For generations of Americans, it was a given that children would live longer than their parents. But there is now mounting evidence that this enduring trend has reversed itself for the country’s least-educated whites, an increasingly troubled group whose life expectancy has fallen by four years since 1990.

Researchers have long documented that the most educated Americans were making the biggest gains in life expectancy, but now they say mortality data show that life spans for some of the least educated Americans are actually contracting. Four studies in recent years identified modest declines, but a new one that looks separately at Americans lacking a high school diploma found disturbingly sharp drops in life expectancy for whites in this group. Experts not involved in the new research said its findings were persuasive.

The reasons for the decline remain unclear, but researchers offered possible explanations, including a spike in prescription drug overdoses among young whites, higher rates of smoking among less educated white women, rising obesity, and a steady increase in the number of the least educated Americans who lack health insurance.

The steepest declines were for white women without a high school diploma, who lost five years of life between 1990 and 2008, said S. Jay Olshansky, a public health professor at the University of Illinois at Chicago and the lead investigator on the study, published last month in Health Affairs. By 2008, life expectancy for black women without a high school diploma had surpassed that of white women of the same education level, the study found.

White men lacking a high school diploma lost three years of life. Life expectancy for both blacks and Hispanics of the same education level rose, the data showed. But blacks over all do not live as long as whites, while Hispanics live longer than both whites and blacks.

“We’re used to looking at groups and complaining that their mortality rates haven’t improved fast enough, but to actually go backward is deeply troubling,” said John G. Haaga, head of the Population and Social Processes Branch of the National Institute on Aging, who was not involved in the new study.

The five-year decline for white women rivals the catastrophic seven-year drop for Russian men in the years after the collapse of the Soviet Union, said Michael Marmot, director of the Institute of Health Equity in London.

The decline among the least educated non-Hispanic whites, who make up a shrinking share of the population, widened an already troubling gap. The latest estimate shows life expectancy for white women without a high school diploma was 73.5 years, compared with 83.9 years for white women with a college degree or more. For white men, the gap was even bigger: 67.5 years for the least educated white men compared with 80.4 for those with a college degree or better.

The dropping life expectancies have helped weigh down the United States in international life expectancy rankings, particularly for women. In 2010, American women fell to 41st place, down from 14th place in 1985, in the United Nations rankings. Among developed countries, American women sank from the middle of the pack in 1970 to last place in 2010, according to the Human Mortality Database.

The slump is so vexing that it became the subject of an inquiry by the National Academy of Sciences, which published a report on it last year.

“There’s this enormous issue of why,” said David Cutler, an economics professor at Harvard who was an author of a 2008 paper that found modest declines in life expectancy for less educated white women from 1981 to 2000. “It’s very puzzling and we don’t have a great explanation.”

And it is yet another sign of distress in one of the country’s most vulnerable groups during a period when major social changes are transforming life for less educated whites. Childbirth outside marriage has soared, increasing pressures on women who are more likely to be single parents. Those who do marry tend to choose mates with similar education levels, concentrating the disadvantage.

Inklings of this decline have been accumulating since 2008. Professor Cutler’s paper, published in Health Affairs, found a decline in life expectancy of about a year for less educated white women from 1990 to 2000. Three other studies, by Ahmedin Jemal, a researcher at the American Cancer Society; Jennifer Karas Montez, a Robert Wood Johnson Foundation Health and Society Scholar at Harvard; and Richard Miech, a professor at the University of Colorado Denver, found increases in mortality rates (the ratio of deaths to a population) for the least educated Americans.

Professor Olshansky’s study, financed by the MacArthur Foundation Research Network on an Aging Society, found by far the biggest decline in life expectancy for the least educated non-Hispanic whites, in large part because he isolated those without a high school diploma, a group usually combined with high school graduates. Non-Hispanic whites currently make up 63 percent of the population of the United States.

Researchers said they were baffled by the magnitude of the drop. Some cautioned that the results could be overstated because Americans without a high school diploma — about 12 percent of the population, down from about 22 percent in 1990, according to the Census Bureau — were a shrinking group that was now more likely to be disadvantaged in ways besides education, compared with past generations.

Professor Olshansky agreed that the group was now smaller, but said the magnitude of the drop in life expectancy was still a measure of deterioration. “The good news is that there are fewer people in this group,” he said. “The bad news is that those who are in it are dying more quickly.”

Researchers, including some involved in the earlier studies that found more modest declines in life expectancy, said that Professor Olshansky’s methodology was sound and that the findings reinforced evidence of a troubling pattern that has emerged for those at the bottom of the education ladder, particularly white women.

“Something is going on in the lives of disadvantaged white women that is leading to some really alarming trends in life expectancy,” said Ms. Montez of Harvard.

Researchers offered theories for the drop in life expectancy, but cautioned that none could fully explain it.

James Jackson, director of the Institute of Social Research at the University of Michigan and an author of the new study, said white women with low levels of education may exhibit more risky behavior than that of previous generations.

Overdoses from prescription drugs have spiked since 1990, disproportionately affecting whites, particularly women. Professor Miech, of the University of Colorado, noted the rise in a 2011 paper in the American Sociological Review, arguing that it was among the biggest changes for whites in recent decades and that it appeared to have offset gains for less educated people in the rate of heart attacks.

Ms. Montez, who studies women’s health, said that smoking was a big part of declines in life expectancy for less educated women. Smoking rates have increased among women without a high school diploma, both white and black, she said. But for men of the same education level, they have declined.

This group also has less access to health care than before. The share of working-age adults with less than a high school diploma who did not have health insurance rose to 43 percent in 2006, up from 35 percent in 1993, according to Mr. Jemal at the American Cancer Society. Just 10 percent of those with a college degree were uninsured last year, the Census Bureau reported.

The shift should be seen against the backdrop of sweeping changes in the American economy and in women’s lives, said Lisa Berkman, director of the Harvard Center for Population and Development Studies. The overwhelming majority of women now work, while fertility has remained higher than in European countries. For women in low-wage jobs, which are often less flexible, this could take a toll on health, a topic that Professor Berkman has a grant from the National Institute on Aging to study.