There are increasing signs that the run in high-yield bonds is reaching levels that are unsustainable relative to other asset classes.

Late last year, something that has not happened in two decades took place, and it is not a good omen for high yield bond markets.

The yield on the benchmark Merrill Lynch high-yield bond index dropped below the earnings yield on the Standard & Poor’s 500 Index (that is, the inverse of the price-to-earnings ratio).