LIFE EXPECTANCY

Life expectancy (or the expectation of life) is the average length of life remaining to be lived by a population at a given age. It is computed in the process of building a life table and can be computed for any age in the life table. Life expectancy at birth is the most commonly presented value because this measure provides a succinct indicator of mortality that reflects mortality conditions across the age range and is unaffected by the age structure of the actual population and thus can be compared across populations. The symbol used to represent life expectancy is ex where x represents an exact age.

LIFE EXPECTANCY IN THE UNITED STATES

In 1996, life expectancy at birth, e0, in the United States was 76.1 years; at age 65, e65 was 17.5 years; and at age 85, e85 was 6.1 years (Anderson 1998). These figures can be interpreted to mean that if a baby born in 1996 were exposed to the mortality conditions existing at each age of the life span in 1996, the baby with an average length life would live 76.1 years.

PERIOD AND COHORT VALUES OF LIFE EXPECTANCY

The 1996 U.S. life table is a period life table, based on cross-sectional data collected over a year; thus, this life table indicates the mortality experience of a hypothetical cohort. No actual cohort ever experiences the mortality in a period or cross-sectional life table; rather, the table indicates mortality conditions if the mortality levels of each age group at the period of time used as a reference were experienced by the hypothetical cohort. Because mortality has been falling over time, period life tables for a cohort’s year of birth have indicated an average expected length of life that is lower than that actually achieved by the cohort. For instance, in 1900 the cross-sectional life table for the United States showed life expectations of 46 for males and 49 for females. On the basis of their actual experience up through the age of 80, the 1900 birth cohort had an average length of life of 52 years for males and 58 years for females (Faber and Wade 1983).

Generation or cohort life tables, like the one mentioned above, based on the experience of an actual cohort are sometimes constructed. These indicate the average length of life actually lived after specific ages for a real cohort. The major difficulty faced in building cohort life tables is obtaining population and death data for a cohort from birth until the last survivors have died—over a 100-year period.

A mistaken notion held by many people is that life expectancy at birth is a good indicator of the age at which an older individual will die. This notion has undoubtedly led to some poor planning for old age because a person who has already reached older adulthood on average will die at an age that exceeds life expectancy at birth by a significant amount. As mentioned above, expectation of life in 1996 was 17.5 years for 65-year-olds, 11.1 for 75-year-olds, and 6.1 for 85-year-olds. With this number of years remaining to be lived on average, 65-year-olds should expect to live to 83 on average. Those who live to 75 should expect to live to 86, and those who live to 85 can expect to live to 91 on average. While expectation of life decreases as age increases, the expected age at death increases for those who survive.

CHANGES IN LIFE EXPECTANCY OVER TIME

As noted above, life expectancy has been increasing over time. This has probably been going on since some time in the last half of the nineteenth century, although reliable data for large sections of the country are not available to track the increase before 1900. In 1900, life expectancy at birth for both sexes was 47.3 years (U.S. Bureau of the Census 1975). This indicates an increase in life expectancy between 1900 and 1996 of 28.8 years. Most of this increase in life expectancy since 1900 is due to declines in mortality among infants and children. These mortality declines were primarily due to the diminishing force of infectious and parasitic diseases which were the most important causes of death among children.

Because life expectancy was low in the past, people often hold the mistaken notion that very few people ever reached old age under high mortality conditions. Yin and Shine (1985) have demonstrated that this mistaken notion was so prevalent that it was commonly incorporated into gerontology textbooks. The fact is that even under conditions of low life expectancy, once childhood is survived, the chances of living to old age are quite high. This is indicated by the fact that life expectancy at the older years has not increased over time nearly as much as life expectancy at birth. For instance, while life expectancy at birth for white males has increased almost 26 years since 1900, from 48.2 to 73.9 years, life expectancy for white males at age 40 has increased almost 9 years between 1900 and 1996, from 27.7 years to 36.4 years; at age 70, the increase for males has been just over 3 1/2 years, from 9.0 to 12.6 (Anderson 1998).

It should be noted, however, that in the past three decades the pace of improvement in life expectancy at the oldest ages has increased. In 1970 expectation of life for white males at age 70 was 10.5 years, indicating an improvement of 1.5 years in the 70 years between 1900 and 1970. Between 1970 and 1998, the increase was 2 years— significantly greater than the improvement during the first seven decades of the century. This reflects the new era of mortality decline in which decreases in mortality are due to decreased mortality from chronic conditions and are concentrated among the old.

A number of authors have studied the relationships between changes in age-specific mortality and life expectancy. Vaupel (1986) concludes that a reduction in the force of mortality of 1 percent at all ages would not produce as much gain in life expectancy today as it did in 1900. This is because we have already made so much progress in lowering infant and child mortality, the ages that have the greatest effect on life expectancy. Vaupel also shows that as mortality moves to lower levels, more progress is made in increasing life expectancy from mortality declines at older ages rather than at younger ages. At the level of mortality now experienced in the United States, much of the future increase in life expectancy will come from mortality declines occurring at ages over 65. This is true because of the prior success in reducing mortality at earlier ages to such low levels.

