Life expectancy in the United States dropped in 2015 for the first time since 1993, according to the Centers for Disease Control and Prevention.

The average life expectancy at birth decreased by 0.1 of a year compared to 2014, which equates to an average of a little under a month. While the change is not particularly large, the drop came as a surprise to many researchers, considering the general upward trend in life expectancy over the past few decades.

The recent drop reflects a number of different causes, according to health researchers. Some pointed to underlying societal trends in recent years, including so-called "diseases of despair," the opioid overdose epidemic, and suicides, reflecting a lack of economic opportunity and hopelessness among white working class Americans.

While the statistical drop last year is a surprise, they also point out that one bad year does not necessarily mean that numbers will continue to drop in the future. The report is mainly based on 2015 death certificates.

The National Center for Health Statistics (NHCS), a part of the CDC, said that life expectancy was 78.8 years in 2015, down from 78.9 in 2014. The cause is hard to pinpoint, with no one leading cause of death.

"This is unusual," lead author Jiaquan Xu, an epidemiologist at the NCHS, told the Agence France-Presse news agency. "2015 is kind of different from every year. It looks like much more death than we have seen in the last few years."

Of the 10 leading causes of death, eight increased in 2015, one stayed statistically the same, and one – cancer – actually decreased. Infant mortality rates, which are "generally regarded as a good indicator of the overall health of a population," according to the report, were basically unchanged from 2014 and therefore did not contribute to the overall drop in life expectancy.

Since there is no one smoking gun for the drop in life expectancy last year, experts cite a number of issues that should be addressed. Beyond treatment of diseases, there is a need to address income inequality and unemployment as well, factors that are associated with drug overdoses, poor nutrition, and suicides, as The Christian Science Monitor's Patrik Jonsson reported last year in a story about the increase in mortality rates of middle-age white Americans since 1999, caused in part by socioeconomic factors.

The study released this week [October 2105] by the Proceedings of the National Academy of Sciences found that the surprising rise in middle-age, white mortality in America from 1999 to 2013 was driven by a rise in suicide, drug abuse, and alcoholism. The trends were strongest among those with the least education and in the predominantly red South and West, with the authors suggesting a vicious cycle of physical pain and addiction to painkillers, compounded by fiscal uncertainty. “[M]any of the baby-boom generation are the first to find, in midlife, that they will not be better off than were their parents,” the authors write.... The Republican push to cut government spending and help business has meant “more risk has been shifting from employers and government to the backs of individuals and families,” says Mark Rank, a sociologist at Washington University in St. Louis and author of “Living on the Edge: The Realities of Welfare in America.” “That means we need to shift our framework from thinking of it as ‘them’ [who are needing help] to thinking it’s ‘us.’ The fact is, it’s middle America that is really experiencing some hard times.” ...The struggle of young people without a college degree in his rural town to find full-time work is a symptom both of economic and cultural tides. Work habits dull for the underemployed. Young, idle men struggle to find life partners, or even get their own place. In the absence of meaning, drugs and alcohol are a potent lure.

The CDC says the country is "in midst of an opioid overdose epidemic," with a record 28,000 people killed in 2014. No figures are yet available for 2015, though the 6.7 percent rise in deaths caused by "unintentional injuries" may be partly related, as the BBC reported.

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With the right social and economic reforms, more Americans could live longer and better lives, increasing overall life expectancy in the United States, say some observers.

"The health-care system is only a part of health," Ellen Meara, a professor at the Dartmouth Institute for Health Policy and Clinical Practice, told The Washington Post.