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# Mortality

Types of death rate

Cause of death

World mortality—situation and trends

Death rates by age

Expectation of life

Death rates by marital status

Death rates by sex

Death rates by cause of death

BIBLIOGRAPHY

Mortality statistics are by-products of the legal process of death registration [seeVitalStatistics]. These data serve various purposes, such as estimating a component of population growth and preparing population projections; delineating health problems, planning public health programs, and assessing health progress; and studying the natural history of disease.

The absolute numbers of deaths are useful as a direct measure of the attrition of the population due to deaths. However, for analytical purposes, death data are generally used in the form of ratios. Properly computed, a death rate expresses the force of mortality on the population at risk.

## Types of death rate

The crudest form of death rate is the total or general death rate. This is the number of deaths occurring in a particular period of time, usually a year, for each 1,000 persons in the area or population. Because the general death rate (often called the crude death rate) is the mean of the death rates by age, sex, color, and other demographic variables weighted by the demographic composition of the population, an area with a young population, for example, would have a low general death rate, and an area with an old population a high general death rate, even if the set of age-specific death rates for the two areas were the same.

In order to take into account the differential mortality by age, sex, or other demographic variable, death rates are usually computed for a specific population class or group. The age-specific death rate is an example of this type of rate. In some cases, comparisons are based on death rates adjusted for differences in population composition. If the rate is standardized for differences in the age composition of two populations, it is called an age-adjusted death rate.

A special kind of death rate is the life table death rate. This is a hypothetical set of derived death rates based on certain assumptions of mortality in a stationary living population unaffected by migration or births. One function of the life table which is of interest is the expectation of life. This is the average number of years that will be subsequently lived by a group of persons who have attained a certain age. The expectation of life at birth is the average age at death of all the 100,000 who start life together in the life table cohort. Another important function is the survival rate, which is the probability that persons of a particular age will survive for a particular period of time, usually a calendar year [seeLifetables].

## Cause of death

An important aspect of mortality statistics relates to data derived from the medical information reported on death certificates. Despite their limitations, statistics on causes of death have contributed a great deal in the past to the field of public health [seePublichealth].

The present statistics on causes of death relate to the “underlying cause of death,” which is the term used to denote the disease or injury that initiated the train of events leading directly to death; in the case of accident or violence, it may also include the circumstances which produced the fatal injury. These statistics have done good service for public health in the past; but, with the lessening importance, at least in the United States, of the acute infectious diseases as compared with the chronic noninfectious diseases, the data have become less and less adequate. The selection of a single disease entity as the “underlying cause” poses a real problem in deaths involving chronic diseases, since in such cases it is frequently difficult, if not impossible, to identify a single underlying cause.

International comparison of cause-of-death statistics also presents a problem. In addition to differences arising from incompleteness of death registration in various countries, there are variations in proportion of deaths attended by a physician, in diagnostic acumen of the clinician in attendance, and in the recording of diagnostic information. International comparisons are further complicated by differences in medical concepts of diseases and in the methods of classifying causes of death. In fact, strict international comparability of cause-of-death statistics is at present a virtual impossibility, and too much significance should not be attached to small differences in rates between countries.

## World mortality—situation and trends

The estimated annual death rate for the world population is 17 per 1,000 population for the period 1958–1962. As might be expected, the death rate varies over a wide range in different parts of the world (see Table 1).

If differences in the age composition of the population in various parts of the world were taken into account, the mortality differential would undoubtedly be much greater than that indicated by the crude death rates shown here. Unfortunately, the

Table 1 — Population estimates, birth rates, and death rates for major regions of the world
PopulationaBirth ratebDeath ratec
a. 1962, in millions.
b. Annual average, 1958–1962, per 1,000 population.
Source: Computed from data in Demographic Yearbook 1963, p. 142. Copyright © United Nations 1964. Reproduced by permission.
Africa2694623
America4303311
North206249
Middle714314
South1534113
Asia1,7644320
Europe4341910
Oceania17248
U.S.S.R.221247
World total3,1353717

data needed to compute age-adjusted death rates are not available for the various regions of the world. In fact, one of the serious problems in international mortality studies is the lack of adequate mortality statistics for a large part of the world. By and large, reliable data are available only for the countries of northern and western Europe, North America, and Oceania. With a few notable exceptions, data for countries in other regions are either very incomplete or nonexistent.

