From the Summary: "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). In both cases, the OASDI tax base is limited to income below a certain threshold, and the HI tax base is not constrained by any income ceiling. The two systems, however, diverge in an important way: The FICA tax is based solely on income from labor, but the SECA tax is based on net business income, which can also include income from capital. Such a difference in the tax code (say, among businesses providing the same goods and services) can prompt people to make choices that they would not otherwise make about self-employment or the organizational form of a business, thereby reducing the efficient allocation of resources."
Congressional Budget Office: http://www.cbo.gov/