The Taxation of Capital and Labor Through the Self-Employment Tax
from CBO Recent CBO Publications
Since 1950, self-employed individuals have been covered by the Social Security system. In many regards, their obligation to pay Self-Employment Contributions Act (SECA) taxes into the Old-Age, Survivors, and Disability Insurance (OASDI) and Hospital Insurance (HI) trust funds and their entitlement to Social Security and Medicare benefits parallel those of workers who are not self-employed and who thus are covered under the Federal Insurance Contributions Act (FICA).
http://www.cbo.gov/publication/43644