The estimated birth rate for the world population is a little more than twice the estimated death rate. The natural rate of population increase (the difference between the birth and death rates) is highest in the Latin American countries, followed by the countries on the African continent and in Asia. Traditionally, a major part of annual population growth comes from the contribution made by births, but one of the significant demographic developments in the recent postwar period is the sharp acceleration in population growth due to the rapid decline in mortality. Virtually all countries, and more particularly the developing countries, experienced unprecedented declines in mortality while their birth rates remained at a high level.

The rate of decline in world mortality following World War n was dramatic, but the death rate began to level off in the 1950s in a number of countries, such as the United States, England and Wales, Sweden, Norway, Finland, the Netherlands, Japan, and Chile. Intensive studies of the mortality trend for the United States (U.S. Dept. of Health, Education, and Welfare ... 1964a), Chile (U.S. Dept. of Health, Education, and Welfare ... 1964b), and England and Wales (U.S. Dept. of Health, Education, and Welfare ... 1965) indicate that a large part of the acceleration in the decline of general mortality was due to the large reduction in the death rate for infective and parasitic diseases as a result of antimicrobial therapy. In the United States, for example, the death rate for infective and parasitic diseases reached a low level, and by the mid-1950s it was no longer significantly influencing the general mortality trend. At the same time, the mortality trend for chronic diseases and for violence was either rising, remaining unchanged, or declining very slowly. This combination of circumstances causes a marked deceleration in the downward trend of the general death rate.

Whether this change in the mortality pattern is transient or permanent is difficult to say. It is obviously not possible for the death rate to decline indefinitely. Further reductions in mortality appear possible in the United States, but it does not seem likely that large declines will occur until a major breakthrough is made in the prevention of deaths from chronic diseases. On the other hand, if the age-specific death rates in the United States were to decline to levels already achieved by several other countries of low mortality, the crude death rate for the United States in 1960 would have been 7.3 per 1,000 population, as compared with the recorded death rate of 9.5 per 1,000 population. For males the expected death rate would have been 7.8, as compared with the recorded rate of 11.0 per 1,000 population. For females the corresponding rates would have been 6.9, as compared with 8.1 per 1,000 population.

The leveling off of the death rate as it reaches its irreducible minimum is readily understandable. However, there seems to be no ready explanation for the change in mortality trends at different levels. For example, the death rate for nonwhites in the United States is still considerably higher than that for whites. Yet the rate of decline of the mortality trend for nonwhites has slowed down in the same manner as that for the whites.

National death rates are also becoming stabilized at different levels. For example, the Scandinavian countries and the Netherlands have achieved much lower age-specific death rates than the United States, whereas the age-specific death rates for Japan and Chile are higher. Yet the death rates appear to be leveling off in all of these countries.

The experience of Chile appears to have important implications for the developing countries. It seems clear that the knowledge and technical means are available for securing significant reductions in the death rate even in developing countries. The institution of mosquito and fly control and/or the widespread introduction of antibiotics for therapeutic purposes will have an immediate impact upon the death rate. However, it would appear that a point of diminishing returns will soon be reached and the decline in mortality come to a halt. Accordingly, the study of mortality trends in Chile points to the importance of planning health activities as a part of the social and economic development of the country (U.S. Dept. of Health, Education, and Welfare ... 1964b).

## Death rates by age

Reference was made earlier to the unsatisfactory nature of the crude death rate, which is significantly affected by the age composition of the population to which it refers. Death rates computed for various age groups, as in Table 2, are, of course, free of this problem.

As indicated by these age-specific death rates, infancy is the most critical period of life, even for

Table 2 – Death rates by age group: United States, 7962
AgeDeath rafe*
*Per 100,000 population.
Source: U.S. Dept. of Public Health Division 1964, Health, Education, and Welfare, Service, National Vital Statistics , pp. 1–5.
Under 1 year2,530.1
1–498.1
5–1443.9
15–24103.5
25–34145.2
35–44298.2
45–54741.0
55–641,692.9
65–743,798.4
75–848,431.5
85 and over20,510.0
Total population945.4

a developed country like the United States. Although data are not available to demonstrate this point, it would not be surprising if one-quarter or more of all live births in many of the developing countries fail to survive the first year of life.

