Gerontology, the study of aging, has become a major focus of attention in science and the professions. With increasing life expectancy and falling birth rates, populations are getting older. Increases in life expectancy in both developed and developing countries and increased needs for services for older persons have contributed to a growing volume of research and education on both basic and applied aspects of aging. Geriatrics, the branch of medicine that treats the clinical problems of late life, is also an area of expanding professional activity. Both gerontology and geriatrics emerged as disciplines immediately after World War II with the establishment of professional societies and specialized journals. Why it took so long, compared with other fields, for academic and professional interests in aging to emerge is an interesting question to pursue.
There have always been speculation and cultural myths about aging and the association of death with advanced chronological age. Gerald Gruman has described many myths about death and aging from ancient times to about 1800. The common interpretation in the Middle Ages was that death was either the outcome of humankind's fate as punishment for sin or an outcome of cosmic forces that were insurmountable. The growth of science in the nineteenth century was accompanied by the conviction that all phenomena of nature were governed by natural laws, and that these laws can be discovered through scientific investigation. The point of view that aging was not a supernatural phenomenon, knowable and explainable by study, was fully expressed by the Belgian mathematician, statistician, and astronomer Lambert Quetelet (1796–1874). In 1835, Quetelet wrote: "Man is born, grows up, and dies, according to certain laws which have never been properly investigated, either as a whole or in the mode of the mutual reactions" (Quetelet, 1968). Quetelet reviewed data on mortality in relation to age, sex, urban, rural, and national differences and found that the duration of human life varied according to the environments in which people lived.
An international exhibition on health in London in 1884 extended further interest in how differences in age affect human functions. The exhibition was sponsored by Francis Galton (1822–1911), a cousin of Charles Darwin. Galton had a broad background in mathematics medicine, psychology, and anthropology. At the exhibition, he took measurements of seventeen different bodily functions, including hand strength, hearing, vision, speed of movement, and vital lung capacity. Over 9,337 males and females were measured. Since Galton was exposed to a large mass of data, and given his back ground in mathematics, he was able to develop the first quantitative measure of the degree of association between two variables, such as age and strength.
Gerontology requires the support of mathematics and statistics to identify and compare the complex sources of variance that influence human aging. Quetelet and Galton were pioneers in creating a quantitative basis for gerontology and replacing older myths. Another contributor to the quantitative approach to aging was Benjamin Gompertz, a British actuary, who, in 1825, described the relationship of mortality to age as an accelerating curve described by exponential equation. The fact that mortality data could be described as an exponential equation did not itself explain why mortality is related to age. It was, however, an early step toward bringing science into discussions about aging.
Similarly, relating health, disease, and changes in function to chronological age does not reveal the causal variables. Modern gerontology recognizes that organizing data by age is but a first step toward explanation. To understand the process of aging and the changes that occur as people age, the causal variables must be understood.
The term "gerontology" was introduced in 1903 by Elie Metchnikoff, a Nobel laureate and professor at the Pasteur Institute of Paris. In America, the emergence of gerontology as a scientific movement can be traced to a small group of
THE BEGINNING OF MODERN GERONTOLOGY
The concerns of public health and medicine in the early years of the twentieth century were focused on the major causes of death at that time, the infectious diseases. Disease was generally regarded as a result of an invasion of the human body by a foreign organism whose influence had to be destroyed. As chronic diseases—heart disease, stroke, cancer, diabetes, and others—began to replace the infectious diseases as the major causes of death, a revision of basic explanatory paradigms had to occur, since the human host was beginning to be regarded as a major element in the cause of the chronic diseases. In the mid-1930s, the Josiah Macy, Jr. Foundation, based in New York, began a series of conferences on aging. The foundation's director was Dr. Ludwig Kast, who believed that degenerative diseases were a manifestation of the process of aging. The foundation had supported studies of degenerative diseases, but Kast encouraged research on aging itself. Thus, work on heart disease was examined in relation to the physiology of aging.
The foundation encouraged E. E. Cowdry, a professor of cytology at Washington University, to organize a book that would embrace not only the biomedical aspects of aging, but include social, psychological, and environmental influences as well. Cowdry's book, Problems of Ageing, was published in 1939. The foundation continued to sponsor conferences on aging, which led to the establishment of The Club for Research on Aging, in New York. By 1940, thinking about aging was becoming more sophisticated. Reflecting the thinking of the times, the U.S. Public Health Service organized a multidisciplinary conference in 1941 on mental health aspects of aging. At the same time, the Surgeon General of the U.S. Public Health Service established the Section on Aging within the National Institutes of Health. Thus, leadership in public health helped to establish aging as an important research topic.
