How Long Will We Live?

WHEN YOU GET REAL OLD, HONEY,” SAID ELIZABETH (BESSIE)

4.1 How Long Will We Live?

LEARNING OBJECTIVES

What is the average and the maximum longevity

•

for humans?

What genetic and environmental factors infl uence

•

longevity?

What ethnic factors infl uence average longevity?

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What factors create gender differences in average

•

longevity?

S

usie is a 51-year-old Chinese American living in San Francisco. Susie’s mother (age 76), father (age 77), and grandmother (age 103), who are all in excellent health, live with her and her husband. Susie knows that several of her other relatives have lived long lives but wonders whether this has any bearing on her own life expectancy.

As we saw in Chapter 1, many more people are living to old age today than ever before. Like Susie, people today have already seen far more older

© Jacques M. Chenet

adults than their great-great-grandparents did. Th e tremendous increase in the number of older adults has focused renewed interest in how long you might live. Susie’s question about her own longevity exem- plifi es this interest. Knowing how long we are likely to live is important not only for us but also for government agencies, service programs, the busi- ness world, and insurance companies. Why? Th e length of life has an enormous impact on just about every aspect of life, from decisions about govern- ment health care programs (how much money to allocate to health care programs to pay for much higher costs to care for more chronically ill people) to retirement policy (debates over the age at which people may collect maximum retirement benefi ts) to life insurance premiums (longer lives on average mean cheaper rates for young adults). Longer lives have forced change in all these areas and will con- tinue to do so for the next several decades.

Life expectancy can be examined from the per- spective of the basic developmental forces, because how long we live depends on complex interactions among biological, psychological, socioeconomic, and life-cycle forces. For example, some people, like Susie, have many relatives who live to very old age, whereas others have relatives who die young.

Tendencies toward long lives (or short ones, for that matter) tend to run in families. As you will see, our

“long-life genes” play a major role in governing how long we are likely to live.

But the world in which we live can aff ect how long we live, too. Environmental factors such as

disease and toxic chemicals modify our genetic heritage and shorten our lifetime, sometimes drasti- cally. By the same token, environmental factors such as access to high-quality medical care can sometimes off set genetic defects that would have caused early death, thereby increasing our longevity. In short, no single developmental force can account for the length of life. Let’s begin by exploring the concept of longevity. To get started, complete the exercise in the Discovering Development feature and see how long you might live. When you have fi nished, continue reading to discover the research base behind the numbers.

Average and Maximum Longevity

How long you live, called longevity, is jointly determined by genetic and environmental factors.

Researchers distinguish between two diff erent types of longevity: average longevity and maximum lon- gevity. Average longevity is commonly called aver- age life expectancy and refers to the age at which half of the individuals who are born in a particular year will have died. Average longevity is aff ected by both genetic and environmental factors.

Average longevity can be computed for people at any age. Th e most common method is to compute average longevity at birth, which is the projected age at which half of the people born in a certain year will have died. Th is computation takes into account people who die at any age, from infancy onward.

Th us an average longevity of 78 years at birth means

Did you ever speculate about how long you might live? Are you curious? If you’d like a preview of several of the key infl uences on how long we live,

try completing the questions at http://www.livingto100.com.

Take notes about why you think each question is being asked.

Once you’re fi nished, submit

your form. Take time to read about each of the topics, then read more about them in the text. Will you live to be 100?

Only time will tell.

D I S C O V E R I N G

D E V E L O P M E N T

Ta k e t h e L o n g e v i t y Te s t

that 78 years aft er a group of people are born, half of them will still be alive. When average longevity is computed at other points in the life span, the cal- culation is based on all the people who are alive at that age; people who died earlier are not included.

For example, computing the average longevity for people currently 65 years old would provide a pre- dicted age at which half of those people will have died. People who were born into the same birth cohort but who died before age 65 are not counted.

Eliminating those who die at early ages from the computation of average longevity at a specifi c age makes projected average longevity at age 65 longer than it was at birth. In the United States, females currently aged 65 can expect to live about 20 more years; men about 16 more years.

