20 CFR §404.1578

1. (a)For monthly benefits payable for months prior to January 1991, we will find that you were disabled and pay you widow's or widower's benefits as a widow, widower, or surviving divorced spouse if—
   1. (1)Your impairment(s) had specific clinical findings that were the same as those for any impairment in the Listing of Impairments in appendix 1 of this subpart or were medically equivalent to those for any impairment shown there;
   2. (2)Your impairment(s) met the duration requirement.
2. (b)However, even if you met the requirements in paragraphs (a) (1) and (2) of this section, we will not find you disabled if you were doing substantial gainful activity.