For the developed countries it is possible to assess the progress made in the reduction of the infant mortality rate. A significant decline in infant mortality has occurred, and remarkably low rates have been achieved by the Netherlands (15.3 per 1,000 live births in 1962), Sweden (15.8 per 1,000 live births in 1961), and Norway (17.9 per 1,000 live births in 1961). A recent study (Shapiro & Moriyama 1963) of the international infant mortality trends indicates that the rate of decline is slowing up in many countries of low mortality.

From a relatively high death rate at infancy, the risk of death drops to a minimum at age ten or so. From then on, there is an increase in mortality with increasing age. This is the typical crosssectional pattern of mortality in countries of low mortality. However, there are a number of countries where the infant mortality rates are lower than that for the United States. Except in extreme old age, lower death rates are also found at other ages in other countries of low mortality.

In countries of low mortality, most of the deaths occur in the older age groups. In the developing countries, by contrast, it would not be unusual for more than half of all deaths to occur among children under five years of age. Under these conditions, it is obvious that the expectation of life at birth could not be very great.

## Expectation of life

The Biblical life span of “three-score years and ten” has become the norm for a number of countries. In Sweden, Norway, Denmark, the Netherlands, and Israel the life expectancy at birth is 70 years or more for both males and females. In other countries, such as the United States, Canada, Czechoslovakia, France, England and Wales, Australia, and New Zealand, the average length of life of 70 years or more for the total population has been attained only because of the favorable mortality experience of females. For example, the average expectation of life at birth in the United States for 1962 is 73.4 years for females and 66.8 years for males. If up-to-date life tables were available for all countries, it is probable that a few other countries could be added to the list above.

The world situation with regard to longevity cannot be described with any precision. However, it seems clear that longevity is at present greatest in the northern and western European countries, Canada and the United States on the North American continent, and Oceania. The average life expectancy is less favorable in the central, eastern, and southern European countries. Still lowei on the scale are the Latin American countries. The average expectation of life for a large part of the Asian population is low, although an average length of life of 60 years or more may be found in such Asian countries as Japan, Nationalist China (Taiwan), and Ceylon. Life table values for many of the countries on the African continent are not available. The question in a good part of Africa, especially in the southern and tropical countries, is not longevity but survival through childhood.

The increase in longevity of the population in the developed countries has been considerable. For example, in the period 1900–1902 the average expectation of life at birth in the United States was 48 years for males and 51 years for females. In a period of some sixty years, the male population gained about 19 years in life expectancy at birth, while the gain for females was about 22 years.

The postwar increase in life expectancy has been spectacular for some countries. For example, the expectancy of life at birth in Ceylon increased from 46.8 years in 1945–1947 for males to 60.3 years in 1954. For females, the corresponding figures were 44.7 years and 59.4 years, respectively. The average annual gain in longevity in Ceylon, as compared with the experience in the United States, is therefore roughly five times greater.

## Death rates by marital status

Almost without exception, the mortality among the married 20 years and over is lower, age for age, than the corresponding death rates for the single, widowed, or divorced. This is true for both males and females. Beyond this, the pattern of

Table 3 — Ratio of death rates of unmarried persons to death rates of married: Sweden, 1959
MALE  FEMALE
AgeSingleWidowedDivorcedSingleWidowedDivorced
* Too few cases for significant comparison with married.
Source: Computed from data in Demographic Yearbook 7961, pp. 592–593. Copyright © United Nations 1962. Reproduced by permission.
20–242.00**2.00**
25–342.502.504.132.402.002.40
35–442.121.562.312.231.772.15
45–541.531.822.581.501.341.59
55–641.271.421.871.281.191.35
65–741.211.291.471.091.171.17
75–841.261.301.411.131.161.10

mortality differentials by marital status varies somewhat by country.

In countries like Sweden, the mortality among divorced males is higher by far than the corresponding rates for bachelors or widowers (see Table 3). For females, the differences in death rates between the single, widowed, and divorced are not so great as those observed for males. The higher mortality among the single has been explained on the basis of selection; that is, those who never marry because of some serious physical impairment or chronic disease have a higher risk of mortality than the married. The single may therefore include among their number a higher proportion of the poorer mortality risks than those who marry. The higher mortality among the widowed has been attributed to the high association of diseases from which both marital partners die or to a less favorable economic situation that they both share.

One of the problems in the interpretation of death rates by marital status is the fact that the informant may not always know the civil status of those living alone. Also, there is the problem of the lack of correspondence between the marital status reported on death certificates and on the census enumeration schedules. Because the married population constitutes a large part of the total population, errors in reporting of marital status affect the data for the married much less than the data for the single, widowed, and divorced.

