16.2: Survival Curves

In the year 1900, the birth rate and the number of births in the U.S. were high, but many children died during the first year of life. Many others died before reaching adulthood. For many who reached adulthood, life still contained many risks. Risks were high because of inadequate public health, crowded urban living, low economic status, limited education, dangerous working conditions, and limited knowledge of techniques for preventing and curing diseases. Death rates remained substantial at all ages. Therefore, the percentage of the population born in 1900 continued to decline significantly as the age of the population increased (Figure 16.1a).

Figure 16.1a Survival curves - 1900-2100 and with artificial increased ML. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission)

During the century following 1900, conditions that contributed to high death rates at all ages improved. Infant mortality rates plummeted, and death rates at all ages declined. By 1950, almost all children survived their first year of life and childhood. This trend continued through 2000. Those that reached adulthood continued to have a much lower death rate. Therefore, the survival curve for a population born in 2000 is more rectangular in shape than is the survival curve for a population born in 1900. Mean longevity at all ages increased (Figure 16.1a).

For the most recent statistics, go to the U.S. Census Bureau web page at https://www.census.gov/topics/population.html

US Census Bureau 2020 census information is at US Census Bureau Reports - 2020 and 2017 .

People making projections believe that many conditions affecting ML will continue to improve, and ML will continue to increase. This book contains many suggestions that can increase ML. If conditions improve, the survival curves for future populations in the U.S. will be even more rectangular (Figure 16.1a). Major changes in immigration since 2000 are likely to have significant effects on the number and percentages of elders in the US population. The effects from the Coronavirus COVID-19 pandemic that began in early 2020 are ongoing. The US Census Bureau is taking these changes into consideration in making population projections.

For activities related to determining life expectancy for an individual, go to https://www.biologyofhumanaging.com/activit.htm#Life Expectancy: Assignment.

For graphs and statistics on life expectancies in the US, go to https://www.biologyofhumanaging.com/plan22.htm#Graphs.

For recent data on life expectancies in the US, go to the NCHS Life Expectancy web page (https://www.cdc.gov/nchs/fastats/life-expectancy.htm)

If ML increases as projected, the total population will gain many more elders 65+, and they will make up a much larger proportion of the total population (Table 1.1 Dead Link). If birth rates do not decline much, the size of the total population will also increase because many more people would be living longer. Using moderate estimates, the U.S. population will increase from 332 million in 2020 to 355 million in 2030, 374 million in 2040, and 389 million in 2050. Using the 2020 population as the reference value, these increases are 7 percent, 12 percent, and 17 percent respectively for the total population. For elders 65+, the corresponding values will be increases of 30 percent, 44 percent, and 53 percent respectively (Table 1.1 Dead Link). If conditions do not improve from those in 2020, mean longevities will not increase, but the numbers of elders and the total population will still increase because of continued births. (See Chapter 1 - Population Trends.)

What would happen if only conditions affecting mean longevity were not changed but fundamental aging processes and age changes could be slowed? Caloric restriction seems to slow age changes, resulting in increases in maximum longevity (XL) for animals. If caloric restriction or some other technique slowed human aging and people adopted that technique, the human XL would increase. Most people would still die at the usual ages from the usual causes of death. These causes are not age changes (e.g., atherosclerosis, Alzheimer's disease, cancer). However, some people would survive longer than ever before. The survival curve would become less rectangular at high ages (Figure 16.1b). This change would add a small number of very old people to the total population. Also, the percentage of elders in the total population would increase slightly, and the total population would increase slightly.

Figure 16.1b Survival curves - 1900-2100 and with artificial increased XL. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission)

What would happen if conditions improved AND aging processes were slowed? Both ML and XL would increase. The survival curve would have little change in shape, but it would become extended (Figure 16.1c). The number of elders would increase; there would be many elders who are very old; and elders would make up a larger proportion of the total population. The total population would increase substantially. The total population would become much larger than if only ML were increased.

Figure 16.1c Survival curves - 1900-2100 and with artificial increased ML and increased XL. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission)

Compare these different results in altering longevity using Figure 16.1a, Figure 16.1b, Figure 16.1c.

If any of these three scenarios occurred globally, the same trends in the total population on Earth would occur. In fact, the first scenario has occurred. Historically, survival curves for humans have become much more rectangular as MLs have increased. The human ML in prehistoric times was probably under 20 years. The ML in the Roman empire at its peak was 23 years. By 1800, the ML in England was approaching 40 years. By 1900, the ML in the U.S. was 47 years. Of course, MLs in less developed areas were probably much lower and continue to be so. However, human MLs are increasing across the globe. Elders are increasing in numbers and as a percentage of the total population. The total human population is growing so rapidly that many people speak of it as a population bomb that is exploding (Figure 16.2). See also the Chapter 16 Supplementary online figures.

Figure 16.2 World population growth, past and projected to 2100. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission)

These trends create complex and difficult challenges. Present examples include providing adequate living accommodations, health care, income, and quality of life for elders. These challenges will likely grow in size and in complexity because aging and age-related changes in diverse realms interact (e.g., biological, social, psychological, economic, spiritual). Moreover, the challenges related to elders amplify challenges for society as a whole. As more elders live longer with less disability and disease, they not only increase problems and challenge, they also can have a larger role in solving and meeting them.