Older adults in Japan may predict their longevity surprisingly well

How old do you expect to live?

Your response could reveal more than you might think.

A study following older adults in Japan for as long as 28 years indicates that people can judge their own longevity with surprising accuracy. Participants who anticipated living longer tended, in general, to do exactly that.

Yet their estimates contained a notable bias: many people underestimated the time still ahead of them. Of the participants whose outcomes could be established, the researchers found that approximately 59 percent lived beyond the age they had expected to reach.

Longevity expectations among older adults in Japan

A team led by University of Tsukuba social scientist Shohei Okamoto examined figures from the Japanese Aging and Health Dynamics Study, a nationally representative survey of people aged 60 and over throughout Japan.

In 1996, researchers asked 2,447 participants the age they believed they would reach. While 393 participants said they did not know, the other 2,054 gave estimates between 63 and 120.

During the following 28 years, 1,514 people who had provided an estimate died. Official resident records or reports from family members supplied approximately 98 percent of the death dates, enabling the team to compare expected lifespans with actual ones.

Those who believed they had more years remaining generally lived longer. This relationship persisted after the researchers adjusted for age, sex, socioeconomic characteristics and health-related factors.

Does that mean that expecting to live for a long time can itself help someone live longer?

The study, which is available as a preprint and has not yet undergone peer review, cannot answer this. As an observational study, it cannot demonstrate that anticipating a longer life lengthens a person's lifespan, or that positive thinking keeps people alive.

Rather, individuals may be aware of aspects of their health, behaviour or family history that influence both their predictions and how long they live. For instance, a person who feels healthy may expect a longer life and also be more likely to have one.

Although the team adjusted for various health factors, characteristics that were not measured may still have influenced both anticipated and actual lifespans.

Nor were these forecasts personalised countdowns. In the team's statistical model, every extra year a participant expected to live was linked with only about 0.12 additional years of estimated survival.

Why many participants underestimated their remaining lifespan

The degree of error differed between groups. Women and people with more years of education were more likely to exceed their own expectations, whereas those in worse health were less likely to do so.

One potential explanation centres on a familiar age: 80.

A large number of participants selected 80 as the age they expected to attain. This was close to Japan's life expectancy at birth for both sexes in 1996, which suggests that people may have based their estimates on a widely recognised population average.

However, life expectancy at birth differs from the remaining life expectancy of someone who has already reached later life. Someone who has made it to age 70 has already passed the early-life risks included in an average calculated at birth.

A calculation based on an individual's present age and sex could therefore be more useful than one figure for the entire population. A general average may also hide the fact that women and people with more education tend to live longer. Nevertheless, the study does not show that using 80 as an anchor caused the inaccuracies.

The education result had a further complication. Participants with more years of education were less inclined to choose ages divisible by 5, including 75, 80 and 85. Still, less rounded estimates were not more accurate: these participants also frequently underestimated how long they would live.

Financial consequences of misjudging life expectancy

Getting longevity wrong may matter for more than simple curiosity about the future.

"Underestimating life expectancy may lead to insufficient retirement funds, while overestimating it could result in unnecessary frugality or even poverty," the researchers write.

The team proposes that clearer details about remaining life expectancy, based on a person's current age and sex, could support better financial decisions.

There are significant limitations to the research. Participants gave their lifespan estimates just once, in 1996, meaning the researchers could not observe how a diagnosis, medical developments or the death of somebody close might alter their expectations.

The dataset also did not include detailed information on how long participants' parents had lived. Furthermore, because the research involved only older people in Japan, its findings cannot automatically be extended to younger people or populations in other places.

The findings do not indicate that people can lengthen their lives merely by expecting to live longer. However, our predictions may hold clues based on what we know, either consciously or unconsciously, about our health and circumstances.

Even so, most participants overlooked one key possibility: their future might last longer than they had imagined.

The study is available on medRxiv.

This article was fact-checked by Michael Irving and edited by Michael Irving. While we take pride in our process, we are only human. If you notice an error, please let us know.