Figure 1 shows the yields to maturity for several categories of long-term bonds from 1919 to 2005. It shows us two important features of interest-rate behavior for bonds of the same maturity: Interest rates on different categories of bonds differ from one another in any given year, and the spread (or difference) between the interest rates varies over time. The interest rates on municipal bonds, for example, are higher than those on U.S. government (Treasury) bonds in the late 1930s but lower thereafter. In addition, the spread between the interest rates on Baa corporate bonds (riskier than Aaa corporate bonds) and U.S. government bonds is very large during the Great Depression years 1930-1933, is smaller during the 1940s-1960s, and then widens again afterward which factors are responsible for these phenomena?