2 Chapter 2: Theories of Aging

Tracy Everitt; Megan Davies; Laurie Wadsworth; and Brittany Yantha

Chapter 2 Learning Objectives

At the conclusion of this chapter, students will be able to:

Learning Objectives

  • Identify the major theories of aging.
  • Compare the similarities and differences between biological and psychosocial theories
  • Describe the process of aging using a biological and psychosocial perspective
  • Explain the rationale for using multiple theories of aging to describe the complex phenomenon of aging


From the beginning of time, preserving youth has been a topic of discussion in science, health care, technology, and everyday life. Few would not want to see what the future holds for our bodies and minds; even more, curiosity surrounds what advances have been made or will possibly be made to alter and slow the aging process. The knowledge these theories provide may help to answer questions surrounding the bodily processes of aging. The purpose of this chapter is to describe the theories of aging and what evidence supports or refutes these theories. 

2.1 The Aging Process

In the past, the maximum life span was thought to be fixed as the process of aging was considered non-adaptive and determined by genes. In the early 1900s, a series of flawed experiments by researcher Alexis Carrel demonstrated that in an optimal environment, cells of higher organisms divided perpetually (Vina et al, 2007). These findings led people to believe that human cells potentially possess immortal properties. In the 1960s, Leonard Hayflick disproved this theory by identifying a maximal number of divisions a human cell could undergo in culture (known as the Hayflick limit) before breaking down by programmed cell death or apoptosis (Vina et al., 2007). This theory sets our maximal life span at around 115 years. 


There are many theories about the mechanisms of age-related changes. No one theory can sufficiently explain the aging process, and they often contradict one another. All valid theories of aging must meet three broad criteria: 

  1. The changes associated with aging that the theory addresses must occur commonly in all humans.
  2. The process must be progressive with time. That is, the changes that result from the proposed process must become more obvious as the person grows older.
  3. The process must produce changes that cause organ dysfunctions and that cause a particular body organ or system to fail.

Theories validated through research can guide healthcare providers in helping elderly patients navigate the changes they experience. There are two predominant categories of theories of aging: psychosocial and biological theories of aging. 

Psychosocial Theories of Aging

The psychosocial theories of aging focus on changes in behaviour, personality, and attitudes as people age. The authors of these theories propose that aging is a lifelong process characterized by major life transitions. These transitions are related to personality or ego development and the accompanying challenges associated with the various life stages. Psychosocial theories of aging speak to how human mental processes, emotions, attitudes, motivations, and personalities influence adaptation to physical and social environmental conditions. Sociological theorists consider how the changing of roles, relationships and status within a culture or society impacts an older adults ability to adapt. These theorists have asserted that societal norms can affect how individuals perceive and enact their role within a community  

2.2 Sociological Theories of Aging

Activity Theory

The activity theory was one of the first aging theories proposed as a result of studying a group of adults. In the early 1950s, sociological theorists believed that society expects retirees to remain active in their communities as they age. Activity theory states that staying occupied and involved is necessary for a satisfying late life (Havighurst & Albrecht, 1953). This theory describes how people can best adjust to the changing circumstances of old age. Activity theory thus strongly supports the avoidance of a sedentary lifestyle and considers it essential to health and happiness that the older person remains active physically and socially. Activity theory suggests that many people are barred from meaningful experiences as they age, but older adults who remain active can replace lost opportunities with new ones. 


This theory is limited because the sorts of activities linked to life satisfaction are not specified. Instead, these theorists believed that the activity itself was associated with psychological health. The thought was that being active helps prolong middle age, which delays the adverse effects of old age.  


Critics of activity theory argue that this approach fails to consider that choices are often limited by ones physical abilities, finances, and access to social resources. There is also the unproven assertion that staying active necessarily delays the onset of the negative effects of aging. Other investigators have suggested that the type of activity does matter and that activities that connect people socially, such as meeting friends for lunch or pursuing hobbies through group activities were more likely to improve life satisfaction than formal or solitary activities. Researchers proposed that successful aging means being able to do things despite the limitations that come with aging (Lange & Grossman, 2010). 

