In February 1637, tulip bulbs sold in Holland for as much as 4,000 guilders each, over 10x the amount a skilled craftsman would earn in a year. Months later, many tulip traders found themselves holding bulbs worth just a fraction of what they had paid for.
As crazy as prices got, tulip mania actually started with good fundamentals. Tulips were a relatively new introduction to Europe, and the flower’s intense color made it a heavily-desired feature of upper-class gardens. Most desirable were the exotic-looking, multi-colored tulips, which was caused by a mosaic virus not identified until the 1970s and now called the “tulip-breaking virus.” At best, tulip bulbs weren’t easy to produce and those with the virus suffered even lower reproduction rates. In the beginning, what occurred in the tulip market was classic supply and demand—a highly sought-after item with limited supply increasing in price. In 1634, that started to change as speculators were attracted to the rising prices, and in late 1636 prices started to accelerate rapidly, to where even single-color tulips were attracting prices of over 100 guilders apiece. The Dutch created a futures market for tulips that enabled traders to purchase and trade contracts to buy bulbs at the end of the season. At the peak, tulips could be traded several times a day without any physical tulips actually being exchanged or either party ever having any intention of planting the bulbs.
Then in February 1637, buyers vanished. Some suspect an outbreak of the bubonic plague as the cause, some a change in demand caused by war in Europe. Any way you look at it, the sentiment for the future price of tulip bulbs took a big U-turn, leaving many investors ruined.
History is full of similar episodes, where investor sentiment got to extreme levels and prices diverged meaningfully from the underlying fundamental value of something, be it stocks, real estate, currency, or even tulip bulbs. Most recently the dot-com bubble in the early 2000s and the housing bubble in 2008 proved that speculation is alive and well.
While periods of extreme sentiment are easy to identify in retrospect, they’re anything but obvious while you’re in them. And while extreme levels of sentiment usually result in big price reversals, more modest levels can mark periods when the market is overbought or oversold, often followed by a market pull-back or rally. Recently, Robert Shiller of Yale University won the Nobel Prize in Economics for his work on irrational markets.
So how can you gauge sentiment? Some of the more popular ones are the Consumer Confidence Index and the Michigan Consumer Sentiment Index, which both try to gauge consumer’s attitudes on a variety of things, including future spending, the business climate, and their level of optimism or pessimism. More direct, and generally more volatile, are the AAII Investor Sentiment Survey and the Wells Fargo/Gallup Investor and Retirement Optimism Index, which ask investors directly about their thoughts on investments. It doesn’t end there. Investors watch Closed-End Fund discounts, Put/Call ratios, even tracking the occurrence of certain words or phrases in the media. In addition, many firms create their own blend of surveys and indexes to best gauge the overall sentiment level.
Sentiment certainly isn’t the be-all, end-all for trading your portfolio. There’s a saying that is attributed to John Maynard Keynes, “The market can remain irrational longer than you can remain solvent.” When sentiment starts moving in one direction, it’s hard to say when the reversal will occur and what will cause it. But knowing where sentiment levels are at any given time can help you get a better understanding of what markets have been doing and what to expect going forward.
 Mackay, Charles (1841), Extraordinary Popular Delusions and the Madness of Crowds, London: Richard Bentley, archived from the original on March 31, 2008.