Coming out of World War II, few Americans could have known about the great era of prosperity soon to arrive.

From 1946 through 1973, our economy expanded by a 3.8% annual pace. Sure, there were a few recessions along the way, but the rapid rise of the middle class, which enjoyed a rising standard of living, helped set the stage for the world's greatest economy. Roughly one-fourth of all global economic activity takes place on U.S. shores.

But as I noted last week, the U.S. is now in its sixth year of weak economic growth. Economists are growing increasingly concerned that we've entered into an extended period of anemic growth. Indeed, the sole purpose of the Federal Reserve's massive stimulus programs was to revive the economy's "animal spirits." But the economy has yet to respond. The Fed is pushing on a string.

Investors may have a hard time seeing this notion as the stock market moves steadily higher. Profits are still rising, thanks to lean corporate expenses, but fully half of the companies in the S&P 500 are expected to boost sales by less than 5% next year.