flashing red

In the go-go days of 1999, Warren Buffett grew very concerned.

Not because his value style of investing had grown unpopular, but because investors were becoming delusional in their zeal for further gains.

In a speech he made to friends, as recounted in a 1999 article in Fortune magazine (that was published just a few months before the market peaked and then plunged), Buffett warned that "once you reach the point where everybody has made money no matter what system he or she followed, a crowd is attracted into the game that is responding not to interest rates and profits but simply to the fact that it seems a mistake to be out of stocks."

A simple test of how much stocks were loved: The aggregate value of the largest 5,000 U.S. companies (as measured by the Wilshire 5000) exceeded the GNP of the U.S. economy. In fact, a market melt-up took this ratio up to 150% by early 2000 (meaning the Wilshire 5000 was 50% larger than the U.S. economy), which set the stage for one of the most painful corrections ever for investors.

This ratio eventually dipped well below 100%, which for Buffett has been seen as a time of deep value for stocks. "If the percentage relationship falls to the 70% or 80% area, buying stocks is likely to work very well for you," he told Fortune in a 2001 follow-up.

Indeed stocks went on to deliver solid gains into that decade, but by 2007, Buffett's handy ratio again flashed red. Stocks were becoming so frothy that this measure once again exceeded 100%. The resulting market blow-off in 2008 was another painful lesson for investors, but at least put the market deep into value territory, setting the stage for the bull market we've been enjoying ever since.

Yet as we head towards the end of 2013, investors need to once again tread cautiously, because Warren Buffett's market valuation tool is again in the red zone. [109%]

The Wilshire 5000 has risen 68% since the end of 2009. Yet the economy has grown just 17%, throwing this key ratio out of whack.

Action to take: The Wilshire-to-GNP ratio is stretched, but it could well go even higher for a while, as was the case in 1999. Yet a clear margin for error has been removed from this market, and there is ample reason to shift your portfolio into a defensive posture. That means missing out on further upside in the aggressive growth segments of the market, but also means a greater chance of capital preservation.