Fashion and style are two different things. It has been said you can look at a centuries-old portrait and have no idea whether the subject’s clothes were in fashion, but you most certainly will know whether the person had style.

Markets run in fashions, too. Consider a two-decade-old example — the chain of causation for the 1987 market crash as described by John Murphy in his 1991 book Intermarket Technical Analysis. Murphy described how dollar weakness led to higher commodity prices, which signaled higher inflation, which in turn led to higher short- and long-term interest rates and eventually to the stock market crash.

At the time the book was published, no one raised an eyebrow at this sound description. However, if you used it as a trading guide between 2005 and 2008, you would have lost a lot of money, because bond yields fell even as the dollar weakened and commodity prices hit a series of new highs.