Many investors are familiar with the VIX, or volatility index, that measures how much investors expect to see the S&P 500 index fluctuate. But far fewer know even a little about a measure of volatility in economic growth called the Economic VIX Index, created by Jim Paulsen, chief investment strategist at investment-management and research firm Leuthold Group. The Economic VIX is important now because its history over the decades since World War II shows that stocks do best when economic volatility in the U.S. is at its...