Prior to the Tax Reform Act of 1986 (TRA 86), interest paid on all types of household debt was deductible from income before the payment of taxes. In 1986, Congress changed the law to phase out the deductibility of interest over a five-year period.' Congress believed deductibility of interest an incentive to invest in durables rather than assets which produce taxable income and, therefore, an incentive to consume rather than save.... By phasing out the present deductibility of personal Congress intended to eliminate from the prior tax law a significant disincentive to (Joint Committee on Taxation [JCT], 1987 p. 263). The other goal of the provision was to raise $9.6 billion per year in tax revenue by 1991. Because Congress determined that encouraging home ownership is an important policy goal, achieved in part by providing a deduction for residential mortgage interest, it chose to retain the residential mortgage interest deduction (JCT, 1987 pp. 263-64). Thus, mortgage interest was fully deductible for interest paid on debt secured by a taxpayer's first or second residence up to his basis in the residence. The Omnibus Budget Reconciliation Act of 1987 (OBRA 87) changed the law so that interest paid was fully deductible on up to $1 million in acquisition debt and $100,000 in home equity debt.2 Debt is categorized as acquisition debt if it is used for the purchase or improvement of a home; home equity debt includes all other debt secured by a home. By keeping the mortgage interest deduction, Congress may have provided a loophole that some taxpayers could exploit. There was no restriction on the use of home equity debt, and taxpayers who owned homes could borrow against their home equity to pay for the same purchases they had previously funded with loans. Homeowners were given an incentive to shuffle their portfolios away from debt into mortgage debt. The widespread introduction of home equity lines of credit in the mid-1980's, which may have been spurred in part by the change in tax law, provided an inexpensive and flexible method for households to make this shift.3 Measuring the extent of portfolio shuffling is important for understanding whether households view mortgage and debt as close substitutes. Clearly the substitution of mortgage debt for debt undermines the goals of Congress to boost saving and increase revenue. Moreover, homeowners already have substantial tax preferences through the lack of taxes on the imputed income from housing and the preferential tax treatment of capital gains on their principal residence. The ability to use deductible mortgage debt to finance purchases provides homeowners another tax advantage relative to renters. Figure 1 plots the percentage change in * Putnam Investments, One Post Office Square, Bos on, MA 02109 (e-mail: dean_maki@putnaminv.com). This paper was completed while I was on the staff of the Board of Governors of the Federal Reserve System. Financial support from the Stanford Institute for Economic Policy Research and the Lynde and Harry Bradley Foundation is gratefully acknowledged. I would like to thank John Shoven, Orazio Attanasio, John Pencavel, Tim Bresnahan, Anne Royalty, Doug Bernheim, Al Teplin, Craig Furfine, Martha StarrMcCluer, Raphael Bostic, Len Burman, Julia Coronado, two anonymous referees, seminar participants, and the Financial Institutions Research Review Group for helpful comments. The views expressed in this paper are those of the author and do not necessarily reflect the views of Putnam Investments or the Federal Reserve Board or its staff. 1 In this paper, consumer interest refers to interest paid on loans that are not secured by a residence. 2 Under TRA 86, interest paid on qualified educational and medical debt secured by a home was also deductible, even if this debt exceeded the household's basis. This provision was not renewed in OBRA 87. Under OBRA 87, home equity debt also could not exceed the difference between the fair market value of the home and the amount of acquisition debt, even if this difference was less than $100,000. 3 Of course, homeowners can also increase their mortgage debt by taking out a traditional home equity loan, by taking cash out when refinancing their mortgage, or by taking out a larger mortgage when they Durchase a home.