Science: Tell me about ... life expectancy

ONE of the most misunderstood statistics in general use is that of "life expectancy". If you tell someone that the life expectancy in country A is 43 years, and in country B is 63 years, they tend to think that in country A adults who are otherwise healthy start falling ill with fatal diseases in their fifth decade, while those in country B carry on happily for 20 more years.

That's the easy way to interpret the phrase "life expectancy at birth" - and it's also completely wrong.

People often get it mixed up with "lifespan". In fact, the maximum possible human lifespan probably hasn't changed in millennia. It's slightly more than 120 years (the oldest confirmed person in modern times, Jeanne Calmant, died last year, aged 122).

So what does life expectancy actually mean? It's how long, on average, a child born alive in that year can expect to live. Like any average, it's only a single figure, and since it's being used to express the huge variation of human lifespans across a population, it tends to mask the truth.

The fact is that country A in the above paragraph will probably be full of hale and hearty 60-year-olds. How have they survived? Because the "life expectancy" considers the age at death of everyone in the country - and if you average the the death of an infant before its first birthday and that of a 70-year-old, you get a life expectancy of just 35 years. However, 35-year-olds tend not to die as easily as 70-year-olds or infant children.

This is where people tend to trip up on this statistic. In countries with comparatively low life expectancy, it is almost always due to high infant mortality. Lack of medical care and malnutrition often conspire to cause many young deaths in developing countries. Paradoxically, this often leads to a vicious circle: because children die young, families try to have many children, hoping that one or two will survive to adulthood. But this stretches their limited resources (such as food, clothing and often health care), which increases the risk that a child will be unhealthy.

In countries which charge for health care, it also increases the chance that they won't get proper treatment. Contraceptive availability usually breaks this loop because it means that having a child becomes a matter of choice, not chance.

Once infant mortality rates drop, life expectancy shoots up. In most Western countries, it is now well over 70. For instance, in the US, the life expectancy for someone born in the 1990s is 74.4 years.

But US citizens now aged 65 can expect to live another 17 years - to 82. And of course there are plenty of centenarians around. But it'll be a long time before our life expectancies approach our potential lifespans.

- Charles Arthur,

Science Editor