Fed Chair Yellen's Warning Adds To Recent Market Jitters

Both stock and bond markets had already been having a rough week, and then on Wednesday, Federal Reserve Chair Janet Yellen added to the jitters.

She warned that stock valuations are "generally quite high," and that "there are potential dangers there."

So if you happen to be an investor who wants to buy low and sell high (and really, who doesn't?), then you might take Yellen's comment as a suggestion that it's time to sell.

And that's just what happened: Measures of U.S. stock prices all slipped — down about 0.7 percent by midday.

Many analysts already had been pointing out that stock prices, relative to corporate earnings, have been higher than their historic norms. So that could mean markets are headed for a rough ride as prices fall back into line with more typical valuations.

But Yellen also called attention to the big picture, noting that while prices might be due for a correction, financial markets are functioning well. "Risks to financial stability are moderated, not elevated at this point," she said.

Yellen made her observations in a discussion with International Monetary Fund Managing Director Christine Lagarde during an event organized by the Institute for New Economic Thinking.

Their conversation came at a time when global markets have been jarred by worries about interest rates, oil prices and economic growth. A lot of investors worry because they are seeing oil prices bouncing up to their highest levels this year at more than $61 a barrel. Also, U.S. interest rate likely will go up soon.

And hiring this spring may be coming in weaker than expected. Private U.S. payrolls rose by only 169,000 jobs in April, according to a report Wednesday by payroll processor Automatic Data Processing and forecasting firm Moody's Analytics. Most economists had expected to see about 205,000 jobs added last month.

There are wildcards out there, too. Just one example: Greece may not be able to make some massive debt repayments that are due soon.

But before you sell anything based on Yellen's advice, remember she and the Fed made similar remarks last July about some stocks: "Valuation metrics in some sectors do appear substantially stretched — particularly those for smaller firms in the social media and biotechnology industries."

And in the nine months following those remarks, increases in the value of such stocks helped drive the Nasdaq Composite Index up about an additional 13 percent.