For people in the United States, average longe- vity has been increasing steadily since 1900; recent estimates for longevity at birth and at age 65 are pre- sented in Figure 4.1. Note in the fi gure that the most

rapid increases in average longevity at birth occurred in the fi rst half of the 20th century. Th ese increases in average longevity were caused mostly by declines in infant mortality rates, brought about by eliminating diseases such as smallpox and polio and through bet- ter health care. Th e decrease in the number of women who died during childbirth was especially important in raising average life expectancies for women, both at birth and at age 65. Advances in medical technol- ogy and improvements in health care mean that more people survive to old age, thereby increasing average longevity in the general population.

Maximum longevity is the oldest age to which any individual of a species lives. Although the bibli- cal character Methuselah is said to have lived to the ripe old age of 969 years, modern scientists are more conservative in their estimates of a human’s maxi- mum longevity. Even if we were able to eliminate all diseases, most researchers estimate the limit to be somewhere around 120 years because key body

19000 20 40 60 80 100

Life expectancy at 65 years Life expectancy at birth

Female

Female Male

Male

1910 1920 1930 1940 1950

Year

Life expectancy in years

1960 1970 1980 1990 2000 2010

Figure 4.1 Average Longevity for Men and Women in the United States 1900–2003.

Source: Data from Centers for Disease Control and Prevention, National Center for Health Statistics, National Vital Statistics System (2008).

systems such as the cardiovascular system have limits on how long they can last (Hayfl ick, 1998).

Genetic theories also place the human limit around 120 years (Barja, 2008).

Whether this estimate of maximum longevity will change as new technologies produce better artifi cial organs and health care remains to be seen.

An important issue is whether extending the life span indefi nitely would be a good idea. Because maximum longevity of diff erent animal species var- ies widely (Barja, 2008), scientists have tried to understand these diff erences by considering impor- tant biological functions such as metabolic rate or brain size (Hayfl ick, 1996). But no one has fi gured out how to predict longevity. For example, why the giant tortoises of the Galapagos Islands typically live longer than we do remains a mystery.

Increasingly, researchers are diff erentiating between active life expectancy and dependent life expectancy; the diff erence is between living to a healthy old age (active life expectancy) and simply living a long time (dependent life expectancy). Said another way, it is the diff erence between adding years to life and adding life to years. One’s active life expectancy ends at the point when one loses inde- pendence or must rely on others for most activities of daily living (e.g., cooking meals, bathing). Th e remaining years of one’s life constitute living in a dependent state. How many active and dependent years one has in late life depends a great deal on the interaction of genetic and environmental factors, to which we now turn.

Genetic and Environmental Factors in Average Longevity

Let’s return to Susie, who wonders whether she can expect to live a long life. What infl uences how long we will live on average? Our average longevity is infl uenced most by genetic, environmental, ethnic, and gender factors. Clearly, these factors interact;

being from an ethnic minority group or being poor, for example, oft en means that one has a higher risk of exposure to a harmful environment and less access to high-quality health care. But it is important to examine each of these factors and see

how they infl uence our longevity. Let’s begin with genetic and environmental factors.

Genetic Factors. Living a long life has a clear, but probably complex, genetic link (Kirkwood, 2008).

We have known for a long time that a good way to increase one’s chances of a long life is to come from a family with a history of long-lived individuals.

For example, if your mother lives to at least age 80, roughly 4 years are added to your average longev- ity (Woodruff -Pak, 1988). Alexander Graham Bell (the same guy who received the credit for invent- ing the telephone) was one of the fi rst people to demonstrate systematically the benefi ts of coming from a long-lived family. Bell considered 8,797 of William Hyde’s descendants and found that chil- dren of parents who had lived beyond 80 survived about 20 years longer than children whose parents had both died before they were 60. Th us Susie’s long-lived family sets the stage for Susie to enjoy a long life herself.

