(1986). The lower estimate is based on national survey data by LaPlante (1989a) and Sirrocco (1987). The two most common developmental disabilities, mental retardation and cerebral palsy, rank first and fifth as chronic conditions causing major activity limitation among persons of all ages (Table 4-1) (LaPlante, 1989a) and rank ninth and eighth, respectively, as conditions that create a need for assistance in carrying out basic life activities.

Disability years, a measure introduced by Houk and Thacker (1989), represents the number of years people survive with disabilities and thus provides an estimate of the public health impact of disability. By this measure, developmental and other childhood disabilities accounted for 35 percent of all disability years in 1986 (Table 4-2). This highlights the importance of preventing childhood disabilities, because significant gains in this area will have a ''multiplier effect," substantially decreasing the number of disability years.

The national costs of caring for children with developmental disabilities are substantial. Data from the 1980 National Medical Care Utilization and Expenditure Survey (NMCUES) indicate that children who experience limitations in normal activities use more medical services than other children, resulting in significantly higher health costs for this group (Newacheck and McManus, 1988).

Of an estimated total of $40.5 billion spent on health care for all children under the age of 21 in 1988 (based on updated 1980 National Medical Care Utilization and Expenditure Survey data), approximately $4.4 billion was spent on children with chronic disabling conditions. Thus an average of $1,406 was spent on each child with a chronic disabling condition, compared with an average of $487 for other children. Four percent of those under the age of 21 accounted for nearly 11 percent of total health care expenditures for that population (Newacheck and McManus, 1988). However, these cost figures underestimate the individual and total charges because of the significant changes in medical care costs and out-of-pocket expenditures since 1980.

A 1986 study of state, local, and federal government expenditures on institutionalization, income maintenance, and special education revealed combined spending of $16.5 billion in 1984, a 23 percent increase over 1979. These figures represent $7.28 billion in federal expenditures, $6.08 billion in state expenditures, and $3.12 billion in local expenditures (Braddock and Hemp, 1986).

The following section provides a descriptive epidemiology of developmental disability.

Epidemiology of Developmental Disabilities

Clinical disorders and diseases associated with developmental disability can be categorized by time of onset as follows: hereditary disorders, early



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