Disengagement Theory

A stark contrast to the activity theory is the disengagement theory, which was proposed in 1961. The Disengagement theory states that aging is characterized by gradual disengagement from society and relationships (Cumming & Henry, 1961). Researchers in favor of this theory believe that both parties desire this separation of older adults from society and that it serves to maintain social equilibrium. The thought is that by disengaging, older adults are freed from social responsibility and gain time for internal reflection, while the transition of responsibility from old to young maintains a continuously functioning society unaffected by members lost to aging. The outcome is a new equilibrium that ideally satisfies older adults and society. This theory states that disengagement enables the older person to die more peacefully, without the stress and distractions of a more socially involved life. The theory suggests that the individual and society mutually withdraw during late adulthood. Older people become more isolated from others and less concerned or involved with life. This once popular theory is now criticized as ageist and used to justify treating older adults as second-class citizens. Critics of this theory argue that the emphasis on social withdrawal is inconsistent with a key element of life satisfaction: engaging in meaningful relationships and activities. It is also argured that the decision to withdraw from society varies among adults, and this theory fails to account for differences in sociocultural settings and environmental opportunities. The many examples of what is termed successful aging are challenging disengagement theory as older adults are actively engaging in a variety of activities that benefit and contribute to their communities and living longer without the prevalence of disease. 

Continuity Theory

The continuity theory, founded in the late 1960s, hypothesizes that personality influences the roles people choose and how they are enacted, influencing ones satisfaction with living. Continuity theory suggests as people age, they continue to view the self in much the same way as they did when they were younger. An older persons approach to problems, goals, and situations is much the same as when they were younger. They are the same individuals, but simply in older bodies. Consequently, older adults maintain and renegotiate their identity even as they give up previous roles and change to new ones. People do not give up who they are as they age. How people define themselves may change with changing societal roles. Hopefully, they can share these aspects of their identity with others throughout life. Focusing on what a person can do and pursuing those interests and activities is one way to optimize and maintain self-identity. 


Continuity theory is the first sociological theory to acknowledge that responses to aging differ among individuals. A positive attitude about adaptation to old age was associated with better perceptions about the quality of life. Active participation in society tends to decline over time, and lifelong participation patterns predict involvement later in life. Critics caution that the social context within which one ages may be more important than personality in determining what and how roles are played. 

2.3 Psychological Theories of Aging

Human Needs Theory

While the activity theory was being developed, the human needs theory was published in 1954 by Abraham Maslow. This theory summarizes a hierarchy of five needs which motivate human behaviour: physiologic, safety and security, love and belonging, self-esteem, and self-actualization. These needs are prioritized so that more basic needs like physiological functioning or safety precede personal growth needs (Maslow, 1954). The movement towards personal growth need fulfillment is a multidirectional and dynamic lifelong process. The need for self-actualization requires the freedom to express and pursue personal goals and be creative in a stimulating and challenging environment that is not accessible to all people. Human needs theory notes that failing to grow toward personal needs leads to feelings of failure, depression, and the perception that life is meaningless. This theory has been applied to varied age groups in many disciplines, as Maslows hierarchy presents a model for the nursing care of older adults in residential settings. 

Theory of Individualism

The theory of individualism (Jung, 1960) is similar to the theory of human needs, which is not specific to aging. This theory proposes that a humans personality develops over a lifetime and is composed of an ego or self-identity with a personal and collective unconscious. The personal unconscious is the private feeling and perceptions surrounding important persons or life events. The collective unconscious is the foundation of personality on which the personal unconscious and ego are built. This theory states that people tend to view life through either their own lens (introverts) or the lens of others (extroverts) (Jung, 1960). As individuals age, they reflect on their beliefs and accomplishments. People are considered to age successfully when they have accepted the past, adapted to their physical decline, and coped with the loss of significant others. This theory states that introspection promotes positive inner growth and is a part of healthy aging. 

