BIOGERONTOLOGIC THEORIES

Theories of age and aging engage a wide variety of phenomena and level s of explanation. There is a general consensus that aging is a complex process or set of processes, involving many ''causal'' inputs and man ifold consequences. Two general theoretical orientations that explicit ly address complexity are sketched here-the differential theory of qua ntitative genetics and systems theory. Among the suggestions derived a re that it may be advantageous to consider aging to be hierarchically organized, with the corollary that subsystems of an organism can have different functional or biological ages, and that several or many indi ces will be required to provide an adequate characterization of a sing le individual. Aging probably proceeds by saltation rather than contin uously. Uncertainty associated with bifurcations in complex systems, t ogether with individual differences in timing and magnitude of step ch anges, may constitute fundamental limitations to the predictability ac ross the full life trajectory. Genetic and environmental influences wi ll differ from hierarchical subsystem to subsystem, and may differ wit hin a subsystem across chronological age. Copyright (C) 1997 Elsevier Science Inc.