Every one of us has wondered at least once how many years we'll live. In 1996, nearly two and a half thousand Japanese people over sixty answered that question in a national survey. Their answers became a kind of mirror reflecting hopes, fears, and habits of thought. Twenty-eight years later, researchers compared those forecasts with actual dates of death and found that the mirror was warped, but not useless.
It turned out that each additional year in one's own forecast lowered the risk of death by 1.6-1.7%. In other words, those who gave themselves more years really did live longer. But the link was weak: if a person added a year to their forecast, actual life lengthened by only 0.12 years on average. Most participants, especially women and people with a university education, lived longer than they had assumed. The probability of outliving one's forecast was 52.8 percentage points higher for women, and each additional year of education added another 1.5 points.
The study was the first to show that a simple point question about one's desired age at death has real predictive power, even though it is systematically biased. Previously, such work was done mainly on Western samples and with probabilistic methods, which are harder for older people. The Japanese context, with its record longevity, adds weight to the results. The patterns identified are not just a statistical quirk but a direct signal for pension advisors and developers of educational programs.
The warped mirror we look into when planning old age can be straightened a little. If we know that women and educated people tend to underestimate their time, while the less educated round off and dodge the question, we can help each group in a targeted way. An accurate longevity forecast is not fortune-telling but a tool that helps people avoid running out of money in their final years.
Nearly 40% of respondents named a "round" age, a multiple of 5 or 10 years, and the most popular answer was 80 years, exactly how long Japanese people lived on average in 1996. Without knowing it, people were using birth statistics as an anchor for their own fate.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.