亮就会越少。但要是人的眼睛发挥的是“正面反馈”机制作用，结果会怎样呢？在太阳光下，瞳孔就会张大，从而对眼睛造成伤害。
Economic bubbles occur when prices trending sharply upward spur positive, rather than negative, feedback. For whatever reason (fear of shortages, greed, an excessively optimistic attitude toward the future, or flaws in the analysis of an asset's underlying value), buyers believe that the value of the asset will continue to rise. If the price rises, overly enthusiastic speculators buy more, or those who missed out on the lower price flock to buy before the price rises any higher. The foremost explanation is the "greater fool theory": Buyers justify their purchases by assuring themselves they will find "a greater fool" who will pay even more. Buyer enthusiasm infects other buyers, amplifying the effect even further. Under the right conditions, prices can reach dizzying heights before falling. One famous example is the tulip-buying bubble which happened in Amsterdam in the 1630s when a single tulip bulb could cost a year's salary.
如果价格急剧上行引起的是正面而非负面反馈，就会产生经济泡沫。因为担心供应不足，因为贪心，或是对其未来过分乐观，或是因为对资产的内在价值的分析存在疏漏——不管何种原因，购买者相信其价值会持续上行。如果价格上涨，狂热的投机者就会买入更多，或者那些错过低价买入的人就会在价格进一步攀升之前蜂拥入市。对此现象的解释莫过于“博傻理论”：买入方深信自己能找到下一个甘愿出更高价格的冤大头，从而认为自己的买卖是合理的。抢购热会相互感染，从而会进一步放大此种效应。在合适的条件下，价格会一路飙升至令人目眩的高位，然后下跌。人尽皆知的例子就是 17 世纪 30 年代发生在阿姆斯特丹的郁金香抢购泡沫，当时，一株郁金香球茎的价格相当于一年的工资。
