How big is the corporate bond market compared to the equity market? [Hint: there's plenty of supply. LOL ]Not enough of the kind of bonds that would be "permitted" in a 401(k) looking for safety. If 401(k)'s were comprised of all sorts of bonds, even lower rated ones, then while the stock/bond combo funds dropped 30% last year, those bonds would have also dropped about 20% last year. The number to look at is the total non-financial corporate bond market with ratings above junk (and perhaps even a few notches above junk).Another issue with solely using corporate bonds in 401(k)'s is periods of time, like now, in which there is a dearth of new bonds being issued, and thus a constraint on supply.