MENLO PARK, CALIF. (CNN/Money) - The guy who championed the idea of buying and holding top-quality stocks for the long-term is still bullish on stocks, newly worked up about the specter of protectionism and is researching what he thinks is the most important economic topic of our day: the aging populations of industrialized countries.

You'll have to wait until September 2004 for the new book from Jeremy J. Siegel, the Wharton professor and author of the famous investing book "Stocks for the Long Run."

But he previewed the results of his latest research at a lecture in San Francisco earlier this week.

First to the topic you really care about now. Stocks. Siegel thinks the S&P 500 Index is worth almost exactly what it should be worth.

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But given the alternative of buying bonds, Siegel's emphatic that stocks are the place to be. "I think the next 10 years are going to be a slam dunk," he said, which to him means mid-single-digit returns.

Siegel also thinks it's important to factor in inflation, which he thinks is certain to come back. The problem, says Siegel, is in the labor market: Companies have grown earnings by not hiring, and that can't last.

At the same time, the Federal Reserve will be slow to raise interest rates, fearful of squelching the recovery.

Finally, Siegel is spending much of his time analyzing the impact of the aging population, in the United States and in Europe and Japan, where the problem is more severe. This makes it particularly important to resist the temptation of protectionism, he figures, since we'll need free trade to support all of our old fogies.

Stocks are good, the trade deficit isn't all that bad and fertility rates aren't what they used to be. That's insight for the long run.

I read in the Wall Street Journal the other day that the total size of the stock market in mainland China is about $450 billion. For comparison's sake, two U.S. companies, GE and IBM, are worth a combined $450 billion.

China's big and growing, but it's got a long way to go.

Adam Lashinsky is a senior writer for Fortune magazine. Send e-mail to Adam at lashinskysbottomline@yahoo.com.

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