AGING AND EVOLUTION
The nineteenth-century interest in the biological evolution of species, expressed by Charles Darwin (1809–1882), and Alfred Russel Wallace (1823–1913), was also accompanied by an interest of a small number of biologists in fitting aging into an evolutionary paradigm. It is not a simple step to account for the natural selection of late-life features, since they appear past the age of reproduction, and therefore out of direct reach of the pressures of natural selection. In 1957, Peter Medawar (1915–1987) reasoned that selective pressures for survival features were maximum at the time of reproduction and then declined. He described this as a result of selective pressures to create a "precession" of positively selected characteristics toward the age of maximum reproduction and a "recession" after the age of reproduction. Natural selection is therefore presumed not to affect late-life characteristics, and a series of unrelated characteristics may appear (e.g., Alzheimer's disease, Parkinson's disease, and other life-limiting conditions). Such diseases are presumably out of reach of selective pressures. This point of view is associated with the idea that life after reproduction is subject to random degradation of the well-functioning organism.
However, this reasoning need not exclude the possibility of indirect selection in which a late-life trait like intact memory and reasoning could operate to meet threats to tribal survival in preliterate societies. That is, tribes that had long-lived elders with intact memories of meeting the problems of families, floods, and warfare could have greater chances of survival. This has been described as a "counterpart theory," in which late-life characteristics of older persons influence the selective survival of the young and those of reproductive age who are dependent upon them for survival (Birren, 1964).
An impressive amount of data has been gathered on the life spans of a wide variety of different species. The comparative biology of aging suggests that most have relatively fixed upper limits of the lengths of life. Particularly, the lengths of life of
An issue facing the translation of the comparative biology of aging into principles about human mortality and morbidity is why different species have characteristic lengths of life. The quest is to identify common or shared interspecies genetic determinants that may have evolved. In contrast, examining individual differences in longevity within a species (intraspecies variability) can involve different traits, or the interaction of traits with the characteristics of a particular environment. Within a species there appear to be simultaneous positive and negative factors contributing to length of life.
ADVANCES IN LIFE EXPECTANCY
Dramatic advances in life expectancy took place in the twentieth century. Life expectancy at birth for the U.S. population rose from 47 years in 1900 to 77 years in 2000. Clearly, genetic selection could not have operated so quickly, and nongenetic factors had to be the cause of this startling increase in life expectancy. A different pattern of the major causes of death emerged during the century. In 1900 the five major causes of death were: (1) pneumonia and flu, (2) tuberculosis, (3) diarrhea and intestinal disease, (4) heart disease, and (5) stroke and brain lesions. By 2000, the five major causes were: (1) heart disease, (2) cancer, (3) stroke and brain lesions, (4) lung disease, and (5) accidents. Presumably, a cleaner environment, in terms of improved water supplies and improved sewage disposal, together with the discovery and use of vaccines and antibiotics, contributed to the spectacular fall of infectious diseases as causes of death. Also, improvements in the transport of fruits, vegetables, and other foods led to the elimination of seasonal dietary deficiencies. General improvements in diets also contributed to the improvement in life expectancy. But the rise in lung disease and cancer as causes of death suggests there may have been simultaneous environmental deficits occurring parallel to these improvements. At any particular time in history there may be simultaneous positive and negative influences on aging, life expectancy, the incidence of particular diseases, and the quality of life.
One of the largest correlates of life expectancy is food intake. In 1935, MacCay, Crowell, and Maynard reported that dietary restriction promotes longer life in mice. Since that time, the effects of restricted dietary intake have been reported in many studies. Little early attention was given to the fact that in addition to the life extension effects of dietary restriction in small animals, many diseases to which the animals are disposed were delayed in their onset. There being no available explanation or mechanism to explain this delay of appearance of diseases, it received little attention. However, in 1954, Denham Harman proposed that free radicals can contribute to the aging of organisms. Free radicals refer to molecules that have one or more electrons in their outer orbits that can interact with DNA, proteins, and unsaturated lipids in cell membranes. They appear to be very reactive at all levels in an organism. In recent years, the identification of oxidant damage from free radicals has led to acceptance of the view that oxidant damage can modulate the expression of the genetic traits of animals, including humans.
Undoubtedly there have been many contributing factors to the relatively slow emergence of gerontology as a subject of study. The implications of aging as a natural phenomenon were to some extent threatening to some religious or philosophical convictions. The large number of children born early in the twentieth century gave rise to the professional specialties of pediatrics, child psychology, and child psychiatry. K. F. Riegel (1977) did a quantitative study of publications in the psychology of aging and found that such publications increased exponentially after 1950, fifty years after child psychology became an established academic field of study.
