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Lifespan vs life expectancy
One is about an individual life. The other is an average built from death rates. They answer different questions.
Lifespan is the length of an actual life. Life expectancy is a calculated average: the mean number of further years lived by a hypothetical group of people who experience a given set of age-specific death rates for the rest of their lives. One is an observation about a person; the other is arithmetic performed on a population's mortality in a particular year.
Why life expectancy is not a forecast
The standard published figure is a period life expectancy. It assumes that the death rates observed in one year stay fixed for the rest of everyone's lives. Actual death rates can change over time, so the figure describes the mortality conditions of a single year rather than the years a person will go on to live. It is a summary of current conditions, not a projection of a life.
In plain English
- Period life expectancy
- The standard figure, calculated as if the death rates observed in one year stayed fixed for the rest of everyone's lives.Why it matters: Death rates will change, so the number is a summary of current conditions rather than a projection.
- Cohort life expectancy
- An alternative figure that follows an actual birth cohort and allows death rates to change over its lifetime.Why it matters: For cohorts still alive, the final value cannot yet be observed and therefore requires assumptions about future mortality.
Swipe the table sideways to see all columns.
| Measure | Question it answers | Main limitation |
|---|---|---|
| Lifespan | How long did this particular person live? | A single case says nothing about the population around it. |
| Maximum lifespan | What upper limit or longest observed length of life is being discussed? | A different question from average life expectancy, and defined in more than one way. |
| Period life expectancy | Under this year's death rates, how many further years would a group average? | Assumes conditions never change, so it is not a prediction. |
| Cohort life expectancy | How long will people born in a given year actually live on average? | Only fully known once the cohort has died; otherwise it rests on projections. |
Why historical figures mislead
Where a life expectancy at birth of around 40 is quoted for a past population, it does not follow that adults typically died at 40. An average at birth is pulled down by deaths in infancy and childhood, and where child mortality was high the average sits well below the ages reached by those who survived childhood. Averages hide the shape of the distribution behind them.
Three claims that misuse the number
- "Life expectancy here is 82, so I have until 82." The figure is an average for a group, and the figure that applies to someone already alive at a later age is a different one.
- "Life expectancy fell this year, so people are dying younger." A period figure moves with the year's death rates; a single-year shock changes the summary without describing anyone's completed life.
- "Humans used to live to 40." That confuses an average pulled down by child deaths with the length of adult lives.
The figure that applies to a person who has already reached a given age is their remaining life expectancy, which behaves differently from the number quoted at birth. Why life expectancy changes with age.