Introduction Adverse changes in the tax treatment of particular securities are often accompanied by grandfather clauses that shield existing holders from the changes. If these holders consider selling their positions, they should realize that any repurchases of these securities will be subject to the new, more severe tax law. By focusing on a particular provision in the Tax Reform Act of 1986, this article illustrates how to derive an optimal investment policy in the presence of grandfather clauses. The Tax Reform Act of 1986 required property-liability insurers to reduce their tax deductions for losses by 15 percent of their tax-free income,(1) effectively raising their marginal tax rate on tax-exempt income. A grandfather clause stated that the rule applies only to income from tax-exempt bonds purchased after August 8, 1986. Effects of this change are quite important: From 1986 to 1989, over 30 percent of property-liability insurers' financial assets were in tax-exempt bonds, and these insurers held about 16.4 percent of all tax-exempt bonds outstanding (Board of Governors of the Federal Reserve System, 1990). The literature relating tax law to investment policy grew along two branches. The first derived tax-induced trading strategies under the assumption that all investors are taxed alike. For example, Constantinides (1983, 1984) and Constantinides and Ingersoll (1984) analyzed optimal deferral and realization of capital gains and losses in stock and bond markets. The second branch focused on the fact that not all investors are taxed alike. Schaefer (1982) showed that differentially taxed investors usually disagree on the relative value of securities and, consequently, choose to hold different sets of securities. Furthermore, the set of securities held by any class of investors may very well change over time. Subsequent work (e.g., Dybvig and Ross, 1986; Dammon and Green, 1987; Dermody and Prisman, 1988) analyzed equilibrium prices in the presence of differential taxation. The analysis here differs from both of these approaches. Whereas the first branch studied tax-induced trading in a particular asset, this article analyzes tax-induced trading across assets. And while the second branch studied the equilibrium implications of differential taxation, this article assumes that price movements can induce differentially taxed investors to switch from one asset to another in order to focus on an interesting investment problem. Furthermore, this is the first study of the effects of a grandfathered tax change on optimal trading strategies. The next section formalizes the investment problem of a property-liability insurer that holds exempt bonds bought before August 8, 1986. Then, using historical parameters, we show that the value of the option to switch between bond classes and the value of the grandfather clause have nontrivial magnitudes. The Model and Optimal Investment Policy This section first develops the model in the absence of capital gains taxation in order to illustrate clearly and simply the methodology and the main results. The second part of the section adds capital gains taxation to the model. No Capital Gains Taxation Assume the existence of two types of perpetuities. Both types pay $1, before taxes, each year, but the income from one type is tax-exempt, and the income from the other type is taxable. In an equilibrium characterized by differential taxation, the prices of the exempt and taxable securities will differ in a way that reflects the level of interest rates and the relative wealths of the different tax classes. Consequently, the difference between the two prices can be modeled as fluctuating randomly over time. For simplicity, we model the spread between taxables and exempts by fixing the price of the exempt perpetuity at some constant, |P.sub.E~, and letting the price of the taxable perpetuity, |P.sub.T~, fluctuate randomly. Now consider the investment decision facing a property-liability insurer. …
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