The great majority of African Americans experience poverty during adulthood, Cornell University and Washington University researchers report. Their startling new findings show that nine out of every 10 black Americans, or 91 percent, who reach the age of 75 spend at least one of their adult years in poverty.

This compares with the study's equally startling findings that "on average 60 percent of all American adults will experience at least one year of living below the poverty line, whereas one third will experience dire poverty." However, the statistics on black American poverty contrast sharply with the findings that by age 75, slightly more than half (52.6 percent) of white Americans will have spent one of their adult years below the poverty line, very close to results the researchers expected. Age 75 is the average American life expectancy.

"When we began this research, we didn't know how the numbers would turn out," says Thomas A. Hirschl, Cornell professor of rural sociology, a department in the university's New York State College of Agriculture and Life Sciences. He and his collaborator, Professor Mark Rank, of Washington University in St. Louis, will publish their report, "The Likelihood of Poverty Across the American Lifespan," in the May issue of the journal Social Work. The study represents the first time that such figures have been calculated within the social and behavioral sciences.

The study says it portrays poverty as a mainstream problem and not merely as an affliction of the underclass. Although the official poverty rate during the 25-year period studied ranged from 11 percent to 15 percent, the percentage of Americans falling into poverty at some point in their adults lives was actually much greater.

The study was based on a nationally representative data sample of 4,800 households and 18,000 individuals, gathered by the Panel Study of Income Dynamics, at the University of Michigan. "The fact that one of two white Americans are eventually touched by poverty is a sizable percentage indeed," according to the report. "Nevertheless, it pales in comparison to the enormity of poverty's grasp within the black population."

By the age of 25, the findings show, about 48.1 percent of black Americans will have experienced at least one year in poverty. By age 40, the number grows to two-thirds, to more than three-fourths by age 50, and more than 90 percent will have lived below the poverty line by age 75.

The researchers say that by age 28, the black population will have reached the cumulative level of lifetime poverty that the white population arrives at by age 75. "In other words, blacks have experienced in nine years the same risk of poverty that whites [experience] in 56 years," the report stated.

The federal government's latest available figure, from 1997, defines poverty as an annual income of $16,276 for a family of four, including two children. Dire poverty is defined by the government as a family with income one-half that of the poverty level. The figures for single individuals are proportionally adjusted for these amounts.

By age 30, according to the report, one-third of black Americans will have spent at least one year of their adult life in dire poverty, rising to about 50 percent by age 45 and more than 66 percent by age 75. In contrast, only 26.6 percent of white Americans will have lived at the dire-poverty level by the age of 75.

''The fact that virtually every [black] American will experience poverty at some point during their adulthood speaks volumes as to the economic meaning of being black in America," noted the report.

The usefulness of the report, Hirschl says, is the hope "that social workers continue to work to ameliorate the conditions that poverty makes, but we also hope to provide rationale for social workers getting involved in poverty policy. Among the industrialized nations of the world, the United States has the least-effective policy for fighting poverty."

The research was partially funded by a grant from the U.S. Department of Agriculture, Cornell's New York State Agricultural Experiment Station in Ithaca and federal Hatch funding.