chronic conditions, thus contributing to the apparent increase in disability without any change in the population's health status. Wolfe and Haveman (1990), in their analysis of trends in work disability from 1962 to 1984, suggest that less stringent eligibility criteria for public disability assistance programs, together with increased willingness to report work limitations, may have acted to increase disability rates through the early 1970s. Subsequent tightening of eligibility criteria appear to have helped reduce the work disability rates from their 1970s peak.

Verbrugge (1984) compares this increase in disability to the decrease in mortality during the same period. After analyzing the possible explanations for these two apparently divergent trends on a disease-specific basis, she concludes that an emphasis on secondary prevention—early detection of chronic disease and intervention to slow its progress—is a major part of the explanation. Based on this analysis, she predicts that health statistics will continue to show increasing morbidity through the turn of the century. Whether this trend continues depends on the success of primary prevention programs to halt the incidence of disease in the future (Verbrugge, 1984).

Other analysts have concentrated on future changes in the prevalence of disability that can be expected due to demographic changes. Assuming that age- and sex-specific disability prevalence rates remain constant, Manton (1989) has estimated that the elderly population with chronic disabling conditions (living in the community and in institutions) could grow by 31 percent to 7.2 million between 1985 and 2000. This compares with a projected 20 percent increase in the nondisabled population. Manton also projects that the most severely disabled population (those with five to six ADL impairments) and the population in institutions could grow even faster. These trends are expected to continue well into the twenty-first century as the baby boom generation ages. In 2060, for instance, the number of people aged 65 or older with chronic disabling conditions could exceed 15 million (Manton, 1989). Schneider and Guralnik (1990) project similar increases in the number of older people requiring nursing home services and experiencing disabling conditions such as dementia and hip fractures.

CONDITIONS LEADING TO DISABILITY

A wide variety of chronic conditions are responsible for activity limitation in the United States. Looking at the single "main cause" of activity limitation as reported by respondents in the NHIS, orthopedic impairments account for 16.0 percent of activity limitations, arthritis for 12.3 percent, heart disease for 11.5 percent. The left half of Table 2-4 gives the 15 single



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