One exciting line of contemporary research, the Human Genome Project, completed in 2003, has mapped all our genes. Th is research and its spinoff s in microbiology and behavior genetics are continu- ing to produce some astounding results in terms of genetic linkages to disease and aging (you can track these through the main website of the Project, http://

www.ornl.gov/sci/techresources/Human_Genome/

research/spinoff s.shtml). Based on this gene map- ping work, attempts are being made to treat diseases by improving the way that medications work and even by implanting “corrected” genes into people in the hopes that the good genes will reproduce and eventually wipe out the defective genes (Kanehisa et al., 2008). Payoff s from such research are helping us understand how increasing numbers of people are living to 100 or older. For example, research on people over age 100 (centenarians) in Italy showed a connection between genetics and ability to cope with disease (Franceschi et al., 2008). Th e oldest- old, such as Suzie’s grandmother, are hardy because they have a high threshold for disease and show slower rates of disease progression than their peers who develop chronic diseases at younger ages and die earlier.

Environmental Factors. Although genes are a major determinant of longevity, environmental factors also aff ect the life span, oft en in combination with genes (Perls & Terry, 2003). Some environmental factors are more obvious; diseases, toxins, lifestyle, and social class are among the most important. Diseases, such as cardiovascular disease and Alzheimer’s dis- ease, and lifestyle issues, such as smoking and exercise, receive a great deal of attention from researchers. Environmental toxins, encountered mainly as air and water pollution, are a continuing problem. For example, toxins in fi sh, bacteria and cancer-causing chemicals in drinking water, and airborne pollutants are major agents in shortening longevity.

Th e impact of social class on longevity results from the reduced access to goods and services, especially medical care, that characterizes most ethnic minority groups, the poor, and many older adults (National Center for Health Statistics, 2008a).

Most of these people have little or no health insur- ance, and many cannot aff ord the cost of a more healthful lifestyle. For example, air pollution, poor

drinking water, and lead poisoning from old water pipes are serious problems in large urban areas, but many people simply cannot aff ord to move. How environmental factors infl uence average life expec- tancy changes over time. For example, acquired immunodefi ciency syndrome (AIDS) has had a devastating eff ect on life expectancy in Africa, where in some countries (e.g., Botswana, Namibia, South Africa, Zimbabwe) average longevity may be reduced by as much as 30 years from otherwise expected levels (Kinsella & Phillips, 2005). In con- trast, negative eff ects of cardiovascular diseases on average longe vity are lessening as the rates of those diseases decline in many developed countries (National Center for Health Statistics, 2008a).

Th e sad part about most environmental fac- tors is that we are responsible for most of them.

Denying adequate health care to everyone, con- tinuing to pollute our environment, and failing to address the underlying causes of poverty have undeniable consequences: Th ey needlessly shorten lives and dramatically increase the cost of health care.

Coming from a family with many long-lived members increases your chances of having a long life yourself.

© Stephen Simpson / Getty Images

Ethnic Differences in Average Longevity

People in diff erent ethnic groups do not have the same average longevity at birth. For example, African Americans’ average longevity at birth is roughly 6.5 years lower for men and about 5 years lower for women than it is for European Americans (National Center for Health Statistics, 2008a). Why do such large diff erences exist? Th e most important reasons are the substantial diff erences in most envi- ronmental variables between European Americans and other ethnic groups.

In the United States, these diff erences can be dra- matic. For example, although African Americans’

average life expectancy at birth is about 6 years less for men and about 4 years less for women than it is for European Americans, by age 65 this gap has narrowed to about 2 and 1.5 years, respec- tively, for men and women. By age 85, African Americans tend to outlive European Americans.

Why the shift over time? Perhaps because of their lower access to good-quality health care in general, those African Americans who live to age 85 tend to be in better health on average than their European American counterparts. But this is just a guess.

Latinos have higher average life expectancies than European Americans at all ages despite having, on average, less access to health care (National Center for Health Statistics, 2008a).

