Bond owners are promised fixed future cash flows in nominal terms. Therefore, their real returns depend crucially on inflation over the life of the bond. Any increase in inflation erodes the purchasing power of future coupon and principal payments. Accordingly, investors generally demand a higher bond yield as their expectations of future inflation rise.
But the expected path of inflation is not the only thing that bond investors care about. They also worry about the degree of uncertainty attached to these expectations. Investors are typically averse to risk. That is, given the choice between two financial investments with the same average expected return, they will prefer the one with less anticipated variability in that return. Consequently, a bond investor has to receive a higher yield as compensation if the uncertainty about the future path of inflation increases because his future returns after adjusting for inflation are more uncertain. This additional compensation, known as a risk premium, is widely viewed as an important determinant of long-term interest rates.