Murray says three factors are largely responsible for the declines in life expectancy -- smoking, obesity and high blood pressure.
"The places where we're seeing drops in life expectancy often have the biggest obesity epidemics," he said.
"The increase in obesity rates are certainly an issue, since being overweight or obese can influence a whole variety of potentially serious and even fatal diseases -- coronary artery disease, diabetes, cerebrovascular disease, chronic renal disease, osteoarthritis to name just a few," said Dr. Henry Black, clinical professor of internal medicine at New York University's Langone School of Medicine.
But some experts say poor access to health care is to blame. A recent study by two faculty members at Columbia University's Mailman School of Public Health found risk factors such as smoking and obesity are not the main culprits in decreasing life expectancy.
"Weaknesses in the health care system such as a rising number of uninsured and fragmentation of care [play] a larger role," said Karen Davis, president of the Commonwealth Fund, a foundation devoted to health care research.
Murray's report also did not find any link to income or the recession.
"There are several examples of counties where life expectancy went down and income went up," Murray said. "It's probably better to have a higher income, but that's not really much of this story."
"One can't easily blame the recent recession for these data, since though we suspect that our citizens are postponing seeing a health care professional since the recession began, I doubt that would be a factor large enough to influence life expectancy this quickly," said Black.
Health policy experts say by focusing on preventing chronic diseases, the U.S. can save valuable health care dollars and the population's health can improve over time.
"We have made progress recently in reducing rates of smoking, but rates of obesity are getting worse," said Robert Field, professor of health management and policy at Drexel University School of Public Health in Philadelphia. "The diseases that these factors cause can take decades to develop, so we won't see improvements in life expectancy for a while."
Life expectancy is also one factor that can determine how successful the health care reform law is.
"If health care reform is to be a success, it should be doing something about our health," said Murray. "Reversing this trend is essentially what reform should be judged against."