From 1988 to 2001, the total number of people receiving SSI rose more than two and one-half times, from about 1.7 million to 4.3 million. During the same period, the number of SSI recipients with mental disorders more than tripled from 411,800 in 1988 to 1.5 million in 2001. About one in four SSI recipients (24.5%) had a mental disorder in 1988, increasing to 35.4% by the year 2001.
The percentage of "disabled workers" (SSDI recipients) with mental disorders has also grown over time, but not as rapidly as the SSI population.
Source: Social Security Bulletins (multiple years), Table 7.F.1
Surveys: Social Security Program Administration Data