Gender Differences in Average Longevity

A visit to a senior center or to a nursing home can easily lead to the question “Where are all the very old men?” Women’s average longevity is about 5 years more than men’s at birth (in 2004), nar- rowing to roughly 1 year by age 85 (Arias, 2006;

National Center for Health Statistics, 2008a). Th ese diff erences are fairly typical of most industrialized countries but not of developing countries. Indeed, the female advantage in average longevity in the United States became apparent only in the early 20th century (Hayfl ick, 1996). Why? Until then, so many women died in childbirth that their average

longevity as a group was reduced to that of men.

Death in childbirth still partially explains the lack of a female advantage in developing countries today;

however, part of the diff erence in some countries also results from infanticide of baby girls. In industrial- ized countries, socioeconomic factors such as access to health care, work and educational opportunities, and athletics also help account for the emergence of the female advantage (Hayfl ick, 1998).

Many ideas have been off ered to explain the sig- nifi cant advantage women have over men in aver- age longevity in industrialized countries (Hayfl ick, 1996). Overall, men’s rates of dying from the top 15 causes of death are signifi cantly higher than women’s at nearly every age, and men are also more suscep- tible to infectious diseases. Th ese diff erences have led some to speculate that perhaps it is not just a gender-related biological diff erence at work in lon- gevity, but a more complex interaction of lifestyle, much greater susceptibility in men of contracting certain fatal diseases, and genetics (Hayfl ick, 1996).

Other researchers disagree; they argue that there are potential biological explanations. Th ese include the fact that women have two X chromosomes, compared with one in men; men have a higher metabolic rate; women have a higher brain-to-body weight ratio; and women have lower testosterone levels. However, none of these explanations has suffi cient scientifi c support to explain why most women in industrialized countries can expect, on average, to outlive most men (Hayfl ick, 1996).

Despite their longer average longevity, women do not have all the advantages. Interestingly, older men who survive beyond age 90 are the hardiest segment of their birth cohort in terms of performance on cognitive tests (Perls & Terry, 2003). Between ages 65 and 89, women score higher on cognitive tests;

beyond age 90, men do much better.

International Differences in Average Longevity

Countries around the world diff er dramatically in how long their populations live on average. As you can see in Figure 4.2, the current range extends

More than 74 years 70–74

65–69 60–64 55–59 50–54 45–49

Less than 45 years Life expectancy at birth in selected countries, 2008

Canada 81.2

Australia 81.5 Iceland

80.5

Mexico 75.8

Spain Tunisia79.9

75.6 Sierra Leone 40.9

South Africa 48.9 Cameroon

53.3

Argentina 76.4 Brazil 71.7

Uganda 52.3 Egypt

71.8

Afghanistan 44.2

Kenya 56.6 Iraq 69.6

Singapore 81.9 Indonesia

70.5 Iran 70.9

South Korea 78.6 North Korea

72.2

Japan 82.1 Pakistan

64.1 Turkmenistan

68.6 Saudi Arabia

76.1 Israel

80.6 Turkey

73.1 Moldova

70.5 Ukraine

68.1 Germany

79.1 France Italy 80.9 80.1

Philippines 70.8

Papua New Guinea

66

Fiji 70.4 Guatemala

70

Nicaragua 71.2

Venezuela 73.5

Colombia 72.5 Honduras

69.4

Bolivia 66.5 Chile

77.2 United States

78.1

India 69.2

China 73.2 Russia 65.9 Greenland

69.5

Guyana 66.4 Cuba

77.3

New Zealand 80.2

Figure 4.2 International data on average life expectancy at birth. Note the differences between developed and developing countries.

Source: http://www.census.gov/ipc/www/idb/tables.html.

from 38 years in Sierra Leone in Africa to 80 years in Japan. Such a wide divergence in life expectancy refl ects vast discrepancies in genetic, sociocultural

and economic conditions, health care, disease, and the like across industrialized and developing nations.

Concept Checks

1. What are some of the main reasons that average longevity increased during the 20th century?

2. What evidence is there that genetics infl uences average longevity? What are the key environmental infl uences on average longevity?

3. What are some of the reasons for ethnic differences in average longevity?

4. What explanations have been offered to account for women’s longer average longevity?