## Death rates by sex

One of the significant constants of mortality statistics in countries of low mortality is the favorable experience among females as compared with that of males. Examination of death rates by sex for a recent year indicates large sex differentials in mortality for the United States and Canada (36 and 38 per cent, respectively) and for New Zealand and Australia (23 and 26 per cent, respectively). In the countries of western Europe the male mortality exceeded the death rate for females by 10 to 20 percent.

The death rate for females is lower than that for males in each age group from birth to the end of the life span in virtually every country of low mortality. Even in the developing countries the mortality experience among females is generally favorable as compared with males, except in the child-bearing ages. Maternal mortality is a significant public health problem in these countries, as it was in the developed countries some forty or fifty years ago.

It is not clear why female mortality is consistently lower than that among males. One obvious explanation is the biological difference between the sexes; however, biological differences do not appear to account for much of the sex differential in mortality. A good part of the difference in the death rate appears to be due to the increasing mortality among males or to the fact that the death rate among females is declining faster than that among males. Whatever the explanation for this phenomenon, the continued occurrence of the large sex difference in mortality as recorded in a number of countries will have important consequences in terms of the sex composition of the population of the future, especially in the older ages.

## Death rates by cause of death

At the turn of the century, infective and parasitic diseases constituted the major public health problems in the world population. Pneumonia and influenza, tuberculosis, diarrhea and enteritis, and the childhood diseases were the principal causes of death in 1900, even in economically developed countries.

The large reduction in mortality since 1900 has been achieved primarily through control of the infective diseases. Although influenza and pneumonia still remain significant public health problems, mortality from the chronic diseases has

Table 4 — Death rate and proportionate mortality for the five leading causes of death: selected countries* of North America, Europe, and Oceania, 1961
Leading causes of deathAverage death rate per 100,000 populationPercent of total deaths
* Australia, Austria, Belgium, Canada, Denmark, Finland, France, German Federal Republic (including West Berlin), Hungary, Italy, Netherlands, New Zealand, Norway, Portugal, Republic of Ireland, Sweden, United Kingdom, and United States.
Source: Compiled from “The Ten Leading Causes ...” 1964a.
Heart disease30031
Malignant neoplasm17218
Vascular lesion of central nervous system13213
Accidents485
Influenza and pneumonia374

come to the forefront. The results of the review of causes of death in selected countries of North America, Europe, and Oceania in 1961 are summarized in Table 4.

From Table 4 it may be seen that more than 60 per cent of all deaths in the developed countries are attributable to the cardiovascular diseases and to malignant neoplasms. Although accidents rank fourth, they constitute the leading cause of death in the age groups 1 to 44 years; malignant neoplasms are the most frequent cause of death in the age group 45 to 64 years; and heart disease the principal cause of death in the population 65 years and over. Similar data for selected countries of Africa, South and Central America, and Asia for 1960 are shown in Table 5.

The number of countries in Africa, Asia, and South and Central America that met the criteria for inclusion in the World Health Organization compilations is limited, and the 12 countries that were selected do not, by any means, represent the mortality problems in the vast population of these continents. Although gastritis, duodenitis, enteritis,

Table 5 — Death rate and proportionate mortality for the five leading causes of death: selected countries* of Africa, South and Central America, and Asia, 1960
Leading causes of deathAverage death rate per 100,000 populationPercent of total deaths
*Mauritius, United Arab Republic, Chile, Colombia, Costa Rica, Guatemala, Mexico, Panama, Trinidad and Tobago, Ceylon, Israel (Jewish population), and Japan.
Source: Compiled from “The Ten Leading Causes ...” 1964b.
Gastritis, duodenitis, enteritis, and colitis959
Heart disease777
Influenza and pneumonia677
Malignant neoplasms485
Accidents384

and colitis were the leading causes of death for half of the selected countries, their average death rate and the proportionate mortality are relatively low. A principal cause of death that accounts for only about 9 per cent of all deaths and five leading causes that constitute no more than one-third of all deaths do not suggest any major health problems. Actually, the averages conceal some of the problems indicated by the data for individual countries. For example, the death rate for gastritis, duodenitis, enteritis, and colitis was 700 per 100,000 population in the United Arab Republic, and 36 per cent of all deaths were charged to these intestinal infections.

