for the substantial majority of older adults. And the prospects of beneficial behavioral and social interventions to enhance well-being in later life continue to improve. The mind set of contemporary gerontology is increasingly interventionist and optimistic about change.

The challenges of individual and population aging are real and should not be underestimated. Demographers have documented, and often viewed with alarm, the increase in dependency ratios (the number of nonworking individuals versus those actively in the workforce) and the age gradient of risk of dependency, and have forecast problems for pension, income maintenance, and health care financing. Epidemiologists have documented both the significance and the persistent age gradient of morbidity and comorbidity and, with behavioral and social scientists, have also documented notable variation in life expectancy and risk of functional disability associated with socioeconomic status, ethnicity, and gender. The documented diversity of older populations has been a significant factor in the growing awareness of gerontologists that chronological age per se is a relatively weak explanatory variable in assessing the prospects of continuing to age well in later life, which is the experience of a substantial majority of older adults (Campbell et al., 1976; Maddox and Glass, 1999).

The established behavioral and social diversity of older adults has had important practical and theoretical consequences in gerontological research. Practically, sampling older populations for research and for generalizing about older adults, who constitute about 13% of the U.S. population currently, typically requires oversampling to insure inclusion of minorities and the very old. Theoretically, repeatedly observed diversity within age cohorts suggests both that behavioral and social factors are involved in aging processes and that such factors are potentially modifiable. The intellectual shot that was heard around the world in gerontology was fired by Stanford's James Fries (1980) in a seminal article on “the compression of morbidity. ” Fries argued that even if the life span is typically limited (he optimistically and incorrectly concluded that the limit is 85 years), the risk of morbidity and related functional disability can be delayed through interventions designed to improve physical activity, social integration, and cognitive performance. Critics initially scoffed and dismissed such optimism, but not for long. The evidence supporting the modifiability of functional disability in later life has continued to accumulate and has laid the foundation for the increasing interest of gerontologists in purposive behavioral and social interventions to improve the well-being of older adults (e.g., Rowe and Kahn, 1987; Berkman, 1988; Manton, 1988; Fries, 1995).

Unlike their clinical colleagues who view interventions at the personal level to be a natural extension of practical knowledge, behavioral and social scientists have moved cautiously in developing enthusiasm for psychosocial interventions. And they have remained particularly cautious about macrosocial legislated interventions such as Social Security and Medicare or Medicaid, both of which are viewed as important and beneficial natural experiments but outside the competence of most behavioral and social scientists to initiate and evaluate as political interventions (see, e.g., Marmor and Okma, 1998). An important turning point in