John Bacon

USA TODAY

If only the good die young, Americans are unfortunately getting better.

U.S. life expectancy dipped by a little more than a month last year from 2014, to 78.8 years, according to a report from the National Center for Health Statistics. It's the first decline in more than two decades. And after years of gains, U.S. life expectancy has been essentially flat for a few years, which means an inauspicious trend could be in the works.

The decline "could be a blip, but even if it’s flat, we have a real problem," said Jonathan Skinner, a professor at the Dartmouth Institute for Health Policy & Clinical Practice.

The culprits for our declining years, the report says, were increases in mortality from heart disease, chronic lower respiratory diseases, unintentional injuries, stroke, Alzheimer’s disease, diabetes, kidney disease and suicide. Not surprisingly, that group plus cancer and the flu make up the top 10 causes of death in the U.S.

Report author Jiaquan Xu said the decline is the first since 1993, when AIDS and a brutal flu epidemic swept the nation. This time, he says, the cause is not so clear.

He did highlight a 3% increase in "unintentional injuries." The heading includes, among other things, traffic accidents and drug overdoses — both of which often involve relatively young victims whose deaths can have a strong impact on the numbers.

Still, heart disease and cancer are the runaway top killers. The death rate from heart disease increased almost 1%. The death rate from cancer actually fell 1.7%.

"Obesity has to be a major factor," said S. Jay Olshansky, a public health researcher at the University of Illinois School of Public Health. "Heart disease, stroke, diabetes — the impact of obesity is across the board. And people are living long enough to show the long-term effects."

Gender matters. For males, life expectancy fell to 76.3 years from 76.5 years. For women, life expectancy decreased to 81.2, down about a month from 2014.

"My major concern is that we know the exact cause of the fall in life expectancy — mortality is rising across a wide variety of illnesses," said Skinner, who was not involved in the study. "It’s not entirely easy to figure out what to do about it."

The march toward a higher life expectancy has been pretty consistent for decades thanks to gains in medical and public health knowledge. And, big picture, the news isn't all bad. Babies born in 2015 are still expected to live about two years longer than babies born in 2000 were expected to live. And a 2015 baby's life expectancy is more than 10 years longer than what those who joined the world in 1950 faced.

The latest numbers technically mean someone born in 2015 is expected to live an average of 78.8 years, although those who turned 65 in 2015 had brighter prospects. If you made it that far, you're expected to live an average of another 19.4 years.

For the record, there were more than 2.7 million U.S. deaths in 2015, 86,212 more than the previous year and a 1.2% age-adjusted increase. That's the first significant increase since 1999, Xu said. Again, he could provide no definitive reason why.

The United States ranked 43rd out of 224 countries for life expectancy in the CIA World Factbook 2015. Leading the pack by far was Monaco, with an estimated life expectancy of 89.5 years. Japan, Singapore, Macau and San Marino rounded out the top five. Most of Western Europe also ranked ahead of the U.S.

Skinner noted that the U.S. is spending 18% of GDP on health care.

"If we’re not getting continued improvement in life expectancy, then perhaps we should redirect some of that towards raising workers’ take-home pay ... and improving educational opportunities," Skinner said.