I got an e-mail this past week noting that Vanguard would be closing its high-yield corporate bond fund to new investors “effectively immediately” and that the fund had received “approximately $2 billion” of flows over the past six months. While growth of Vanguard’s assets under management is almost always a good thing, a fund shuttering its doors to new flows makes one wonder just how frothy credit markets have become. Let’s take a step back, though, and look at high-yield fixed income as an asset class. I find that many investors simply don’t understand what returns high-yield bonds have historically generated or just how closely correlated they are with the equity markets.