Six highly regarded measures of stock valuations show the market has been overvalued for months, says Mark Hulbert, editor of Hulbert Financial Digest.

"While that doesn't mean a bear market is imminent, there is a high probability that investment returns over the next decade will be below average, according to Yale University economics professor and Nobel laureate Robert Shiller," Hulbert writes in The Wall Street Journal.

Hulbert presents data comparing the six valuation indicators to their levels during the 35 bull-market peaks since 1900. Five of the six metrics are higher than they were during 82 to 89 percent of past peaks. That includes:

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• The cyclically adjusted price-earnings ratio, which includes 10 years of earnings (86 percent of previous bull-market peaks had lower valuations than the current market).
• The dividend yield (86 percent).
• The price-sales ratio (89 percent).
• The price-book ratio (82 percent).
• The Q ratio, which represents a company's market capitalization divided by the replacement cost of its assets (89 percent).

The sixth statistic is the traditional price-earnings ratio, which is based on trailing or forward 12-month earnings. It's higher than in 69 percent of prior peaks.

"The stock market isn't poised to produce returns that are in line with even its long-term annualized average of around 10 percent, much less the 20 percent-plus returns we have seen over the past five years," Hulbert warns.

"Unfortunately, none of the valuation ratios help us determine the path the market will take in producing these anemic returns. It could turn in low but positive returns during the next 10 years, or it could soar first and then plummet — as it did in the late 1990s and early 2000s."

Renowned activist investor Carl Icahn apparently is concerned about valuations too.

"In my mind, it is time to be cautious about the U.S. stock market," he told Reuters. "While we are having a great year, I am being very selective about the companies I purchase."

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