in corporate income tax, the deductions from gross income allowed investors in exhaustible mineral deposits (including oil or gas) for the depletion of the deposits. The theory behind the allowance is that an incentive is necessary to stimulate investment in this high-risk industry.

The depletion allowance is similar to the depreciation (q.v.) allowance afforded other firms for their investments. There are substantial differences, however. One is that it is difficult to estimate what proportion of a mineral deposit has been exhausted. Another is that the value of the deposit is often substantially larger than the amount invested. The search for a deposit entails considerable risk, but once it is found, it may justify high levels of investment even without tax incentives.

The first depletion allowance in the United States, called the “discovery depletion,” was enacted in 1918 to stimulate oil production for World War I (even though the war had just ended). Discovery value proved too hard to estimate, however, so this was changed in 1926 to the “percentage depletion” for oil and gas property, under which the corporation deducts a fixed percentage of its sales as a depletion allowance, regardless of the amount invested. In addition, producers can deduct their capital costs, thus gaining a double benefit. After 1931, Congress expanded the use of “percentage depletion” to many other extractive industries, such as those concerned with metals, sulfur, and coal.

Proponents of the depletion allowance claim that special treatment for the oil and gas industry is justified because of the high risks involved and because reliable oil supplies are vital to national defense. Opponents argue that overly beneficial depletion allowances lead to over-investment in the favoured industries and excessive exploitation of some minerals while distorting allocation of resources. After years of debate, the depletion allowance for oil and gas was reduced from 27.5 percent to 22 percent in 1969 and completely eliminated for certain large producers in 1975. Only small, independent companies and royalty owners, as well as owners of geopressurized methane gas wells, were allowed a percentage depletion, but it was to decline gradually to 15 percent beginning in 1984.