Adequate mortality statistics for these regions would delineate existing public health problems more clearly. If such statistics were available, it is likely that other infective diseases, such as tuberculosis, dysentery, typhoid, and measles; parasitic diseases, such as schistosomiasis and malaria; and possibly malnutrition and other dietary deficiency diseases would figure prominently as causes of death.

With the availability of knowledge and means for controlling most of the important infective and parasitic diseases, prospects are good for rapid reduction in mortality from these diseases. The resultant increase in survival of the population will bring new problems to the regions affected. These are the problems of the chronic noninfectious diseases with which the developed countries are now struggling.

Iwao M. Moriyama

## BIBLIOGRAPHY

Campbell, Hubert 1965 Changes in Mortality Trends: England and Wales, 1931-1961. U.S. National Center for Health Statistics, Vital and Health Statistics, Series 3, No. 3. Washington: Government Printing Office.

Demographic Yearbook 1961. 13th ed. 1961 New York: United Nations. → Special Topic: Mortality Statistics. Prepared by the Statistical Office of the United Nations in collaboration with the Department of Social Affairs.

Demographic Yearbook 1963. 15th ed. 1963 New York: United Nations. → Special Topic: Population Census Statistics II. Prepared by the Statistical Office of the United Nations in collaboration with the Department of Social Affairs.

Shapiro, S.; and Moriyama, I. M. 1963 International Trends in Infant Mortality and Their Implications for the United States. American Journal of Public Health and the Nation’s Health 53, no. 5:747-760.

The Ten Leading Causes of Death for Selected Countries in North America, Europe and Oceania, 1954-1956, 1964a World Health Organization, Rapport epidemiologique et demographique 17:54-112.

The Ten Leading Causes of Death for Selected Countries in Africa, South and Central America and Asia, 1954-1956, 1960, 1961. 1964k World Health Organization, Rapport epidemiologique et demographique 17: 118-152.

U.S. Dept. of Health, Education, and Welfare, Public Health Service, National Center for Health Statistics 1964a The Change in Mortality Trend in the United States, Prepared by Iwao M. Moriyama. National Center for Health Statistics, Series 3, No. 1. Washington: Government Printing Office.

U.S. Dept. of Health, Education, and Welfare, Public Health Service, National Center for Health Statistics 1964b Recent Mortality Trends in Chile. National Center for Health Statistics, Series 3, No. 2. Washington: Government Printing Office.

U.S. Dept. Of Health, Education, And Welfare, Public Health Service, National Center For Health Statistics 1965 Changes in Mortality Trends in England and Wales, 1931–1961. Prepared by H. Campbell. National Center for Health Statistics, Series 3, No. 3. Washington: Government Printing Office.

U.S. Dept. of Health, Education, and Welfare, Public Health Service, National Vital Satistics Division 1964 Vital Statistics of the United States 1962. Volume 2: Mortality. Part A. Washington: Government Printing Office.

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# MORTALITY

Genes are the ultimate time travelers. They transcend the bounds of time by hitching a ride in sexually reproducing species such as humans, but then discard the human body later in life as if it was a used car that had passed its warranty period. Once immortality became a fundamental property of deoxyribonucleic acid (DNA), at some time in the distant history of life on earth, the carriers of these genetic codebooks for constructing living organisms, including humans and other sexually reproducing species, became disposable. The timing with which death occursboth for individuals, as measured by their lifespan, and collectively for populations, as measured by life expectancydefines the concept of mortality.

Although it is not possible to know with certainty when any single individual will die, it is known with surprising accuracy when death occurs for members of a population when viewed as a group. In humans and a large number of other species, scientists have demonstrated that the risk of death is highest just after birth, declines to its lowest point near the time of sexual maturation (puberty), and then increases exponentially until extreme old age.

Why is this age pattern of death so common among sexually reproducing species? Early in life, death rates are high because newborns are subject to mortality risks from infectious and parasitic diseases, predation, and congenital malformations. Puberty is the time of lowest mortality because, from an evolutionary perspective, this is the moment at which the investment in the next generation has reached its maximum. This implies that the body design of humans and other living things are constructed with the ultimate goal of reproduction in mind (e.g., the passage of genes from one generation to the next), so this time of life is the most highly protected of all times in the life span. Following puberty, the risk of death from intrinsic (aging-related) causes increases exponentially because of a combination of wear and tear to the physical components of the body; accumulated damage to DNA, cells, tissues, and organs, highly efficient but nevertheless imperfect maintenance and repair mechanisms; and because of the presence of lethal inherited genes that "leak" into the gene pool of every generation.

