World stock markets have been plummeting over the last year due to worries about global economic contraction. One of the big drivers has been a dramatic drop in the price of oil, from over $100 a barrel two years ago to well under $30 a barrel today. The sharp decline has played havoc with the domestic fossil fuel industry, caused the loss of lots of high paying jobs in the U.S. oil patch, and hurt earnings of companies from giant Exxon to small independent drillers. Stock prices recently have followed the daily declines in oil prices. I began to hear the argument last year that the drop in oil prices will have a good effect on American consumers. I was skeptical. I reasoned that the loss of high-paying jobs more than offset the extra few hundred dollars left in each American driver’s pocket. Count me as a believer today. Retail gasoline prices...