Murray and his team also found counties where, even as wealth increased, life expectancies slipped backward or barely moved.

In Calvert County, a fast-growing area, women's life expectancy was almost 21/2 months shorter in 2007 than in 1997. Yet over the past decade, the median household income in the county increased more than 30 percent, far outpacing the nation, where the median income grew less than 20 percent.

The research is being published in the journal Population Health Metrics.

Life expectancies for black men and women improved faster than for whites since 1987, although blacks continue to have lower life expectancies overall than whites do, the data showed.

Nationwide, women's life expectancy at birth in America hit 81.3 years in 2007, placing the country 35th in the world. That's down from 20th in 1987, according to United Nations data.

Men's life expectancy was 76.7 years, or 24th best, up from 32nd two decades earlier.

The backsliding for women began before 1997, but researchers found that it has accelerated in the past decade. Only 227 counties saw women's life expectancy decline between 1987 and 1997, according to the study.

The grim trend is fueled largely by the persistent influence of smoking, high blood pressure and obesity, according to Murray and other population health experts.

American women historically smoked more heavily than women in other countries, particularly after World War II, said Samuel Preston, a University of Pennsylvania demographer who co-chaired a 2011 National Academies panel that looked at life expectancies in high-income countries.

That had a delayed effect that drove up lung cancer rates among women as this generation aged. The trend may ease as that age group passes and the effects of more recent efforts to reduce smoking are felt, Preston said.

But Preston cautioned that the impact of other unhealthy lifestyles may undermine that progress. "In place of smoking, we have substituted obesity," he said.

The proportion of Americans who are classified as obese hit 34 percent in 2010, more than double the rate in 1980.

Women in southwest Florida's Collier County live to be 86 years old on average, tops in the nation. Collier is among a cluster of counties in South Florida with high life expectancies.

Twelve states, including Pennsylvania, Connecticut and the rest of New England, have no counties where life expectancies slipped backward.

Life expectancies for men and women in Los Angeles County and Cook County, Ill., which includes Chicago, both rose in the last two decades.

Los Angeles County has among the highest life expectancies in the country despite a poverty rate above the national average. Countywide, women live more than a year longer than they do nationally, and men live more than eight months longer on average.

This may be evidence of what demographers and public health officials call the "Hispanic paradox," a long recognized phenomenon in which Latino immigrants are generally healthier than non-Latinos of similar income.

Nearly half of Los Angeles County's 10 million residents are Latino, and more than a third are foreign-born, according to census data. By contrast, less than a sixth of the population nationally is Latino, and less than an eighth is foreign-born.

One explanation of that phenomenon is that the people who become immigrants tend to be healthy. "These are not random people. They are the healthiest people who could get here," said Carmen Nevarez, former president of the American Public Health Association.

But David Hayes-Bautista, who heads the Center for the Study of Latino Health and Culture at UCLA, said other factors, including social support networks, diet and even physical labor, may play a role as well, because not all immigrants have as good health outcomes as Latinos.

Some rural parts of the nation also have done well. Indeed, some of the highest life expectancies in the country are found in rural parts of Minnesota, Wisconsin and Iowa. "These are not wealthy communities," Murray said, noting that many benefit from a "cohesive community" that may improve health outcomes.

Sun reporter Meredith Cohn contributed to this article.

noam.levey@latimes.com