CALCULATION OF LIFE EXPECTANCY WITHIN THE LIFE TABLE

These observations about changes in life expectancy should make clear that life expectancy at birth is heavily weighted by mortality conditions at the youngest ages. A brief explanation of the life table and how life expectancy is calculated demonstrates why this is the case.

The life table is a statistical model that provides a comprehensive description of the mortality level of a population. Life table measures are particularly valuable because they are succinct indicators of mortality that reflect mortality conditions across the age range, are unaffected by the age structure of the actual population, and thus can be compared across populations. Life table measures can also be used to describe the characteristics of the stationary population that would result from an unchanging schedule of age-specific mortality rates in a closed population with a constant number of births.

There are a number of functions that appear in most life tables and for which conventional notation is widely recognized: qx, lx, dx, Lx, Tx, and ex. Each of these measures provides information useful in describing some aspect of the mortality conditions and/or characteristics of the stationary population. The definitions and interpretations of the life table functions follow below. In order to clarify the interpretation of the abridged life table functions, the life table for the U.S. population for 1996 is used as an example (Table 1).

nqx is the probability of dying between exact age x and x + n. As shown in Table 1, the probability of dying in the first year of life is 0.00732. This is higher than at subsequent ages until age 60 to 65, when the probability of death is 0.06649.

lx is the number of survivors reaching exact age x out of the original life table population. The size of the original life table population, the radix or lo, is usually assumed to be 100,000; however, this is a convention and other values can be used. Mortality conditions in 1996 were such that out of 100,000 births, 99,268 would reach age 1. This column of the life table can be used to compute how many people who reach a given age will survive to a later age. For instance, among the 80,870 people who reach age 65, 33,629 people or 42 percent will reach age 85 with mortality conditions as shown in Table 1.

Table 1:ndx is the number of deaths in the life table population between exact age x and x + n.

In the sample life table, 732 of the 100,000 births would die between ages 0 and 1 and 7,814 would die between ages 65 and 70.

nLx is the total number of years lived by the life table population between exact age x and x + n. Between birth and age 1, the life table population represented in Table 1 would live 99,370 years. This column also can be interpreted as the number of people in the stationary population at each year of age.

Tx is the total number of years lived after exact age x by the life table population surviving to age x, or the number of people in the stationary population age x and older. The 100,000 entrants to the life table in Table 1 would live a total of 7,611,825 years, and the 80,870 who reach age 65 would live a total of 1,416,873 more years.

ex is the expectation of life at exact age x or the average length of life remaining to be lived for the life table population which survives to exact age x. ex is computed from the Tx and lx columns of the life table: ex = Tx/lx. As indicated earlier, at birth, the life table population in Table 1 has a life expectancy of 76.1 years.

DIFFERENTIALS IN LIFE EXPECTANCY

There are large differentials in life expectancy among demographic and socioeconomic groups in the United States. Males have lower life expectancies than females throughout the age range. Males’ lower chances for a longer life are thought to result from a combination of biological differences and lifestyle factors. In 1996, e0 was 73.1 for males and 79.1 for females (Anderson 1998). By age 50, the difference is narrowed to 4.3 years, with a life expectancy of 27.2 for men and 31.5 for women. At age 85, men can expect to live another 5.4 years, while women can expect to live 6.4 years.

There is also a significant difference in life expectancy between whites and African Americans in the United States. This is assumed to result primarily from the difference in socioeconomic status and accompanying life circumstances that exist between African Americans and whites in the United States. In 1996, life expectancy at birth was 76.8 for whites and only 70.2 for blacks. At age 65, white life expectancy was 17.6 years; while for blacks of that age, it was 15.8 years. At the oldest ages, a crossover in mortality rates by race has been observed in the past. After the age of crossover, African-American mortality rates are lower than white mortality rates. In 1987 this was true at ages above 83. In the past, this crossover has shown up repeatedly in comparisons of African-American and white mortality in the United States and has been attributed to the ”survival of the fittest” among the black population (Manton and Stallard 1981). Recently, however, doubt has been raised as to whether the crossover is real or is a statistical artifact resulting from age misstatement by older African Americans in both the census and vital records of deaths (Coale and Kisker 1986; Elo and Preston 1994; Preston et al. 1996). Interestingly, Hispanics appear to have life expectancy values that are higher than non-Hispanic whites (Anderson et al. 1997).

