The rich pay less and the poor pay more in taxes as a proportion of their income than they did two decades ago, according to a Brookings Institution study released yesterday.

However, the impact of that change has been offset to some extent by government aid, which has redistributed income in the other direction -- and more efficiently, the study found.

In one of the most sweeping efforts to date to count who gets what share of income and how much of it goes for taxes, Brookings economist Joseph A. Pechman found three factors responsible for the gradual decline in progressivity in the tax system.

The corporate tax and property taxes both have become less important, which ultimately reduces the tax burden on higher-income families, according to the study. The personal and business tax cut of 1981 exacerbated that shift, the study said. On the other hand, payroll taxes -- principally for Social Security, which is not an economically progressive tax -- have increased, placing a larger share of the tax burden on lower-income taxpayers.

The poorest one-tenth of the population paid 16.8 percent of income in federal, state and local taxes in 1966, according to one set of assumptions used by the study. By 1985, that will have risen to an estimated 21.9 percent. Other assumptions yield only slightly different results.

The top tenth of taxpayers, on the other hand, paid 30.1 percent of income in taxes in 1966 and an estimated 25.3 percent in 1985. The share of taxes paid by the second-lowest tenth also rose, while the share of the taxpayers in the middle changed very little.

This happened despite the fact that, before taxes, the distribution of income by class was almost unchanged during the 19-year period covered by the book.

In 1980, families earning more than $44,900, the top 20 percent of taxpayers, made 48.7 percent of all income. The top 1 percent, earning more than $175,000, received 11 percent of the money. On the bottom, the 20 percent making less than $11,850 pulled in 4.1 percent of all U.S. income.

"The distribution of income before taxes was virtually the same in 1985 as it was in 1966," Pechman wrote in "Who Paid the Taxes, 1966-1985." "As a result of the decline in progressivity of the tax system, the distribution of income after taxes was more unequal in 1985."

Some of Pechman's results were surprising: Even though the distribution of before-tax income has remained roughly the same, the poor would have lost ground if it hadn't been for such government transfers as Social Security and food stamps.

In other words, wages, salaries and investment income alone are expected to yield less equal results in 1985 than in 1966. Transfer payments appear to be the only reason why poorer families maintained their share of national income.

The explanation, Pechman suggested, may be partly because the unemployment rate is higher now than it was in the 1960s. Joblessness averaged 3.8 percent in 1966 and 7.5 percent in 1984. In addition, he said, the increase in two-earner families probably raised income in the higher brackets, while the rise in the number of single parents reduced it in lower categories.

The fact that government programs have offset this inequality means that further, proposed cuts in transfer programs would make the poor even worse off, Pechman said.