Practice with respect to personal deductions also varies widely. In the United States, for example, such deductions include interest paid on home mortgage debt (but not other personal debt), unusually high medical expenses, philanthropic contributions, and state and local income and property taxes. In Great Britain, on the other hand, virtually no deductions are granted that do not in some measure bear a direct relation to the production of earned income. In South America, where multischedule income taxes are common, some countries allow virtually no deductions, whereas in others the latitude permitted in the deduction of personal expenses is very great.

In those countries that allow the deduction of extraordinary medical expenses, a stated percentage of the taxpayer’s income often has to be used for this purpose before any deduction can be taken. In the United States only those expenses that exceed a small percentage of adjusted gross income (less than 10 percent) are deductible, and a similar rule is observed in Germany. On the other hand, in the Netherlands the whole expense becomes deductible once the minimum has been exceeded. The justification for a deduction of this type is that medical expenses are not generally controllable and, when incurred above a certain normal level, reduce an individual’s ability to pay taxes relative to others at the same income level.

The justification for deduction of contributions to religious, charitable, educational, and cultural organizations is usually found in the encouragement of socially desirable activities rather than in any allowance for differences in taxable capacity. The contributions that qualify for this deduction vary from country to country, and total charitable contributions are usually limited to some percentage of the taxpayer’s income. In Japan contributions made to government, to local authorities, or to institutions for scientific research are deductible from taxable income but only to the extent that such payments exceed the lesser of either a certain percentage of income or a specific amount of yen. This, of course, denies any deductions to taxpayers whose contributions amount to only a small fraction of their incomes.

A third type of deduction—one that serves neither to relieve hardship nor to encourage voluntary support of socially desirable activities—is allowed in some countries for certain kinds and limited amounts of personal saving. These have included (1) social security contributions and compulsory contributions to private pension funds, for which deductions are allowed in Japan, France, the Netherlands, and Belgium, and (2) limited amounts of life insurance premiums, which are deductible in Great Britain, Japan, France, and Germany. Deductions also have included limited amounts of savings earmarked for the construction of dwellings or placed in savings deposits. One justification for these allowances has been that they encourage low-income taxpayers to seek the protection afforded by life insurance, pension plans, and accumulated savings; another reason is that they channel the personal savings of such individuals into banks and other financial institutions where they can be used to support capital expansion. Over the years various countries have introduced special tax-privileged savings plans. In none of these countries, however, were they found to be particularly effective in increasing total personal saving. Such plans may simply redirect a given amount from one form of saving to another.

Another frequently permitted deduction, the justification for which is not entirely clear, is that allowed for interest paid on personal indebtedness. In the case of interest paid on home mortgages, it is generally regarded as one of several special tax concessions granted to homeowners. The United States imposed increasingly strict limits on the deductibility of interest, beginning in 1921 (with denial of any deduction on debt incurred for the purpose of acquiring tax-exempt securities) and culminating in 1986 (with the denial of deductions for interest on consumer debt, along with provisions intended to prevent investment interest deductions from sheltering income from other sources). In Canada the interest deduction is denied in cases of consumer debt and of most home mortgages. Loan interest is deductible in Germany as a “special expense,” as it is in France on moneys borrowed from third parties.

Still another deduction that does not appear to have much to do with the determination of true income is the deduction for taxes paid. Among the justifications offered for this type of deduction, the most widely accepted is that it contributes to fiscal coordination in a federal system and avoids extremely high rates in the case of overlapping income taxes. In the United States state and local taxes on property and income are deductible. These deductions are tantamount to subsidies from the national to the subnational governments. Foreign taxes on real property and income are also deductible, although most taxpayers elect instead to credit their foreign income taxes against their U.S. tax. Japan and Germany also allow deductions for local taxes, although Japan specifically excludes the income taxes of prefectural and municipal inhabitants from the exemption allowed for other taxes. Ordinarily, the tax paid with respect to income in one year is not allowed as a deduction in determining the same tax the following year.

One way of limiting the use of itemized personal deductions for taxpayers whose total deductions are small—and thereby minimizing costs of compliance and administration—is to provide an optional standard deduction. Examples of this practice are found in the United States and Germany.

The taxation of capital gains and losses presents a special set of problems to which different countries have found different answers. An increase in the value of a capital asset—a share of stock, a corporate or government bond, or perhaps a piece of real estate—increases the net worth of its owner, and it thus can be seen as a form of income. There is a problem, however, of valuing all of the capital assets a taxpayer may own so as to be able to determine how much his net worth has increased or decreased during the taxable year. Moreover, imposing taxes on gains that have not been realized may create cash-flow problems for taxpayers. In practice, these problems have usually been avoided by taking into account only those gains and losses that have been realized in the form of cash or its equivalent. Taxing gains only at realization allows taxpayers the benefit of postponing taxation, however. In the United States this problem of deferral is aggravated by allowing gains on assets transferred at death to escape tax permanently. An alternative would be to require that accrued but unrealized gains be taxed, either periodically or at death, as if they had been realized through a sale, a policy known as “constructive” realization.

Even realized capital gains may present a problem of valuation. During an inflationary period (or for some time thereafter), an increase in the monetary value of an asset may not mean that there has been an increase in the real (inflation-adjusted) value of the asset. Some economists believe the solution to this problem is to adjust the cost of the asset for inflation, as is done in several Latin American countries.

Another problem that arises in the taxation of capital gains is that of determining the appropriate rate at which realized gains should be taxed. One answer to this is that they should be treated no differently from other forms of income, an approach followed by the United States with the Tax Reform Act of 1986. The difficulty with this answer is that under many circumstances it is unfair to the taxpayer and may also have undesirable economic effects. If capital gains that have accrued over a number of years are taxed at regular progressive income tax rates in the year of their realization, the tax on them may be higher than it would have been if the unrealized gains had been taxed annually as they accrued. The knowledge that capital gains are subject to very heavy taxes upon realization can deter individuals subject to high-bracket rates from making investment decisions that are socially desirable. This difficulty is usually handled by taxing such gains at a relatively low rate or by excluding a stated percentage of the gain from taxable income. In either case this special treatment commonly applies only to long-term gains involving assets that have been held for a minimum length of time.

Some countries, including Canada, France, and Germany, do not tax capital gains unless they arise out of a business. The line between a business transaction and a personal one is not easy to draw, however. Moreover, the exemption of capital gains tempts taxpayers to recharacterize ordinary taxable income as tax-exempt capital gains income. Complexities then arise as taxpayers who want to minimize taxes match their wits against lawmakers and tax administrators who must prevent abuse.

Countries that do not, in principle, tax individuals on their capital gains also do not allow capital losses to enter into the determination of taxable income. Those that do tax capital gains ordinarily take capital losses into account only as offsets to capital gains. Even then, deductions of losses are usually limited to prevent abuse.