People with more than a high school diploma can expect to live up to seven years longer than their less-educated counterparts, a study from Harvard Medical School and Harvard University suggests.

Additionally, the Boston-based study found that the gap in life expectancy between education groups has been growing in the past two decades.

"We like to think that as we as a country get healthier, everyone benefits," said study co-author David Cutler, Harvard University's dean for social sciences at the Faculty of Arts and Sciences, in a release. "Here we've found that you can have a rising tide that only lifts half the boats — and the ones lifted are the ones doing better to begin with."

To predict life expectancies, Cutler and Ellen Meara, assistant professor of health-care policy at Harvard Medical School, combined death certificate data with census population estimates and data from the National Longitudinal Mortality Study, looking at the years 1981 through 1988 and 1990 through 2000. Researchers looked only at "non-Hispanic blacks and whites to limit the impact of immigration on estimates."

The study, published in the March/April edition of the journal Health Affairs, found that  "between the 1980s and 2000, life expectancy increases occurred nearly exclusively among high-education groups."

From 1980 to 1990, highly educated individuals increased their life expectancy by nearly a year and a half, while it grew only half a year for less-educated people, the authors report. The gap widened even more in the next 10 years, as the better-educated group gained 1.6 years while the other group's life expectancy remained unchanged.

As of 2000, the better-educated group could expect to live to age 82, while the other group's expectancy remained at 75.

A well-educated white man lived on average 5.8 years longer than his less-educated counterpart in 1990, and the difference had grown to 7.8 years by 2000. For black men, the gap in 2000 was 8.4 years, up 1.3 years from 1990.

By 2000, educated white women could expect to live five years longer than women who did not finish high school.

The researchers said that the longevity gap appears to be caused in part by better access to information about diseases and medical advances.

"As information about how to live longer, healthier lives becomes available and technologies become available to help you do things like quit smoking or lead a less sedentary lifestyle, we have to some extent figured out successful ways to do this," Meara told AFP. "But we've only brought it to certain parts of the population."

Smoking was another prime cause of the gap, the study said, citing the fact that while adult cigarette consumption has fallen by half in the U.S. since 1964, much of the decline occurred among the more educated group.

For example, lung cancer and chronic obstructive pulmonary disorder, which includes bronchitis and emphysema, accounted for a quarter of the gap for women over age 45, the study found, adding the disparity was more moderate with men.

"The growing education-related gap in mortality for smoking-related causes supports the long-standing paradox that prevention can widen disparities in health across education and income groups," the study said.

The study also noted that obesity is more common among the less educated and "recent research suggests that obesity might contribute to nearly as many deaths as tobacco does."

The authors conclude that "larger and better-targeted efforts to push successful health interventions into less-educated groups" may be needed to help close the education-related longevity gap.