Declines in death rates from the four leading types of cancer in the United States since the early 1990s have been driven largely by progress among college-educated men and women, researchers said on Tuesday.

The study, published in the Journal of the National Cancer Institute, was the latest to illustrate how a person's health can be closely tied to socioeconomic factors such as education and income level.

Researchers at the American Cancer Society and Emory University in Atlanta calculated death rates for lung, breast, prostate and colorectal cancer by level of education among U.S. blacks and whites ages 25 to 64 for 1993 through 2001.

Death rates for each of these types of cancer decreased from 1993 to 2001 in men and women with at least 16 years of education - a college degree - except for lung cancer among black women, for whom death rates were stable, they found.

By contrast, among people with less than 12 years of education - those who did not finish high school - a statistically significant decrease in death rates during the same period was registered only for breast cancer among white women, according to the study.

Death rates among people with less than 12 years of education increased for lung cancer in white women and for colon cancer in black men and were stable for the rest of the cancer types, the researchers said.

"The recent reductions in death rates from major cancers in the United States have bypassed less-educated working-age people, suggesting that persons in lower socioeconomic groups have not yet benefited equivalently from recent advances in prevention, early detection and treatment of the major fatal cancers," the researchers wrote.

The researchers offered several possible explanations.

They noted that less-educated adults are more likely to smoke. They added that in breast cancer, less-educated women may be less likely to get a mammogram that could provide early detection of the disease. And less-educated people may be less likely to get colorectal cancer screening tests.

Less-educated people may be less likely to get the best types of cancer treatment, they added.

Income levels track closely with education levels, they noted, and many lower income people are less likely to have health insurance than people who are more highly paid.