Dynamics of functional impairment in late adulthood.

The inconclusive debate about Fries' theory of "natural death and the compression of morbidity" has diverted attention from two key issues--the modifiability and related dynamics of functional impairment in late adulthood. Evidence is presented from a large (N = 11,000) panel of adults aged initially 58-63 over the period of a decade which documents substantial modification of functional status. Further, patterned changes in functional status are related to the dynamics of income, historically a major determinant of functional status, and independently to educational attainment. The dynamics of functional status, of income, and of their relationship are explored using discrete time hazard models in an event history analysis incorporating time-varying income and functional status indicators. The importance of income and of education as positive, independent predictors of functional status and patterned changes of functional status is confirmed. The implications of this finding for differentiating distal (e.g. education) and proximate (e.g. income) measures of socioeconomic status and for assessing competing theories of social causation and social selection are discussed.