The bond market has consistently trumped returns from stocks over the past decade. From Dec. 31, 1999, through May 29, 2012, the Barclays Capital U.S. Aggregate bond index — which covers the investment-grade bond market, including government and corporate bonds as well as mortgage- and other asset-backed securities — returned 104.1 percent to investors. By comparison, thetotal return of the S&P 500 was 14.2 percent, including share price gains and dividends.

No wonder, then, that many investors today prefer the merry-go-round to the market roller-coaster.

If the world economy worsens, corporate profits could as well.

Investors are not heartened by stock market valuations that are trading at a discount to long-term average price-to-earnings ratios, according to Stovall.

For instance, the S&P 500 index is trading at 13.2 times trailing 12-month operating earnings . That's a 26 percent discount to the median P/E of the last 25 years.

Role Reversal Time

Some market watchers say enough is enough, though.

“Investor behavior is paradoxical. It’s mathematically impossible for bonds to keep appreciating anything like they have over the past several years,” says financial planner Jonathan M. Bergman, citing financial guru Jeremy Siegel, author of "Stocks for the Long Run.”

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