Scientists have demonstrated that the rate of increase in the death rate following puberty is often calibrated to the length of each species' reproductive window, which is the average duration of time that elapses between puberty and menopause. In other words, animals like mice that experience puberty within weeks after birth tend to age much more rapidly and live considerably shorter lives than sea turtles, which do not experience puberty until about fifty years after birth. As a result of these differences in the rate of aging across species, one day in the life of a human is, in terms of percentage of life span, equivalent to about one week in the life of a dog and one month in the life of a mouse.

Death is an event that can and does happen at every conceivable age in a genetically diverse population. The death rate (also referred to as the mortality rate) for a population may be calculated in its simplest form as the number of deaths that occur in a given year divided by the population at risk of death, the product of which is then multiplied by a standard number (such as one thousand) to give the statistic more intuitive meaning. For example, in the United States in 1995 there were 2.3 million deaths and 262.8 million people alive in the middle of that year. This means that the crude death rate for the United States in 1995 was 8.8 deaths per thousand people [(2.3 / 262.8) × 1,000 = 8.8]. Death rates may also be calculated for people of various age groups or by single-year-of-age, and are often used to estimate the life expectancy of a population.

The various ages at which death occurs provides useful information about the longevity attributes of a population. For example, if one were to imagine a hypothetical group or cohort of one hundred thousand babies born in any given calendar year, and one applied to those babies throughout their lives the death rates that prevailed at every age in that year, it would be possible to plot on a graph the hypothetical ages at which all of the babies would have died. This is known as the distribution of death for a population. Although the distribution of death in 1900 was characterized by high mortality early in life, for those who lived beyond the perilous early years, the modal age at death for females was about 73 years of age (see Figure 1). A comparable distribution of death was observed for males in that year.

The opposite of the distribution of death is a plot of the number of people that are expected to survive from one year to the next. This is known as a survival curve. The survival curve is another useful tool for examining age patterns of death and survival in a population because it provides summary statistics that are easy to interpret and understand. For example, from the survival curve for U.S. females born in 1900 it may be determined that, based on the death rates that prevailed in that year, 58 percent would have been expected to survive to the age of fifty (see Figure 2). By contrast, an estimated 95 percent of the female babies born in the United States in the year 2000 are expected to survive to at least their fiftieth birthday. This demonstrates the dramatic improvements in survival that occurred at younger ages during the twentieth century. In 1900 in the United States the survival curve for females illustrates that the median age at death (the age at which 50 percent of the babies born in that year will still be alive) was fifty-eight years of age. By the year 2000 the median age at death for females in the United States was eighty-three years. As shown in Figure 2, based on death rates observed in the U.S. in 2000, an estimated 86 percent of all the female babies born will survive at least to their sixty-fifth birthdaya dramatic improvement that occurred during the twentieth century. Both curves provide actuaries, demographers, and other scientists with valuable information that can be used to compare the same population across time, or different populations during the same time period.

## The Gompertz Equation and its relationship to mortality

In 1825 an English actuary by the name of Benjamin Gompertz made an important discovery. Gompertz's job as an actuary for an insurance company was to calculate the risk of death for people of different ages in order to determine how much to charge for life insurance. (The exact same kinds of calculations are made by actuaries today.) Using data from various parts of England, where he lived, Gompertz discovered that the risk of death increased in a predictable fashion with age. His calculations led him to conclude that the death rate doubled about every ten years between the ages of twenty and sixty, which was the primary age range for people purchasing insurance annuities at that time. The mathematical formula Gompertz used to predict this exponential rise in mortality after age twenty has become known colloquially as the Gompertz equation, and it has remained an integral part of mortality computations conducted by actuaries and demographers ever since the early nineteenth century.

What made Gompertz's discovery so interesting was not just the fact that he devised a formula that accurately portrayed the dying-out process of humans, but that he and others believed that the same formula could be used to characterize death rates for other species. In fact, for more than a hundred years following Gompertz's discovery, numerous investigators from a wide range of scientific disciplines speculated that the Gompertz formula described a fundamental principle of death for all living things, a principle that became known as the Universal Law of Mortality. Recently, scientists have used death statistics for such species as humans, mice, and dogs to demonstrate that there is evidence to support the idea that age patterns of death occur in a consistent way across species, despite the fact that there is a wide variation in the observed lifespans of different species. In other words, there is scientific evidence to suggest that Gompertz was rightthere appears to be a nearly universal age pattern to the dying out of living things.

