Romola Davenport & Jim Oeppen
Campop’s studies of mortality suggest that, in England, average life expectancy at birth varied between 35 and 40 years in the centuries between 1600 and 1800. It is a common misconception that, when life expectancy was so low, there must have been very few old people. In fact, the most common age for adult deaths was around 70 years, in line with the Biblical three score years and ten. So what does ‘life expectancy’ actually measure?
What is life expectancy?
To understand life expectancy, we can imagine a group of 1,000 babies born at the same time. We can measure how long each one lives. Figure 1 shows the lifespans for these infants as horizontal bars that indicate the length of life, arranged from top to bottom in order of lifespan. Their lifespans follow the pattern of mortality in England in 1841.
As you can see, in 1841 a lot of children died in the first five years of life. Of 1,000 babies, 138 (nearly 14 percent) died before reaching their first birthday. By age five, over a quarter of the original 1,000 babies were dead.
However, after the first five years, the rate of attrition eased. Children who made it to their fifth birthday had a 50:50 chance of making it to their 60th birthday. Of the original 1,000 babies, 38 percent survived to age 60, and nearly 10 percent to age 80.
So why was life expectancy only 42 in 1841? Because life expectancy is the average of all the different lengths of lives in the population. When mortality is high in infancy and childhood, then many of these lives are very short, and these many short lives really bring down the average age at death.
Calculating life expectancy
To calculate life expectancy, we take all the ages at which people died, add them up, and then divide by the number of people. For example, if we had a ‘population’ of two people, one of whom died on their first birthday and the other who died on their 100th birthday, their average life expectancy would be their ages at death added together and divided by two (101/2 = an average life expectancy of 50.5 years). But neither individual died in their 50s, or anywhere near their 50s. The average is not a good indicator of mortality risk in this case, because the length of life is so variable in this population.
On the other hand, if we have a population of two people, one of whom died on their 80th birthday and the other on their 100th, then average life expectancy would be 90 years, a much more representative estimate of average years lived. The latter case is much more like most populations in the world today. As life expectancy has risen, the benefits have been felt first at younger ages, and death has become increasingly concentrated in late adulthood.
Changing life expectancy over time
In the early 1600s (the first period for which we can calculate life expectancy in the English population) there was a huge peak of deaths in infancy, but then deaths were strung out across the whole life course between birth and 110 years of age. That is, the length of life was very unpredictable in the 1600s, and the risk of death was fairly high at all ages.
By 1800, this pattern had begun to shift. Mortality had become more concentrated at the oldest and youngest ages. In personal terms, this meant that fewer young children experienced the loss of their parents, fewer young adults were widowed, and fewer elderly parents experienced the untimely deaths of their adult children.
By the 1960s, deaths in childhood and early adulthood were relatively rare, and most people could expect to live into their 60s, 70s or 80s. Life expectancy was around 72, and this is a much better reflection of the ages to which most people could expect to live.
Today, when the death distribution is compressed and dominated by the adult peak, average life expectancy at birth is a much more representative statistic than in the past when the average fell between two peaks (infancy and old age). Nevertheless, most people die above the average age, and the most common age at death is almost 90.
It’s a bit more complicated…
So life expectancy is a kind of summary measure of mortality patterns in a population. It allows us to compare mortality trends over time, and between populations. But it is not a measure of the lifespan of a population, or even of the most common age at death.
Calculating life expectancy in real populations is also not quite as straightforward as we have suggested. Take the life expectancy of the English population in the 1960s. This doesn’t actually apply to the cohort of people born in 1960, because to calculate life expectancy for a real cohort we would have to wait until they were all dead in order to know how long they had lived!
So to calculate life expectancy for the people born in 1960, we would take all the deaths that occurred in that year and use these to measure the risk of dying at each age in 1960. We then apply these risks to an imaginary population that was born in 1960 and work out the average age at which they would have died if they had faced these risks at each age. This captures the particular mortality patterns of the year 1960, and is given the technical term ‘period life expectancy’. This is what people usually mean when they refer to life expectancy.
Demographers are, however, also interested in the life expectancy of cohorts of real people. For example, we can follow cohorts with unusual experiences, such as men born in the last years of the 19th century who were of recruitment age in World War I, and compare how they fared compared with other cohorts born before and after them.
Great expectations
The modern rise in life expectancy has provided enormous social and economic benefits. Not only do we live longer, but there has been a massive reduction in uncertainty with respect to both our own lifetimes and the lifetimes of our family and friends.
Further reading
Davenport, R.J. (2021) ‘Patterns of death, 1800 – 2020: Global rates and causes’ in P.N. Stearns (ed.) The Routledge History of Death Since 1800. Routledge.
Wrigley, E.A., R.S. Davies, J.E. Oeppen and R.S. Schofield (1997) English Population History from Family Reconstitution. Cambridge University Press.