In 1956, the Social Security Act was amended to provide monthly disability insurance benefits to qualifying individuals under a uniform national program administered by the Secretary of Health, Education, and Welfare. Under this program, a claimant is entitled to disability benefits if he is unable to "engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to be of long continued and indefinite duration." This definition and its accompanying statutory standards were purposely made conservative in order to minimize the problems inherent in initiating the program; it was contemplated that improvements and adjustments would be made when experience with the administration of the program was acquired.
Michigan Law Review,
Social Security Disability Determinations: The Burden of Proof on Appeal,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol63/iss8/9