## The biology of life span

Why do people and other living things endure as long as they do? Why aren't we immortal? The answer to the most basic question of why we age is still an unsolved problem in biology, as the late famous biologist Sir Peter Medawar said in 1951. However, scientists are quickly closing in on at least some of the possible reasons why aging occurs. One of the most prominent theories of aging today is known as the free-radical hypothesis. During the process of metabolizing food and water and operating the machinery of life in a toxic world, damaging substances known as free radicals are generated. Although the human body has a highly efficient mechanism to protect itself from these damaging substances, it is not perfect. It is this lack of perfection that leads to accumulated damage to the DNA contained within the nucleus and the mitochondria (energy factories) of most cells. The level of damage moves up the scale of biological organization from DNA to cells, tissues, organs, organ systems, and ultimately to the whole organismcontributing to a degradation in the functioning of biological systems and an increased susceptibility to the diseases now associated with aging. Even though the damage that occurs to DNA is itself repaired with near perfection, it is the lack of perfection that is the basis for the free-radical hypothesis of aging.

There are a number of other prominent theories about the mechanisms of aging. Among them are the wear-and-tear theories and the discovery of an attribute of nuclear DNA known as the telomere. If the human body is viewed as a living machine with pulleys, pumps, levers, and hinges, much like that of a man-made mechanical device, it is evident that such machines cannot be operated indefinitely because of wear and tear. There are changes that occur in most human biological systems with the passage of time, including the loss of bone and muscle mass, increased brittleness of the circulatory system, and a degradation of the immune and reproductive systems.

Telomeres are the end caps of nuclear DNA, and they are known to shorten in length with each cell division. When they become short enough, the cell experiences a phenomenon known as programmed cell death, or apoptosis. An enzyme referred to as telomerase is known to be present in larger quantities in cells that are protected from aging, such as eggs, sperm, and stem cells, but there is no evidence so far to suggest that adding telomerase to other cells in the body would extend length of life. Although some scientists believe that this is one of the major biological mechanisms that contributes to aging, most people tend to die well before telomere shortening poses a serious problem for the whole organism.

## Mortality in the twentieth and twenty-first centuries

During the twentieth century, humanity witnessed the most dramatic declines in death rates and increases in life expectancy at birth than at any other time in history. Based on prevailing death rates in 1900, male and female babies born at that time were expected to live to 46.4 and 49.0 years, respectively. Now that the twentieth century has passed, it is known that babies born in the United States in 1900 fared a little better than predicted at the time because of unanticipated declines in death rates that occurred at every age throughout the century. There were three main forces that led to these declines in mortality. The first, which occurred early in the century, was a rapid decline in the risk of death among infants and children. The combination of improved sanitation, refrigeration, the more widespread use and distribution of clean drinking water, and the development of controlled indoor living and working environments led to rapid declines in the risk of waterborne and airborne infectious and parasitic diseases (IPDs). Infants and children benefitted the most from these developments because their immature immune systems placed them at a higher risk of death from IPDs. Examples of some of the IPDs that waned early in the twentieth century include diphtheria, tuberculosis, smallpox, and cholera. The second force that led to declining death rates was the more widespread use of hospitals for childbirth, which contributed to declines in both maternal and infant mortality. The third factor was the introduction of antibiotics in the middle of the century, which has saved people of all ages from a wide range of bacterial infectious diseases. The fourth factor, which led to declining death rates at middle and older ages in the latter third of the twentieth century, occurred as a combination of improved lifestyles, advances in surgical procedures, the development of pharmaceuticals, and a host of other advances in the biomedical sciences. In this case, death rates from such chronic degenerative diseases as heart disease and some cancers were observed to have declined during this period. As evidence for the magnitude of the changes in mortality that occurred throughout the twentieth century, consider the fact that life expectancy at birth rose by thirty years during this time, which was an increase of magnitude and speed that exceeded that observed during the previous 100,000 years.

