Perhaps the most important concept to understand about bonds is that they are debt. Debt is a loan and borrowers do not pay more than they are contractually obligated to. The implications is that even under ideal conditions, future returns are both known and limited. Below are several reference points that investors can use to predict future returns.
Starting yields have been a fairly good predictor of Treasury returns:
Within credit, starting spreads have been good predictors of future returns:
Spreads are also indicative of future excess returns:
The above charts convey an important message: be aggressive when future returns are high and defensive when future returns are low. Fortunately, fixed-income investors can get a sense of future returns in the present (as a point of comparison, this is very different from equity markets where expensive assets can get more expensive and cheap assets can get cheaper). Invest accordingly.