Life expectancy

World map showing Human Life expectancy

Life expectancy is the average number of years remaining for a living being (or the average for a class of living beings) of a given age to live. Life expectancy is also called average life span or mean life span, in distinction to maximum life span. Life expectancy should not be confused with median survival time (the time at which 50% of a cohort will have died).

Although it is common usage to talk about life expectancy of any living being ranging from trees, insects, dogs, stroke victims, to mine workers, this article focuses on human life expectancy in general, that is, the aging and longevity profile of the human species.

Overview

Human life expectancy at various ages and under different circumstances is carefully studied by the insurance and actuarial professions, and is calculated on the basis of historic data as shown on the mortality or annuity table used as a reference.

By way of example, if people that are aged 60 live 10 more years on the average in a country, the life expectancy of people aged 60 in that country is said to be 10. If an age is not specified, life expectancy is understood to be from birth; therefore, a statement such as "the life expectancy of group A is higher than that of group B" represents data indicating that members of group A live longer on the average than members of group B. The fact that an average is calculated over a subset of the population makes life expectancy a statistical measure.

Notice that the life expectancy is heavily dependent on the criteria used to select the group. In countries with high infant mortality rates, the life expectancy at birth is highly sensitive to the rate of death in the first few years of life. In these cases, another measure such as life expectancy at age 10 can be used to exclude the effects of infant mortality to reveal the effects of other causes of death. Typically, life expectancy at birth is specified. To calculate it, it is assumed that current mortality levels remain constant throughout the lives of the hypothetical newborns.

Life expectancy over human history

One of the biggest jumps in life expectancy coincided with the introduction of sewers, which greatly reduced the spread of disease. In the last few centuries a strong statistical effect was caused by the near elimination of infant mortality in the Western world and elsewhere. On a world-wide scale, extreme poverty still remains a barrier to increasing life expectancy in developing nations.

Life expectancy before the 'health transition' of the modern era is thought to have varied between about 20 years and 35 years, depending upon particular circumstances. It has been suggested that life expectancy fell with the introduction of plant and animal domestication because of:

higher infection rates caused by the increase in human settlement size and density,
poorer nutrition due to reduced meat intake and a poorer vegetable diet.^[1]

Life expectancy recovered somewhat in the Bronze Age but it is only in recent centuries (since 1800) that it has dramatically increased. These changes are the result of a combination of factors including nutrition and public health, and medicine only marginally. The most important single factor in the increase is the reduction in death in infancy. The greatest improvements have been made in the richest parts of the world.

Life expectancy increased dramatically in the 20th century, especially in developed nations. Life expectancy at birth in the United States in 1901 was 49 years. At the end of the century it was 77 years, an increase of 57%. Similar gains have been enjoyed throughout the world. Life expectancy in India and the People's Republic of China was around 40 years at midcentury. At century's close it had risen to around 63 years. These gains were due largely to the eradication and control of numerous infectious diseases and to non-sustainable advances in agricultural technology (such as chemical fertilizers).

Basic life expectancy numbers tend to exaggerate this growth, however. The low level of pre-modern life expectancy is distorted by the previous extremely high infant and childhood mortality. If a person did make it to the age of forty they had an average of another twenty years to live. Improvements in medicine, public health, and nutrition have therefore mainly increased the numbers of people living beyond childhood, with less effect on overall average lifespan.

The major exception to this general pattern of improvement has been in those countries worst hit by AIDS, principally in Sub-Saharan Africa, which have seen significant falls in life expectancy due to the disease in recent years. Another exception is Russia and other former USSR republics after the collapse of the Soviet Union. Life expectancy of men dropped to 59.9 years (below the official retirement age), of women to 72.43 years (1999).

In recent years, obesity-related diseases have become a major public health issue in many countries. The prevalence of obesity is thought to have reduced the potential for longer life expectancy by contributing to the rise of cancers, heart disease and diabetes in the developed world.

Throughout human history most of the increase in life expectancy arose from preventing early deaths. However, many scientists believe this will not stay true in the near future as medical advancements aimed at better monitoring day to day, medically significant test values, and simple intervention such as blood pressure and clotting level control will prevent many sudden deaths or strokes. It is widely believed by researchers, that a full half of the North American and Japanese babies born since 2000 will live to an age of 90, and 10% to 100 years of age. It is hoped, with that extended lifespan, more productive and non-debilitated years will be added to the extreme upper end of middle age.

Timeline for humans

Homo sapiens live on average 37 years in Zambia and on average 81 years in Japan. The oldest confirmed recorded age for any human is 122 years, though some people are reported to have lived longer. The following information is derived from the Encyclopædia Britannica, 1961:

Humans by Era, Average Lifespan (in years)

Neanderthal, 20 (Note: Neanderthal is actually a different species from modern humans but is still considered to be a hominid)
Neolithic, 20
Bronze Age, 18^[2]
Classical Greece, 28
Classical Rome, 28
Medieval England, 33
End of 19th Century, 37
Early 20th Century, 50
Circa 1940, 65
Current (in the Western world), 77-81

Note: These represent the life expectancies of the population as a whole. In many instances life expectancy varied considerably according to class, and knowledge of the environment.

Variations in life expectancy in the world today

There are great variations in life expectancy worldwide, mostly caused by differences in public health, medicine and nutrition from country to country.