Children's susceptibility to infectious diseases contributed to a health focus on early life, and little attention was given to phenomena of aging and later-life morbidity. Also, the dominant views
Economic factors undoubtedly played a role in the slow emergence of gerontology as an area of study. In the early twentieth century, few institutions, private or nonprofit, were devoted to the care and treatment of the aged. Pensions, social security, disability insurance, retirement communities, assisted living facilities, adult education programs, and many other programs emerged later, increasing the need for knowledge about the characteristics of the older population. As the institutional lag in serving older persons began to ebb, research on aging began to grow.
The elevation of the Section on Aging to the National Institute on Aging in 1975 by the U.S. Public Health Service within the National Institutes of Health was a landmark in the growing support of research on aging. Gerontology was coming of age. Handbooks on the biological, psychological, and social science aspects of aging made their appearance in 1977, providing further evidence of the significant growth of the study of aging after 1950.
The complexity of aging as a set of interacting phenomena presented early life scientists with questions that could not easily be answered with the methodology of the time. In the past it was much easier to study single organs and their functions in isolation than to study them in an aging human organism. Important shifts in the major causes of death over the last hundred years indicate that aging is a highly dynamic phenomenon. Early studies of heart disease, however, did not recognize the contribution of the social environment and the behavioral dispositions of individuals as contributing factors to disease. For example, bereavement was found to have an effect on the mortality of the surviving spouse, increasing the awareness of the complex interactions in aging.
AGING AS AN ECOLOGICAL PHENOMENON
In the early twentieth century, the realization began to grow that aging is also an ecological phenomenon. Longitudinal studies of human populations in the latter part of the twentieth century have provided considerable evidence of the range and plasticity of human variability in the way aging is manifest. Since experimental studies of human longevity and aging are not morally possible, longitudinal studies have been important in providing evidence of the relative effects of environment and heredity on mortality, morbidity, and functional characteristics. Further advances in our understanding of human aging have been provided by longitudinal studies of identical twins reared together or apart over their life spans. No longer are simple assumptions acceptable about the contributions of "nature" and "nurture" to human aging. Contemporary questions focus on the relative magnitudes of different influences. The lifespan identical twin studies of the Karolinska Institute in Stockholm, Sweden, have provided evidence that both genetic and environmental factors influence individual variability in the expression of late-life characteristics. Such findings make it impossible to explain human aging solely in terms of genetic inheritance or environmental influences.
A strong force in attributing human aging to genetic inheritance is the fact that the various species have characteristic lengths of life relative to one another. Even if one increases the length of life of rats by 100 percent, they still do not live as long as cats or dogs. Thus there is a hierarchy of life spans, which presumably has evolved by natural selection of animals exposed to different environmental pressures such as availability of food, extreme temperatures, and predation.
To answer questions raised by gerontology about how long people live—and how well they live—it is clear that some answers will come from the bottom up; from the study of elemental subcellular and cellular biological processes. Other answers will come from the top down; from the organization of our behavior through experience and the interaction of "software and hardware" in the nervous system. For example, smoking and high alcohol consumption have been found to shorten life and predispose people to diseases of the heart, lungs, and liver. Behavior and lifestyle
People with higher levels of education appear to have a greater capacity for self-regulation, and they live longer, on average. Being aware of different sources of information, they tend to seek medical assistance sooner and have more medical diagnoses than the less educated, and they spend fewer days in hospitals. Those with less education tend to initiate interventions only when there are health crises accompanied by a higher risk to survival. In contrast, the highly educated seek early interventions, when desired outcomes are more likely.
The "software" of the nervous system can also acquire different tendencies to self-destruction through suicide in different societies. Suicide is about the ninth leading cause of death in America, but is more common in Hungary and Finland. In the history of Japan, ritual suicide (hara kiri) was a justified accompaniment of loss of face or disgrace. The tendency to violence and homicides also varies among different cultures, which again illustrates the interaction of the environment and length of life. While the major causes of death are chronic diseases, culture modulates our disposition to such illnesses, as well as to suicide and homicide.
In the past, religions have placed a high emphasis on individual fates being determined by a higher power. Prayer has therefore been assumed to have great intervention potential. In recent years there has been more exchange between the cultures of religion and of science. The result has been research showing that participation in religious activities does indeed have beneficial effect on aging. Some of the benefits may result from selection of the subjects studied—better adjusted and more socialized adults may be more likely to belong to a religious community. Also, a belief in a higher power by itself may reduce stress and promote health. Regardless of the preferred interpretations of causality in the relationships of longevity, health, lifestyle, culture, spirituality, and religion, the fact that information is being gathered and exchanged is likely to be of use in improving human well-being in later life.
