Last week on Sound Money, I interviewed John Bogle, the founder and chairman of the Vanguard Group. In the course of our conversation, I asked the octogenarian financier for his forecast of stock market returns over the next decade. He replied that a 4 percent to 9 percent range was reasonable. Later in the show, I picked a 4 percent to 6 percent return when answering a listener question. These figures sparked several e-mail protests, including this thoughtful one from Ohio:

The listener said, "I am quite puzzled and in fact alarmed by John Bogle's expectations for stock valuation growth over the next decade. Even more disconcerting is your own lower prediction of 4-6 percent over the foreseeable future. I'm curious as to why you think returns will be so low for such an extended period of time. Over the past 75 years, equity appreciation has averaged about 11 percent annually. This is documented history. Why would you expect the long term future to underperform so drastically?"

My answer: The essence of investing is uncertainty. The fog is especially heavy at tumultuous times like this. And the trajectory of stock market returns is far more complicated than suggested by the nearly 11 percent average annual gain of the past 75 years. Bear markets follow bull markets, and stocks languish for long periods of time.

The past offers up valuable clues that suggest caution. For instance, stock returns are often less than stellar following the demise of a great bull market. It took about two decades for investors to recover their enthusiasm for equities after the stock market peak of 1901. Stocks returned a -0.02 percent, after adjusting for inflation, from 1901 to 1921. The Kennedy-Johnson bull market reached its high in early 1966 with the Dow Jones Industrial Average hovering around 1,000. Seventeen years later, the Dow was still struggling to break 1,000. And the Dow had declined some 60 percent after taking the high inflation of that era into account.

Well, the stock market put on a once-in-a-lifetime performance over the past two decades with a real 12 percent annual gain. It's a safe bet that returns will pale over the next 10 years compared to the previous 10 years.

Here's one way to gauge future returns. In the long run, stock prices reflect corporate earnings growth, which, in turn, is linked to the economy's growth rate. Let's do the numbers. Economists at the Federal Reserve Board estimate that the U.S. economy can grow at a 3.5 percent rate without generating inflation. Add to that the current dividend yield of 1.5 percent, and a real return of 5 percent or so over the next five to 10 years is a sensible forecast.

By the way, that's not a bad return on investment at all. But it does mean we're all going to have to save more and spend less to prepare for our golden years.