The search for a general, theoretical framework of aging has long been of interest to both geriatrics and gerontology. Before 1900, Weismann proposed the first evolutionary theory of aging and argued that biological aging and death are adaptations that ensure species renewal and are programmed within the organism itself (1889). Mutation Accumulation Theory later explained aging as a process that, due to lessening effects of natural selection on post-reproductive individuals, mutations continue to accumulate over time, increasing mortality in later life (Medawar, 1946). Finally, Antagonistic Pleiotropy describes natural selection as having a bias toward youth that selects against longevity when genes that have pleiotropic effects are advantageous in the young but disadvantageous in the aged organism (Williams, 1957). For an expanded review of these and other proposed evolutionary theories of aging, see Gavrilov and Gavrilova (2002) or Crews (2003).
Why organisms have a short or a long life and why they grow old and die remain questions that are best addressed within an evolutionary framework (Gavrilov and Gavrilova, 2002). While the search for an evolutionary, universal theory of biological aging continues, current directions focusing on physiological aging, in addition to asking why organisms age and die, are also continuing to identify factors that act to further extend life (i.e., via caloric restriction, maintenance of cortical function, and tissue rejuvenation, etc.). Additionally, however, some contemporary investigators are adjusting the focus of their aging investigations to describe biological senescence as part of a general, life history model of inquiry. Although in our view, this methodological approach to aging investigation is a step in the right direction, it can also be limiting unless it addresses individual, behavioral variation (personality) and its effect on successful aging.
All biological organisms develop, mature, and age over the life span. Although the processes of human development, maturation, and reproduction have been broadly researched, aging studies have, for the most part, limited their focus to longevity and biological senescence (i.e., on geriatrics). Aging, however, is a complex interaction of factors that go beyond clinical aging and biological senescence, and should be investigated from a broader, multidimensional perspective if we hope to enhance our expanded longevity. In contrast to geriatrics, gerontology is interested in understanding the processes of normal, undiseased aging, and draws on collaborations from diverse disciplines, both biological and behavioral (Crews, 2003). It is argued here that it is essential to expand aging research designs to include both biology and behavior whenever possible, so that a better understanding of growing older as the culmination of multiple processes can be achieved.
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