Stages of Personality Development Theory

The stages of personality development theory, founded in 1963 by Erik Erikson, focuses specifically on the development of ones personality. This theory states that personality develops in eight sequential stages, each associated with a life task at which one may succeed or fail. Reaching old age is the final stage of development, where individuals search for the meaning of their lives and evaluate their accomplishments. From this period of introspection, feelings of satisfaction lead to integrity, while dissatisfaction creates a sense of despair (Erikson, 1963). Older adults specifically face additional challenges or life tasks, including physical and mental decline, accepting the care of others, and detaching from life. Theorists of the stages of personality development define integrity vs. despair” to include three more challenges: creating a meaningful life after retirement, dealing with an empty nest as children move away, and contemplating the inevitability of death. The stages of personality development theory are widely employed in the behavioural sciences. In nursing, this model is used as a framework to examine the challenges different age groups face. Research has found that older adults who expressed elevated energy and meaning in life described a sense of connectedness, self-worth, love, and respect that was absent among participants who felt unfulfilled. 

Biological Theories of Aging

Biological theories of successful aging explain the physiological processes that change with aging. These theories delve into how aging is manifested on the molecular level in the cells, tissues, and body systems; how the body-mind interaction affects aging; what specific biochemical processes impact aging; and how ones chromosomes impact the overall aging process. Some aging theorists divide the biological theories into two categories: 

  1. A stochastic or statistical perspective, which identifies episodic events that happen throughout one’s life that cause random cell damage and accumulate over time, thus causing aging.
  2. The non-stochastic theories, which view aging as a series of predetermined events happening to all organisms within a time framework.

Others believe aging is more likely the result of both programmed and stochastic concepts as well as allostasis, which is the process of achieving homeostasis via both behavioural and physiological change. 

2.4 Stochastic Theories

Stochastic theories focus on the study of animals, which reflect that the effects of aging are primarily due to genetic defects, development, environment, and the inborn aging process. There is currently no set of statistics to validate these findings with human organisms. 

Free Radical Theory

The free radical theory was developed in 1956. This theory posits that aging is due to oxidative metabolism and the effects of free radicals. The term free radical describes any molecule with a free electron, which makes it destructively react with healthy molecules. Due to the extra electron, free radical molecules have an extra negative charge (Hayflick, 1985). This unbalanced energy makes the free radical bind itself to another balanced molecule as it tries to steal electrons. In so doing, the balanced molecule becomes unbalanced, thereby becoming a free radical. Free radicals can enter reactions with other molecules, such as proteins, lipids, DNA, RNA or along membranes, causing cellular damage. This damage accumulates over time and is thought to accelerate the process of aging. Free radicals causing extensive cellular damage to DNA can cause DNA strands to break, causing base modifications that lead to gene modulation. Cellular membrane damage can cause other chemicals to be blocked from their regular receptor sites, thus impacting other processes that may be crucial to cell metabolism (Hayflick 1985) 


It is known that diet, lifestyle, drugs such as tobacco and alcohol, and radiation are all accelerators of free radical production within the body. Older adults are also more vulnerable to free radical damage as oxidative damage is known to increase with age. Some strategies to delay mitochondrial decay include decreasing calories to lower weight; maintaining a diet high in nutrients, especially antioxidants; avoiding inflammation; and minimizing the accumulation of metals in the body that can trigger free radical reactions (Harman, 1994). Additionally, studies have demonstrated that mitochondrially targeted antioxidant treatments may decrease the adverse effects of Parkinsons Disease, Alzheimers disease, and cardiovascular disease.  

Error Theory

The error theory, also named the Orgel theory (author, 1970), states that cells accumulate errors in their DNA and RNA protein synthesis that cause the cells to die. Environmental agents and randomly induced events can cause errors, with ultimate cellular changes. For example, substantial amounts of x-ray radiation can cause chromosomal abnormalities. Aging would not occur if destructive factors such as radiation did not exist and cause errors such as mutations and regulatory disorders. Critics of this theory explain that all aged cells do not have errant proteins, nor are all cells found with errant proteins old. 

