How Is Life Expectancy Calculated?

average human lifespan determined through number of factors

By Jonny Lupsha, Wondrium Staff Writer

The average life expectancy has more than doubled in 400 years. Whereas people were once expected to live just 35 years, that number is now more than 79 years. How do we determine life expectancy?

Happy family celebrating life with sparklers at night, partying.
Life expectancy is a demographic term meaning the average number of years a person can expect to live during the specific time period when they were born. Photo by AlessandroBiascioli / Shutterstock

In the 1800s, in light of cholera and smallpox outbreaks, death rates spiked, causing the average expected lifespan to drop considerably. The 1918 flu pandemic followed suit, as did the novel coronavirus pandemic in some areas. In 2020, the death rate in New York City climbed more than 50% over the previous year, while life expectancy dropped from 82.6 years to 78 years.

How is life expectancy calculated? As you can imagine, statisticians don’t just pick groups of newborn babies and follow them around for 80 years on end to see how long each of them lives. In his video series The Skeptic’s Guide to Health, Medicine, and the Media, Dr. Roy Benaroch, Adjunct Assistant Professor of Pediatrics at the Emory University School of Medicine, explains.

What Is Life Expectancy and How Is It Calculated?

“The term ‘life expectancy’ means the number of years a person can expect to live,” Dr. Benaroch said. “Though it’s usually expressed from birth—that is, a person born today can expect to live, on average, [79] years—it can be expressed from a certain current age. For example, if you are 20 years old now, you can expect to live another 64 years, or until the age of 84.”

However, Dr. Benaroch expressed that when he used the term “life expectancy,” he meant from birth.

The most common way to determine an average life expectancy, also known as a “period life expectancy,” is to use a snapshot of time. This means looking today at the chance of dying during every year of life. After this, the researcher would take a hypothetical group of babies and mathematically run them through the years of their life to see how the group decreases in number year-over-year. But there’s a catch.

“This kind of computation does not take into account how trends in the chance of death are changing, or how they will change over the period of someone’s life—it assumes that mortality patterns present at the time of birth remain constant in the future,” Dr. Benaroch said. “But it’s the method most commonly used by both U.S. and international health authorities.”

What Does Life Expectancy Data Tell Us?

In 2017, the UK publication Telegraph ran an article about life expectancy in the UK. According to Dr. Benaroch, the article also pointed out an important fact about life expectancy trends.

“Over the last 100 years, we’ve gotten used to reports about an increasing lifespan—that is, every generation lives longer than the generation before it,” he said. “But those increases are stalling. As the article reports, between 2006 and 2011 life expectancy at birth increased by more than a year for men and nine months for women. But over the most recent five-year span, the life expectancy increase has been only three months for men, and half that for women.”

You may have noticed a pattern in Dr. Benaroch’s statement. Traditionally, women have longer lifespans then men. Yet in both cases here, the average increase in life expectancy for men is greater than that of women. The expected length of life from 2006 to 2011 for men increased by one-third longer than it did for women. From 2011 to 2016, both expectancies had smaller increases, but women’s lifespans increased by only half the rate that men’s did.

“The traditional gap between women and men’s lifespan seems to be shrinking,” Dr. Benaroch said. “And not because men are living healthier. It’s because women, worldwide, may be starting to, well, act more and more like men. More smoking, more drinking, more road accidents, and more homicides. These had been mostly male problems, but maybe not so much anymore.

“As lifestyles become more similar between men and women, so does their longevity.”

The Skeptic’s Guide to Health, Medicine, and the Media is now available to stream on Wondrium.

Edited by Angela Shoemaker, Wondrium Daily