Social Security payroll taxes are collected under the authority of the Federal Insurance Contributions Act (FICA). The payroll taxes are sometimes even called "FICA taxes." But what is FICA? Is it a separate law, apart from the Social Security Act?

In the original 1935 law, the benefit provisions were provided under  Title II of the Social Security Act (which is why Social Security sometimes calls Social Security the "Title II" program). The taxing provisions were under a separate title, Title VIII. There is a valid reason for this,  which pertains to the constitutionality of the law. As part of the 1939 Amendments, the Title VIII taxing provisions were taken out of the Social Security Act and placed under the Internal Revenue Code. Since it wouldn't make any sense to call this new section of the Internal Revenue Code "Title VIII," it was renamed the "Federal Insurance Contributions Act." The payroll taxes collected for Social Security are of course taxes, but they can also be described as contributions to the social insurance system that is Social Security. Hence, the name "Federal Insurance Contributions Act." So, FICA is nothing more than the tax provisions of the Social Security Act, as they appear in the Internal Revenue Code.