Over the past several years, bond yields have correlated pretty well with equity prices—yields rise and fall along with the ups and downs of the stock market. Higher stock prices reflect increased optimism (or less pessimism) about the future, and bond yields move in synch because a more healthy economy increases the odds of higher inflation and a tighter Fed. Most of the time the correlation between stocks and bond yields has been 0.6 to 0.8, interrupted with a few relatively brief periods of negative correlation, as can be seen in the two charts below. Over the past few months, we have once again entered one of those periods where the correlation has gone to zero. Equity prices are up, but bond yields remain very low and relatively stable. As the above chart suggests, either bond yields should be at least 100 bps higher, or equity prices should be a lot lower.
In my view, this disconnect reflects a buildup of tension in the market—something is likely to break pretty soon. Bond yields have been depressed because risk-averse investors have been seeking shelter from a potential Eurozone collapse that might trigger another global recession/depression/deflation. But equity prices have been rising because in the meantime, while the world waits for the Eurozone to implode (and we've been waiting for at least 18 months now), the U.S. economy continues to improve. Bonds are the doomsday trade, while equities are more realistic about what's happening right now.
I can think of an alternate explanation, but it's not very convincing. Maybe bond yields are low because the market is absolutely convinced that no matter what happens to the economy, inflation is going to be very low for a very long time, and central banks, and particularly the Fed, are going to remain at or close to their "zero bound" for as far as the eye can see. To counter this explanation, I note that break-even spreads on TIPS reflect inflation expectations to be in a 2.0- 2.7% range, which is pretty close to the average rate of CPI inflation over the past two decades (2.5%), and during that time 30-yr bond yields have averaged 5.5%. In other words, I find no evidence to suggest that bond yields today are priced to deflationary concerns, so their low levels therefore more likely reflect an intense risk aversion on the part of investors.
In any event, it's unusual and I think unnatural for capital markets to be so schizophrenic. The assumptions driving bonds and stocks should ordinarily reflect the same world view, but these days they don't seem to.
My money is on bonds catching up to stocks—bond yields are more likely to rise than stock prices are to fall. What could trigger this? Maybe the Fed bows to the same realities that are driving equity prices higher, and informs us that with the economy doing better on the margin, the Fed mostly likely won't have to keep yields at zero for the next several years as the market currently believes it will. Or maybe Greece finally executes its default, but the world does not come to an end (after all, a huge default has been fully priced in for a very long time, so it should not prove surprising or disruptive when it finally happens). Or maybe the simple passage of time without anything disastrous occurring will do the trick—markets can't stay priced to disaster forever if a disaster doesn't occur.