Wear and Tear Theory

The wear and tear theory of aging was first introduced in 1882 and compares the aging experience of the human body to the experiences of nonliving things that people observe around them (i.e., components of an aging car brake due to repeated use). Cells and tissues have vital parts that wear out, resulting in aging (Van Cauter, Leproult, & Kupfer, 1996). This theory states that body parts eventually wear out from repeated use, killing the parts and then the body. Over time, cumulative changes occur in cells and damage cellular metabolism. For example, the cell becomes unable to repair damaged DNA, resulting in the cell aging. Cells in heart muscle, neurons, striated muscle, and the brain cannot replace themselves after they are destroyed by wear and tear. Some speculate that excessive wear and tear caused by exercising may accelerate aging by increasing free radical production. Studies have found that aged cells have lost the ability to counteract mechanical, inflammatory, and other injuries due to their senescence. No one theory of aging incorporates all the causes of aging, but a combination of factors likely contributes 

2.5 Non-Stochastic Theories

The nonstochastic theories of aging are founded on a programmed perspective that is related to genetics or ones biological clock. It is suggested that aging is more likely to be an evolved characteristic resulting from complex processes and not a series of random events. 

Programmed Theory

Robust evidence supports the suggestion that aging is somehow programmed into each species, including humans. This may be controlled neurologically or hormonally through the hypothalamus. Other studies suggest that the thymus, a gland that atrophies at about the onset of adolescence, controls the aging process suggesting that aging occurs more readily in the absence of the thymus gland.   In studies of cultured cells, however, cells divide a certain number of times, implying that the cell’s life cycle is controlled within the cell, rather than externally by the hypothalamus or thymus.  


Scientists agree that although normal cells may have a limited number of times they can divide, this is rarely if ever, reached in the body. It is believed that cells have a finite doubling potential and become unable to replicate after they have done so several times. Human cells age each time they replicate because of the shortening of the telomere. Telomeres are the most distal appendages of the chromosome arms. The enzyme telomerase allows human cells grown in the laboratory to continue replicating once they normally stop dividing. Normal human cells do not have telomerase. It is hypothesized that some cancer, reproductive, and virus cells are not restricted, having infinite doubling potential. These cells become immortal due to the presence of telomerase, which adds back DNA to the ends of the chromosomes. Although it is unknown what initial event triggers apoptosis, it is acknowledged that apoptosis is the mechanism of cell death (Thompson, 1995).  

Gene Theory

The gene theory states that aging is programmed due to one or more harmful genes within each organism. The gene theory suggests that human life span is an inherited trait. This theory explains that each cell, or the entire organism, has a genetically programmed aging code that is stored in the organisms DNA. This theory suggests that aging is the result of genetic influences that predict physical condition, disease occurrence, cause and age of death, and other factors contributing to longevity. Studies show that identical twins die at similar ages compared to fraternal twins or siblings.  

2.6 Implications for Healthcare

Understanding theories of aging can assist healthcare providers in supporting their clients. For example, psychosocial theories help to describe the development tasks and challenges older adults face and the importance of finding and accepting meaning in ones life. These theories recommend that social support systems, activity and role engagement support healthy aging and suggest that societal expectations can influence adjustment to aging and life satisfaction (Lange & Grossman, 2010). Healthcare providers can learn from these theories to help minimize challenges of aging by connecting older adults to resources. For example, dealing with loss of friends, spouse, and other important relationships can lead to isolation and depression. Connecting older adults to their communities through senior centers, online groups, adult education programs, or volunteer groups can help them to explore new passions and develop new relationships 



This chapter has been adapted from ‘Biology of Aging’ by Jessica Kelly: https://library.achievingthedream.org/herkimerbiologyofaging/

Creative Commons Attribution: BY


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