He and another researcher, Mohammad Siahpush, a professor at the University of Nebraska Medical Center in Omaha, developed an index to measure social and economic conditions in every county, using census data on education, income, poverty, housing and other factors. Counties were then classified into 10 groups of equal population size.

In 1980-82, Dr. Singh said, people in the most affluent group could expect to live 2.8 years longer than people in the most deprived group (75.8 versus 73 years). By 1998-2000, the difference in life expectancy had increased to 4.5 years (79.2 versus 74.7 years), and it continues to grow, he said.

After 20 years, the lowest socioeconomic group lagged further behind the most affluent, Dr. Singh said, noting that “life expectancy was higher for the most affluent in 1980 than for the most deprived group in 2000.”

“If you look at the extremes in 2000,” Dr. Singh said, “men in the most deprived counties had 10 years’ shorter life expectancy than women in the most affluent counties (71.5 years versus 81.3 years).” The difference between poor black men and affluent white women was more than 14 years (66.9 years vs. 81.1 years).

The Democratic candidates for president, Senators Hillary Rodham Clinton of New York and Barack Obama of Illinois, have championed legislation to reduce such disparities, as have some Republicans, like Senator Thad Cochran of Mississippi.

Peter R. Orszag, director of the Congressional Budget Office, said: “We have heard a lot about growing income inequality. There has been much less attention paid to growing inequality in life expectancy, which is really quite dramatic.”

Life expectancy is the average number of years of life remaining for people who have attained a given age.

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While researchers do not agree on an explanation for the widening gap, they have suggested many reasons, including these:

¶Doctors can detect and treat many forms of cancer and heart disease because of advances in medical science and technology. People who are affluent and better educated are more likely to take advantage of these discoveries.

¶Smoking has declined more rapidly among people with greater education and income.

¶Lower-income people are more likely to live in unsafe neighborhoods, to engage in risky or unhealthy behavior and to eat unhealthy food.

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¶Lower-income people are less likely to have health insurance, so they are less likely to receive checkups, screenings, diagnostic tests, prescription drugs and other types of care.

Even among people who have insurance, many studies have documented racial disparities.

In a recent report, the Department of Veterans Affairs found that black patients “tend to receive less aggressive medical care than whites” at its hospitals and clinics, in part because doctors provide them with less information and see them as “less appropriate candidates” for some types of surgery.

Some health economists contend that the disparities between rich and poor inevitably widen as doctors make gains in treating the major causes of death.

Nancy Krieger, a professor at the Harvard School of Public Health, rejected that idea. Professor Krieger investigated changes in the rate of premature mortality (dying before the age of 65) and infant death from 1960 to 2002. She found that inequities shrank from 1966 to 1980, but then widened.

“The recent trend of growing disparities in health status is not inevitable,” she said. “From 1966 to 1980, socioeconomic disparities declined in tandem with a decline in mortality rates.”

The creation of Medicaid and Medicare, community health centers, the “war on poverty” and the Civil Rights Act of 1964 all probably contributed to the earlier narrowing of health disparities, Professor Krieger said.

Robert E. Moffit, director of the Center for Health Policy Studies at the conservative Heritage Foundation, said one reason for the growing disparities might be “a very significant gap in health literacy” — what people know about diet, exercise and healthy lifestyles. Middle-class and upper-income people have greater access to the huge amounts of health information on the Internet, Mr. Moffit said.

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Thomas P. Miller, a health economist at the American Enterprise Institute, agreed.

“People with more education tend to have a longer time horizon,” Mr. Miller said. “They are more likely to look at the long-term consequences of their health behavior. They are more assertive in seeking out treatments and more likely to adhere to treatment advice from physicians.”

A recent study by Ellen R. Meara, a health economist at Harvard Medical School, found that in the 1980s and 1990s, “virtually all gains in life expectancy occurred among highly educated groups.”

Trends in smoking explain a large part of the widening gap, she said in an article this month in the journal Health Affairs.

Under federal law, officials must publish an annual report tracking health disparities. In the fifth annual report, issued this month, the Bush administration said, “Over all, disparities in quality and access for minority groups and poor populations have not been reduced” since the first report, in 2003.

The rate of new AIDS cases is still 10 times as high among blacks as among whites, it said, and the proportion of black children hospitalized for asthma is almost four times the rate for white children.

The Centers for Disease Control and Prevention reported last month that heart attack survivors with higher levels of education and income were much more likely to receive cardiac rehabilitation care, which lowers the risk of future heart problems. Likewise, it said, the odds of receiving tests for colon cancer increase with a person’s education and income.