Sandra Evans' article ''Child Care Proposals on Hill Would Hit Area Middle Class'' {Aug. 20} implied that the primary reason the House adopted a phaseout of child care tax benefits was to fund new day-care programs for low-income families. This is not true.

Day-care tax write-offs were scrapped in the House primarily because many believe tying tax benefits to day-care expenses penalizes families that make economic sacrifices to care for their own children.

According to the most recent Census Bureau report on child care practices, 74 percent of all families with children under 14 and 70 percent of those with preschool children have no day-care expenses. Despite the fact that these families pay a significant ''opportunity cost'' (in forgone wages) to care for their own children, some believe they should be denied dependent care tax benefits because they do not have any out-of-pocket day-care expenses.

This argument appears reasonable until one considers that it could be used to justify a tax break for just about any expense. A taxpayer who eats at restaurants or calls out to Domino's could argue for a tax break because he uses his after-tax earnings to pay for meals that other families prepare for themselves. Similar claims could be made by those who hire housekeepers, butlers, gardeners, interior decorators and chauffers.

Thus, those who advocate linking tax benefits to day-care expenses must demonstrate that paid forms of day care are superior to unpaid family care arrangements. It is here that their case breaks down, for the raging debate in child development circles centers around whether or not non-familial group day care is bad for kids -- not whether or not it is good.

It is a lamentable fact of modern life that not all parents enjoy the economic freedom to provide primary care for their own children -- or live close enough to their parents or other relatives that they can rely on extended family assistance. But public policies that skew benefits in favor of paid day care only serve to increase the number of families that lack this freedom. For as the old cliche goes, you get less of what you tax, and more of what you subsidize.

Business groups and feminists are right to be concerned about the tax treatment of America's families with young children. America's families are seriously overtaxed. But in defending the status quo, they must explain why a family with an annual income of $90,000 should be entitled to day-care tax benefits worth up to $2,025 when a $30,000-a-year family that cares for its own children is eligible for $0.

WILLIAM R. MATTOX JR. Director of Policy Analysis Family Research Council Washington