Invest in stocks? Beware of saros cyles.

Let workers invest part of what they now pay in Social Security tax. People will gladly accept reduced retirement benefits, say proponents, because they''ll make more money by investing their private accounts in equities.

They could make much, much more money, judging from long-term returns cited by Ben Stein in his Sunday New York Times column. Over 20-year periods from 1871 to 2002, an investor who owned stocks that performed like the S&P 500, reinvested dividends and sat tight would have seen his nest egg grow an average of 600%.

What's more, Stein says, stocks these days are cheap, less expensive than they were five years ago.

But what if stocks remain cheap for years and years to come? That's the fear of those who see 18-year saros cycles echoed in the stock market.

As NASA helpfully explains, saros cycles run for roughly 18 years and relate to the movement of the moon and the recurrence of eclipses. Three saros cyles make one Kondratieff cycle. Astrologers and technical market analysts labor mightily to churn out short-term market forecasts based on these cycles, with predictably mixed results.

Yet in the long run -- well, let's look at the record:

In 1929, prices in the stock market soared sky high, then crashed. Dismal economic times delayed a new bull market until after World War II. Even when the war ended, investors feared a renewed Depression. Stock prices didn't start perking up until 1947. Eighteen years.

From 1947 onward, the bulls stampeded. Yields on risky stocks actually dropped below yields on presumably safer bonds.. People started talking about "growth stocks." By 1965, the Dow had soared to 1,000. Another eighteen years!

It would be a long time before 1,000 on the Dow was seen again. Stock prices seemed to fluctuate endlessly and aimlessly from 1965 through the 1970's. (But if you adjust the stock averages for inflation, you'll see another stock market crash, this time in slow motion.) It wasn't until late 1982 that a new bull market began. Almost eighteen years!

You know the rest of the story. From 1982 onward, the stock market made millions of Americans affluent if not downright rich. The Dow soared past 10,000 and kept on going. The unprecedented party didn't end until the dot.com bubble burst in 2000. Yet another eighteen years!

Will these 18-year cycles persist? Will investors have to wait until 2018 for the next significant bull market? If so, millions of neophyte investors could very well lose their patience and their Social Security money, requiring an eventual Congressional bailout at the expense of American taxpayers.

But not to worry. All this saros-cycle stuff is nonsense, right? Perhaps. But there's a simpler, more plausible explanation for the regularity of the market's long-term ups and downs. The average investor probably starts investing in stocks or stock funds in her late 20s or early 30s and starts moving into bonds when in her 60s. That's just about long enough for two saros cycles, one down, one up.

So maybe it's not the moon, just human nature.