This complex picture defines the subject of gerontology. The study of the biology of aging exists at many levels, from cell parts to whole organisms. In the case of humans, the various critical organs, such as the heart, liver, kidneys, immune system, and the nervous system, have often been studied in isolation. There are both interactions and factors, however, that effect all organs. One of the general factors coming into prominence is the energy-producing component of cells, the mitochondria. Energy is needed by the whole organism for the development, maintenance, and repair of organs, tissues, and cells. Aging of the mitochondria can have widespread consequences throughout an organism. Clearly, new knowledge about aging must be sought at all levels—biological, psychological, social, and cultural. Risk factors are being identified that provide insights into the causes of late-life disabilities and provide clues about ways to ameliorate or control their consequences.
New theories of aging are being advanced as our data and our understanding of the human organism improves. One of the earliest theories was that of Raymond Pearl (1879–1940), who, in 1928, held that the metabolic rate of different species underlay the difference in their length of life. Longevity, he proposed, is inversely proportional to the metabolic rate per unit of body mass. Presumably animals are born with a capacity for a fixed amount of irreplaceable energy. Pearl viewed rate of energy expenditure as predictive of the length of life of a species.
Energy is needed at all levels of an organism, and it ultimately depends upon the functioning of the mitochondria. Thus, aging of the mitochondria can have widespread consequences throughout an organism. The fact that the mitochondria has its own DNA that lies outside of the DNA on the chromosomes, is important. The mitochondrial DNA is transmitted to the fertilized ovum solely from the mother. Is has a structure different from the chromosomal DNA, but it too is susceptible to the effects of damage from oxidants. One general consequence of the metabolism of large amounts of food is the production of oxidants that can interfere with the process of energy release in the cells of the body's critical organs and reduce their level of functioning. In a sense, this is a general environmental interference with genetic expression in the cells of vital organs of the body.
The oxidative damage done by free radicals may also interfere with the signaling or communication between biological systems, which may include immune system in older persons, and its failure to recognize the necessary proteins of a body and attack them. Recent work suggests that the body's inappropriate production of oxidants and the capacity of the body to control oxidative stress is a key feature of aging and the length of life (Finkel and Holbrook, 2000). Also, there is the question of the extent to which the life-limiting effect of oxidant damage results from the oxidant level itself, or from a change in the capacity of the organism to manage it. In either case the result may be individual organ failure or a life-limiting disease.
Gerontology continues to be an expanding field of study and a rising area of public interest. Academic positions are increasing, and even more increases are seen in private-sector employment in service areas such as retirement housing, assisted living, exercise programs, health care, adult education, travel, and entertainment. Also expanding are training activities related to aging in the professions; and new academic and professional journals are appearing. One consequence of this rapid growth is a rise in research on specialized aspects of aging. The rapid growth in published literature has been accompanied by a lag in integrating data from different disciplines. There is a need for integration of information within and across academic disciplines. One might expect to see more emphasis on meta-analysis of many studies, not only on narrow topics, but on broader issues related to genetic, environmental, and behavioral factors.
Many of the factors influencing aging have lower and upper limits that have yet to be defined and measured. The benefits of exercise, for example, conceivably have a ceiling beyond which further activity can have negative outcomes. Such a ceiling is in contrast to the "floor" of low physical activity, which leads to disuse atrophies in the organism and can contribute to episodic risk factors such as falling. In a similar manner, cognitive activity and the use of memory have both optimum upper limits of use for maintenance of function and floor effects leading to regression of function.
As a species, humans have undoubtedly been selected for rapid and sustained physical activity to avoid predators and seek food sources. The capacity of the human nervous system for strategic control of the environment has led to a drop in the need for physical activity and for high food availability. Individuals tend to lower their physical activity when it is not needed, and to overconsume food in relation to metabolic needs. Cultural controls have to be cultivated in these areas to maximize potentials for long and useful lives, along with better health promotion and disease prevention efforts.
With the rising interest in promoting the well being of aging populations, new metaphors are being introduced to motivate people to undertake lifestyle changes. Such metaphors appear to be designed to cast a motivating optimistic aura about aging. Terms like "successful aging," "productive aging," and "vital aging" do not in themselves identify the important variables in human aging. Rather, they reflect a rising interest on the part of the research community to attract public interest to areas of research thought to be useful in an aging population. The proliferation of terms used in gerontology and in reference to aging led to the development of a Thesaurus of Aging Terminology by AARP.
The understanding of aging and the analysis of its complexity requires the consideration of many contributing variables and their interactions. It is inherently a multidisciplinary and interdisciplinary field of research. There appears to be little doubt that the twentieth century will introduce new concepts and theories about aging, and that the sciences concerned with gerontology will advance our understanding of aging and lead to further increases in life expectancy and improved quality of life.
JAMES E. BIRREN
(SEE ALSO: AARP; Aging of Population; Cohort Life Tables; Cohort Study; Dementia; Epidemiologic Transition; Geriatrics; Life Expectancy and Life Tables; National Institute on Aging; Physical Activity; Rates: Age-Adjusted; Rates: Age-Specific; Widowhood)
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