Only a handful of the nation's high-income taxpayers are exceptionally generous charitable givers, according to a study released yesterday by the Yale University Program on Non-Profit Organizations.

The study also found that the average low-income taxpayer gives a higher proportion of income to charity than the average middle-income person.

The study, carried out by Gerald Auten of Bowling Green State University, the U.S. Treasury's Office of Tax Analysis and Gabriel Rudney of Yale, was based on an analysis of federal tax returns for 1971-75. It focused largely on returns of those with income of at least $100,000 a year, but looked at taxpayers at all levels of income.

The study defined an exceptionally generous giver as one who donated at least one-fifth of his or her income to charity -- such as hospitals, churches, schools and medical research -- over the five-year period.

Only 4 percent of those with incomes of $100,000 or more fell into the generous-giver category in 1971-75.

However, 30 percent of these well-to-do taxpayers fell into the nongenerous category, which included those giving less than 1 percent of their incomes to charity over the five-year period.

Nonetheless, the study found that low-income taxpayers and the wealthiest, those earning more than $1 million a year, were both more generous than middle-income taxpayers.

For example, the taxpayer whose annual income averaged $5,000 to $10,000 from 1971 to 1975 gave 3.5 percent of it to charity. Those earning $20,000 to $50,000 gave 2.2 percent, while those with incomes of $1 million or more averaged 6.4 percent.

The study also found that wealthy givers have highly volatile donation patterns, sometimes giving a high proportion of income and sometimes a small proportion.

It had once been thought that their patterns were fairly steady and changed largely in response to modifications in tax incentives and personal income levels. Rudney said other reasons why wealthy taxpayers may change their contribution levels include a shift in need for operating funds and capital for a business; luxury consumption patterns; plans for leaving the money after death; gifts to relatives such as wedding presents, and a desire to take advantage of investment opportunities with funds that otherwise might have been donated.

Although only a small proportion of high-income givers are exceptionally generous, the over-$100,000 class as a whole in 1971-75 was extremely important to charitable institutions because those taxpayers accounted for nearly a fifth of all charitable contributions.