new york - Humanity is hitting the upper limit of life expectancy, according to a new study.

Advances in medical technology and genetic research - not to mention larger numbers of people making it to age 100 - are not translating into marked jumps in lifespan overall, according to researchers who found shrinking longevity increases in countries with the longest-living populations.

'We have to recognize there's a limit' and perhaps reassess assumptions about when people should retire and how much money they'll need to live out their lives, said S. Jay Olshansky, a University of Illinois-Chicago researcher who was lead author of the study published Monday by the journal Nature Aging.

Mark Hayward, a University of Texas researcher not involved in the study, called it 'a valuable addition to the mortality literature.'

'We are reaching a plateau' in life expectancy, he agreed. It's always possible some breakthrough could push survival to greater heights, 'but we don't have that now,' Hayward said.

What is life expectancy?

Life expectancy is an estimate of the average number of years a baby born in a given year might expect to live, assuming death rates at that time hold constant. It is one of the world's most important health measures, but it is also imperfect: It is a snapshot estimate that cannot account for deadly pandemics, miracle cures or other unforeseen developments that might kill or save millions of people.

In the new research, Olshansky and his research partners tracked life expectancy estimates for the years 1990 to 2019, drawn from a database administered by the Max Planck Institute for Demographic Research. The researchers focused on eight of the places in the world where people live the longest - Australia, France, Hong Kong, Italy, Japan, South Korea, Spain and Switzerland.

The U.S. doesn't even rank in the top 40. But is also included 'because we live here' and because of past, bold estimates that life expectancy in the U.S. might surge dramatically in this century, Olshansky said.

Who lives the longest?

Women continue to live longer than men and life expectancy improvements are still occurring - but at a slowing pace, the researchers found. In 1990, the average amount of improvement was about 2½ years per decade. In the 2010s, it was 1½ years - and almost zero in the U.S.

FILE - Seniors practice tai chi in a cultural center in Brussels, June 28, 2017.

The U.S. is more problematic because it is harder hit by a range of issues that kill people even before they hit old age, including drug overdoses, shootings, obesity and inequities that make it hard for some people to get sufficient medical care.

But in one calculation, the researchers estimated what would happen in all nine places if all deaths before age 50 were eliminated. The increase at best was still only 1½ years, Olshansky said.

Eileen Crimmins, a University of Southern California gerontology expert, said in an email that she agrees with the study's findings. She added, 'For me personally, the most important issue is the dismal and declining relative position of the United States.'

Why life expectancy may not be able to rise forever

The study suggests that there's a limit to how long most people live, and we've about hit it, Olshansky said.

'We're squeezing less and less life out of these life-extending technologies. And the reason is, aging gets in the way,' he said.

It may seem common to hear of a person living to 100 - former U.S. President Jimmy Carter hit that milestone last week. In 2019, a little over 2% of Americans made it to 100, compared with about 5% in Japan and 9% in Hong Kong, Olshansky said.

It's likely that the ranks of centenarians will grow in the decades ahead, experts say, but that's because of population growth. The percentage of people hitting 100 will remain limited, likely with fewer than 15% of women and 5% of men making it that long in most countries, Olshansky said.