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I. INTRODUCTIONOBSERVED DIFFERENTIALS among yield series for different types of long-term instruments-U.S. government bonds, municipal bonds, corporate bonds and residential mortgages-vary considerably over time. Many factors can contribute to observed long-term yield spreads, the most important of which are "technical" factors that relate to differences in the particular characteristics of instruments or in the investors that purchase them. These include the tax status of various types of income accruing to the security and the degree of certainty associated with that income. Long-term yield spreads might also be affected by relative security supplies and relatively exogenous demands, e.g., Federal agency demands for residential mortgages, particularly if institutional constraints on permissible yields or on the various sectors that purchase the long-term instruments exist. Failure to consider the effects of all of these factors has frequently created misunderstanding both in the financial press and in academia about the causes of observed yield spreads.
For example, attempts have been made to measure the impact on long-term yield spreads of individual factors-such as default risk or relative security supplies-by comparing yield series without proper regard for the concurrent impact of other technical and/or fundamental factors.This paper deals exclusively with long-term corporate and U.S. government bond yields. The most widely used series are Salomon Brother's Aa deferred call new issue utility yield and the Federal Reserve Bulletin's average yield on bonds maturing or callable in 10 years or more.' The spread between these series is shown as the solid line in Chart 1. (The dashed line will be discussed later.) The spread has moved over a wide range, rising sharply from 40 basis points in late 1965 to 235 basis points in the second quarter of 1970, subsequently falling to 140 basis points in early 1973, and then rising to almost 300 basis points in the third quarter of 1974. The central question addressed in this paper is what have been the relative contributions of taxes, risk, and relative security supplies as determinants of these movements?The general procedure used to deal with this question is as follows: First, direct adjustments are made to the yield series to account for some factors affecting them differently. These include the unusual tax treatment of some of the income from