INTERNATIONAL DIFFERENCES

In general, the life expectancy of a country is related to its level of socioeconomic development. Most countries that are classified as ”more developed” have higher levels of life expectancy at birth than most of the countries classified as ”developing”; however, within each of these groups of countries there is quite a bit of variability in life expectancy. While the United States has a high level of life expectancy compared to that of the developing countries of the world, the United States ranks quite low in life expectancy among developed countries and relative to its income level. A recent United Nations listing of the developed countries by level of life expectancy at birth ranks U.S. males as nineteenth and U.S. females as fourteenth (United Nations 1997). The countries with higher life expectancy for women include Japan and the Scandinavian countries. For men, most European countries including some in southern Europe have higher life expectancies at birth than the United States. The low ranking of the United States is attributed, in part, to the inequities in mortality among subgroups of the population, especially the high level among African Americans, and also to the high level of violent deaths. In recent years Japan has become the world leader in life expectancy at birth with values of e0 of 76.4 for men and 82.9 for women in 1995 (Ministry of Health and Welfare, Japan 1999). These values exceed 1996 U.S. values by 3.3 years for men and 3.8 years for women. The success of the Japanese in raising their levels of life expectancy has been due to large declines in mortality from cerebrovascular disease and maintenance of low levels of heart disease relative to other developed countries (Yanagishita and Guralnik 1988).

RELATED CONCEPTS

There are some other concepts that are related to life expectancy and are sometimes confused with life expectancy. One is ”life span.” The life span of a species is the age to which the longest-lived members survive. The life span of humans is thought to be approximately 115 years; however, Madame Jeanne Calment, whose age was well documented, died in 1997 at the age of 122. Current thinking is that while life expectancy has increased dramatically over the last century, the life span of humans has not changed over time; however, this does not mean it will never change. If discoveries are made in the future that enable us to retard the aging process, it may be possible to lengthen the human life span in the future.

”Life endurancy” is a related concept that, like life expectancy, is computed from the life table. This is the age at which a specified proportion of the life table entry cohort is still alive. For instance, in 1990 the age at which 0.1, or 10 percent, of the life table population remained alive was 90 years for men and 96 years for women. Life endurancy has been increasing over time and is expected to continue to change with changes in survival rates. In 1900 the 10 percent survival age was 81 and 82 for men and women, respectively (Faber and Wade 1983).

Finally, ”healthy or active life expectancy” is a subset of total life expectancy. Total life expectancy at any age is the sum of two parts: healthy life expectancy and unhealthy life expectancy. While the concept of health life expectancy was introduced in the 1960s (Sanders 1964) and developed in the 1970s (Sullivan 1971a, 1971b), it has only become widely adapted by governments and international organizations in the 1990s.

Interest in healthy life expectancy has grown recently as people have recognized that gains in total life expectancy today may not mean the same thing as in the past. Past gains in life expectancy came about largely because fewer people died of infectious diseases, either because they did not get the diseases or they received treatment that prevented death. People thus saved from death were generally free of the disease. Under these circumstances gains in life expectancy were accompanied by better health in the population surviving. Now, with gains in life expectancy being made because of declining death rates from chronic diseases especially among the old, it is not clear that the surviving population is a healthier population. This is because generally there is no cure for the chronic diseases, and for many their onset has not yet been prevented. People may be saved from death but they live with disease. This is the basis for questioning whether the additions to life expectancy are healthy or unhealthy years.

Crimmins and colleagues (1997) estimated that healthy life expectancy or disability-free life expectancy at birth in the United States in 1990 was 58.8 years for men and 63.9 years for women. The difference between blacks and whites in disability-free life expectancy at birth was even greater than the difference in total life expectancy. In 1990 black disability-free life expectancy for males at age 20 was 37.9 years while that for whites was 45.8 years (Hayward and Heron 1999). Studies that addressed the issue of changes in healthy life expectancy for the 1970 and 1980 period generally found that when healthy life was defined as nondisabled life, active life expectancy had not increased (Wilkins and Adams 1983; Crimmins et al. 1989). More recent studies have found increases in active life expectancy (Crimmins et al. 1997; Robine and Mormiche 1994).

Healthy life expectancy can be defined in many ways. Examples include average length of life free from a disability that causes a person to alter his or her normal activity; average length of life free of dependency on others for the performance of basic activities necessary to living, such as eating, bathing, and getting in and out of bed; and average length of life without disease (Bebbington 1988; Colvez et al. 1986; Crimmins et al. 1997; Crimmins et al. 1994; Rogers et al. 1989). Some measures of healthy life combine multiple indicators of health using weights; for instance, the U.S. National Center for Health Statistics measure combines self-assessed health and disability in its indicator of healthy life (Erickson et al. 1995).

There are multiple methodological approaches to estimating health expectancy. Most can be described under one of two headings: the Sullivan method or the multistate method (Sullivan 1971a; Schoen 1988). Microsimulation techniques have also been employed recently (Laditka and Wolf 1998).