There are also variations between groups within single countries. For example, in the United States during the early 20th century there were large differences in life expectancy between people of different ethnicity, which have since lessened. Significant differences still remain in life expectancy between men and women in the US and other developed countries, with women outliving men. These gender differences have been lessening in recent years, with men's life expectancy improving at a faster rate than women's. Poverty has a very substantial effect on life expectancy. In the United Kingdom life expectancy in the wealthiest areas is ten years longer than the poorest areas and the gap appears to be increasing as life expectancy for the prosperous continues to increase while in more deprived communities there is little increase.^[3]

Life expectancy may also be reduced for people exposed to high levels of highway air pollution or industrial air pollution. Occupation may also have a major effect on life expectancy. Well-educated professionals working in offices have a high life expectancy, while coal miners (and in prior generations, asbestos cutters) do not. Other factors affecting an individual's life expectancy are genetic disorders, obesity, access to health care, diet, exercise, tobacco smoking, and excessive drug and alcohol use.

Life expectancy of animals and plants

The vast majority of animals have shorter life expectancies than humans do, and typically the lifespan of the animal increases with size. Some examples include:

Cats have lifespans in the 14-20 year range.
Large herbivores (cattle, horses, camels, deer) in the 30-50 year range.
Birds usually live 10-30 years, with parrots - particularly macaws - and several seabirds notable exceptions, with lifespans ranging from 40-80 years.
Elephants have lifespans of 50-80 years.
Blue whales can live from 40 to 80 years.
Bowhead whales are thought to live up to 200 years, with a few individuals found to have century-old harpoons embedded in their blubber.
Dogs live 10-25 years.
Some turtles live to 150 years or more.
Certain trees have almost outlived recorded human history; the baobab tree can live for 1,000-4,000 years, although it is understandably difficult to measure this. Similarly long-lived are olive trees, domesticated in the Mediterranean. Several olive trees still alive today were nurtured by the ancient Greeks.
Many corals can potentially live for over 100,000 years. However, there is no consensus among marine biologists how to determine age of a coral, and whether or not it is really a single organism.

Evolution and aging rate

The different lifespans of different plants and animals, including humans raises the question of why such lifespans are found.

The evolutionary theory is that organisms that are able by virtue of their defenses or lifestyle to live for long periods whilst avoiding accidents, disease, predation etc. are likely to have genes that code for slow aging- good repair.

This is so because if a change to the organism (for example a bird might evolve stronger wings) may mean that it is exceptionally capable of escaping from predation, then it will live longer, and die of old age. So a member of the population with the better wings who by chance has genes that code for better repair will outlast its contemporaries and have more successors. Its genes will tend to dominate more and more of the gene pool and genes for slower aging and slower reproduction rate will dominate.

Conversely a change to the environment that means that organisms die younger from a common disease will mean that organisms that have genes that code for putting more energy into reproduction than repair will do better.

The support for this theory includes the fact that better defended animals, for example cats, live longer and functionally age slower than less well defended animals such as dogs; and even small birds that can fly away from danger live for a decade or more whereas mice which cannot, die of old age in a year or two. Turtles are very well defended indeed and live for over a hundred years.

Calculating life expectancy

The starting point for calculating life expectancy is to calculate the crude death rates of people in the population at each age. For example, if one observed a group of people who were alive at their 90th birthday, and 10% of them were dead by their 91st birthday, then the crude death rate at age 90 would be 10%.

These crude death rates can be used to calculate a life table, from which one can calculate the probability of surviving to each age. In actuarial notation the probability of surviving from age x to age n+x is denoted $\,_np_x\!$ .

The life expectancy at age x, denoted $\,e_x\!$ , is then calculated by adding up these probabilities at every age. This is the expected number of complete years lived (one may think of it as the number of birthdays they celebrate).

$\,e_x = \sum_{t=1}^{\infty} \,_tp_x\!$

Because the age is rounded down to the last birthday, on average, it can be expected that people live half a year beyond their final birthday, and half a year is added to the curtate life expectancy to calculate the full life expectancy.

Life expectancy is by definition an arithmetic mean. It can be calculated also by integrating the survival curve from ages 0 to infinite (the ultimate age, sometimes called 'omega'). For an extinct cohort (all people born in year 1850, for example), of course, it can simply be calculated by averaging the ages at death.

Note that no allowance has been made in this calculation for expected changes in life expectancy in the future. Usually when life expectancy figures are quoted, they have been calculated like this with no allowance for expected future changes. This means that quoted life expectancy figures are not generally appropriate for calculating how long any given individual of a particular age is expected to live, as they effectively assume that current death rates will be "frozen" and not change in the future. Instead, life expectancy figures can be thought of as a useful statistic to summarise the current health status of a population. Some models do exist to account for the evolution of mortality (Lee-Carter model).

References

^ Galor, Oded and Moav, Omer, "Natural Selection and the Evolution of Life Expectancy" (October 12, 2005). Minerva Center for Economic Growth Paper No. 02-05 http://ssrn.com/abstract=563741
^ James Trefil, "Can We Live Forever?" 101 Things You Don't Know About Science and No One Else Does Either (1996)
^ Department of Health -Tackling health inequalities: Status report on the Programme for Action

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