S. Jay Olshansky

See also Life Expectancy; Life Span Extension; Longevity: Reproduction; Longevity: Selection; Longevity: Social Aspects; Theories of Biological Aging.

## BIBLIOGRAPHY

Carnes, B. A.; Olshansky, S. J.; and Grahn, D. "Continuing the Search for a Law of Mortality." Population and Development Review 22 (1996): 231264.

Gompertz, B. "On the Nature of the Function Expressive of the Law of Human Mortality and on a New Mode of Determining Life Contingencies." Philosophical Transactions of the Royal Society of London 115 (1825): 513585.

Kirkwood, T. B. L. "Comparative Life Spans of Species: Why Do Species Have the Life Spans They Do?" American Journal of Clinical Nutrition 55 (1992): 1191S1195S.

Makeham, W. "On the Law of Mortality and the Construction of Annuity Tables." Journal of the Institute of Actuaries 13 (1860): 325358.

Medawar, P. B. An Unsolved Problem in Biology. London: Lewis, 1952.

Olshansky, S. J., and Carnes, B. A. The Quest for Immortality: Science at the Frontiers of Aging. New York: Norton Press, 2001.

Olshansky, S. J.; Carnes, B. A.; and Cassel, C. "In Search of Methuselah: Estimating the Upper Limits to Human Longevity." Science 250 (1990): 634640.

Olshansky, S. J.; Carnes, B. A.; and Desesquelles , A. "Prospects for Human Longevity." Science (2001): 14911492.

Shryock, H., and Siegel, J. "Mortality." Chapter X in The Methods and Materials of Demography. Academic Press, 1976.

U.S. Bureau of the Census. Statistical Abstract of the United States. Washington, D.C.: U.S. Government Printing Office, 2000.

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## mortality

mortality, mortality rate The death-rate, usually standardized by age and sex, to facilitate comparisons between areas and social groups. It provides a measure of health risks, improvements in the quality of health care, and the comparative overall health of different groups in the population. It is thus used as a reliable indicator of social and economic change, and of comparative standards of living, as well as by epidemiologists who are interested in monitoring the risk of death from infectious diseases and other causes. A variety of mortality rates are used, each with its own purpose, with the overall national death-rate used as the starting-point for comparisons between areas and social groups in a society.

The crude death-rate is the number of deaths in a year per 1000 population in a defined geographical area. In effect a refined version of the absolute number of deaths, this is not very informative, as so much depends on the sex-ratio and age-structure of a population. Crude death-rates can be multiplied by Area Comparability Factors to produce corrected rates which are comparable one with another and enable direct comparisons between areas. More commonly, age-standardized death-rates are calculated separately for men and women, to produce overall Standard Mortality Ratios (SMR) for each sex, or for both sexes combined, for a given area or social group. The SMR compares age-specific death-rates for a given area or social group with national average age-specific death-rates. It is computed as the actual or observed number of deaths in the group of interest, divided by the expected number of deaths, multiplied by 100. (The expected number of deaths is the number that would have occurred if age-specific death-rates in the group of interest were equal to the national averages for the year.) Age-specific crude death-rates and SMRs can also be calculated to identify the age-groups accounting for mortality rates above or below the national average. Five-year and ten-year age-bands are normally used, but broader bands are sometimes used for age-standardization calculations. Mortality rates are also calculated for specific causes of death, such as cholera, cancer, or suicide; and to monitor the control of infectious diseases, improvements in health care, or the social consequences of high unemployment.

Some mortality rates are already age-standardized. The infant mortality rate is the number of deaths within the first year of life divided by the number of live births in the same year times 1000. The neonatal mortality rate is the number of deaths within the first four weeks of life divided by the number of live births in the same year times 1000. The perinatal mortality rate is the number of still-births plus the number of deaths within the first week of life, divided by total births (still-births and live births) in the same year, again times 1000. The maternal mortality rate is the number of maternal deaths divided by total births times 1000. See also LIFE-TABLE; MORBIDITY STATISTICS.

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## mortality

mor·tal·i·ty / môrˈtalətē/ • n. (pl. -ties) 1. the state of being subject to death: the work is increasingly haunted by thoughts of mortality. 2. death, esp. on a large scale: the causes of mortality among infants and young children. ∎  (also mortality rate) the number of deaths in a given area or period, or from a particular cause: postoperative mortality was 90 percent for some operations.

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## mortality

mortality: see vital statistics.

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## mortality

mortality See death rate.