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Life expectancy prospects trimmed in NZ

Estimations of future life expectancy of New Zealanders have declined over the last few years.  Each cohort is still expected to live longer than previous cohorts – people born in 2000 should live longer on average than people born in 1990, for example –  but the expectations for everyone are not as optimistic as they were. What is behind this and what does it mean?

First, some definitions.  A cohort is a group of people born in the same year.  The life expectancy of that cohort at a certain age, for example age 65, is the remaining average lifespan of the members of that group who survived to their 65th birthday.  It is a guide to the average length of time from age 65 to death. This is useful for retirement planning, and for the full story, see here.

Obviously while any members of a cohort are still living, the actual average cohort lifespan is not known, so demographers and actuaries will make estimates based on what mortality data and modelling expertise they have.

In New Zealand, StatsNZ publishes this information in tables for cohorts born from 1876 until a couple of years ago.  As this information is updated each year, we can see how the expectation of future average lifespans is changing over time. 

The story of reducing life expectancy. The charts below show cohort life expectancy at age 65 from three recent updates* published in 2018, 2021 and 2023. 

Since 2018, expectations for lifespan have been trimmed.  For example, a female reaching age 65 from the cohort born in 1960 was expected in 2018 to live for 24.8 years in the 2018, 2019 and 2020 updates, but only 24 years in the 2021 update, then a reduction of another half-year in 2022, and still at 23.5 years in the 2023 update.

Why expectations have been trimmed. A change in expectations is a consequence of inputs to the model and assumptions made.

Inputs to the estimation of future life expectancy include data on actual population mortality as measured by death rates.  As previous posts have shown, the rate of improvement in mortality in New Zealand is slower in the second decade of this century than the first.  I examined the reasons for this long-term slowdown in this post, from 2017. 

More recently, Covid-19 has not been a driver for mortality improvement slowing in New Zealand, which is one of few countries which avoided excess deaths in 2020 and 2021.  See here for a fuller review of the  impact of Covid-19 on mortality.

Assumptions made by StatsNZ  as they make their estimates for future mortality include a consideration of international trends. A stalling since 2010 is observed across all advanced countries and is in the context of very steep declines in the second half of the 20th century. The current lull in mortality improvement in these countries has been explained as being between ‘waves’ of medical technology innovation.

The common causes of these international trends are relevant in New Zealand. Up until the early 21st century there were great advances in the treatment of communicable diseases and cardiovascular disease and a dramatic drop in the number of smokers.  Since then, the rate of mortality improvement has slowed, especially in cardiovascular disease, with a widening socio-economic divide.  Increasing prevalence of obesity and diabetes underlie this trend. 

There may soon be new medical technologies to start another wave of rapid improvement, but the timing and reach of any improvements are uncertain.

What does this mean for how long New Zealanders may live?  Even if at a slower rate than previously, New Zealand mortality is expected to keep improving which means longer lives on average for each generation. 

The chart below shows this as average lifespans for the population of Aotearoa born from 1950 onwards, from birth and from their 65th birthday.  This post explains further.



* From StatsNZ New Zealand Cohort Life Tables published in each year, using the median scenario of the National population projections in each update

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