A Wall of Worry

Bets against the market, in the form of short sales and open interest in put options, are at record levels. Analysts have more stocks at "hold" than at "buy" for the first time in a decade. Some 75% of mutual fund flows are heading overseas.

Volatility, a measure of investor fear, has increased sharply. In a bull market, the time to worry is when investors get too confident, not when they're too cautious, so it still may be the time to buy stocks.

Even though stocks have reached record levels, valuations are still a long way from the overheated levels of the late 1990s.

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