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In July, Federal Reserve Chief Janet Yellen warned of the “stretched” values of junk bonds. Few seemed to care, and among the unconcerned were millions of retirees who had reached for these bonds’ higher yields in order to maintain their lifestyle. Now, a reckoning may be at hand.

Yellen’s mid-summer warning on asset prices was reminiscent of the former Fed chief Alan Greenspan’s “irrational exuberance” comment regarding stock prices in 1996. Few listened then, either. It turns out that the Greenspan warning was way early. But the dotcom collapse hit later with devastating results.

Yellen’s remarks may be timelier. High-risk, high-yield corporate bond prices have been falling amid the strongest selling in 18 months. Since June, investors have pulled $22 billion out of the market and prices have dropped 8%. The pace of the decline has quickened since October.

The junk bond selloff began in the energy sector, where oil prices recently hitting a five-year low set off alarms about the future profits—and ability to make bond payments—of some energy companies. In the past month, the selling has spread throughout the junk-bond universe, as mutual fund managers have had to sell to meet redemptions and as worries about further losses in a possibly stalling global economy have gathered steam.

The broad decline means that junk bond investors have little or no gain to show for the risks they have been taking this year. Investors may have collected generous interest payments, and so not really felt the sting of the selloff. But the value of their bonds has fallen from, say, $1,000 to $920. The risk is that prices fall further and, in a period of global economic weakness, that issuers default on their interest payments.

Retirees have been reaching for yield in junk bonds and other relatively risky assets since the financial crisis, which presumably is partly what prompted Yellen’s warning last summer. It’s hard to place blame with retirees. The 10-year Treasury bond yield fell below 2% for a while and remains deeply depressed by historical standards. By stepping up to the higher risks of junk bonds, retirees could get 5% or more and live like it was 10 years ago. Many also flocked to dividend-paying stocks.

So far, taking these risks has generally worked out. Junk bonds returned 7.44% last year and 15.8% in 2012, according to Barclays, as reported in The Wall Street Journal. Meanwhile, stocks have been on a tear. But the backup in junk bond prices this fall should serve as a warning: Companies that pay a high yield on their bonds—and many that pay a fat stock dividend—do so because they are at greater risk of defaulting or going bust. That’s the downside of reaching for yield, and it doesn’t go away even in a diversified mutual fund.