Period vs Cohort Life Expectancy

Why Do Period and Cohort Life Expectancy Give Different Numbers?

A financial planner tells you to budget for 30 years of retirement, based on a lifespan of 85. You check a government website. It says 79. Both figures are correct. One is a period calculation. The other is a cohort projection. The gap between them explains why you get two answers, and which one you should actually use.

Period life expectancy takes death rates from a single year and applies them across every age at once. It answers: "If the conditions of this year stayed frozen forever, how long would a newborn live on average?"

Cohort life expectancy follows a real group born in the same year, tracks the death rates they actually experienced as they aged, and projects future improvements for the years they still have ahead. It answers: "What is the average lifespan likely to be for people born in this specific year, given that conditions change over time?"

Both are averages. Neither predicts your individual lifespan. But they answer different planning questions.

Period Life Expectancy: A Snapshot of Today's Death Rates

Period life expectancy is the figure you see in news headlines. The UK Office for National Statistics (ONS) publishes it annually. It is calculated as soon as the year's death data is in. That timeliness is its strength. Its weakness is that it assumes nothing changes for the next 80 years.

If a pandemic hits in 2020, period life expectancy for 2020 drops sharply, because the calculation assumes those elevated death rates will persist forever. It does not credit the fact that a vaccine arrived in 2021. Period figures react instantly to a bad year, which makes them volatile and, for planning purposes, overly pessimistic about the long term.

Cohort Life Expectancy: Tracking a Real Generation

Cohort life expectancy for someone born in 1960 cannot be fully calculated until the last person born in 1960 has died. That is decades away. So cohort figures for living generations are always part projection. Actuaries at the ONS and the Institute and Faculty of Actuaries model future mortality improvements based on past trends, medical advances, and behavioural changes.

Cohort figures are usually higher than period figures in countries where health is improving. If you are a 40-year-old in the UK today, your cohort life expectancy is higher than what period life expectancy would suggest, because period figures do not credit the medical advances that will occur over the next 40 years. The ONS now emphasises cohort figures more than it historically did, precisely because period calculations underestimate how long today's adults will live.

How Period vs Cohort Life Expectancy Plays Out for Two Birth Years

Take a baby born in the UK in 1960. The period life expectancy at birth for that year was about 71 years for a boy. That figure assumed the death rates of 1960 would persist. In reality, the cohort born in 1960 experienced falling infant mortality, declining heart disease deaths, and better cancer treatment. Their actual average lifespan will be higher than 71. The cohort figure for the 1960 birth year, projected, is around 77 for men.

Now take a baby born in 2020. The period life expectancy at birth is about 79 years for a boy. But if health continues improving, the cohort figure for the 2020 birth year is projected to be around 83. That is four extra years that the period snapshot misses.

The gap between period and cohort figures is roughly the rate of mortality improvement multiplied by the average remaining lifespan. In a country where death rates are falling by 1 percent per year, a newborn's cohort figure will be several years higher than the period figure.

Which One Should You Use for Planning?

For personal financial planning, use cohort figures. Period figures will understate how long you are likely to live, which means you risk outliving your savings. The Society of Actuaries and the UK Institute and Faculty of Actuaries both use cohort-based mortality tables for pension and annuity pricing. Insurance companies do not use period figures to set premiums. They know their clients will live longer than the snapshot suggests.

For public health surveillance, period figures are fine. They tell you how many people died this year and at what ages. They are a useful temperature check of current conditions.

For comparing countries, period figures are the standard. The World Health Organization and the UN use period life expectancy for international comparisons because it is available quickly and uses a consistent method across all countries.

For understanding your own timeline, use a life clock that lets you input a custom life expectancy. The default figures on most life expectancy tools are period-based. If you are 40, healthy, and in a country with improving mortality, add 3 to 5 years to the period figure for a more realistic planning figure.

Why Headlines Use Period Figures (and Why That's Misleading)

Period figures are available immediately after the data year ends. Cohort figures require decades of waiting or complex projections. Journalists need a figure today, so they use period figures.

This creates a systematic bias in public understanding. Every headline about life expectancy is slightly too low for anyone under 50. When you see "Life expectancy in the UK falls to 79," that is a period figure for the current year. It does not mean a 40-year-old today should expect to die at 79. Their cohort figure is higher. The headline misleads by implying a frozen future.

The ONS now publishes cohort life expectancy tables alongside period tables. Its "National Population Projections" use cohort methods. The shift reflects a recognition that period figures are inadequate for long-term planning.

How the UK ONS Is Shifting Toward Cohort Emphasis

The ONS has moved to publishing "period life expectancy" and "cohort life expectancy" as separate distinct statistics. Its main life expectancy bulletin now explicitly notes the difference. The ONS's "Mortality Projections" for the UK use a cohort-based model that projects future improvements.

This is a change from 20 years ago, when period figures dominated all official publications. The shift happened because pension actuaries and government treasury models kept under-projecting how long people would live, which led to underfunded pension schemes. The cohort method, though less certain in the short term, produces figures that better match what actually happens to real generations.