A month ago, when the stock market suffered its first correction—a 10% drop—in years, many found comfort in the economy’s apparent strength: Surely a severe bear market can’t begin when the economy is growing so strongly?

Their argument provides false comfort, however, since the stock market usually tops out well before the economy slips into a recession. And by the time you know the economy is in a recession, odds are overwhelming that a bear market (which is confirmed when stocks drop 20% from the high) will have long since...