The Social Security Trust Fund was established in 1939 to receive monies collected for Social Security through payroll taxes. The monies in this fund are managed by the Department of the Treasury; they are not, nor have they ever been, put into the "general operating fund."

However, whether the Social Security Trust Fund can truly be said to be "independent" is problematic. The Social Security Act specifies that the monies in the fund may only "be invested in securities backed by the full faith and credit of the Federal government," such as treasury bills, treasury notes, and treasury bonds, as well as special issue bonds. So, essentially, the government can "invest" Social Security funds by lending them to itself, then spending that money on programs not related to Social Security (e.g., defense, foreign aid, education). The government "pays back" this money when the Social Security program redeems the bonds, but critics of the program contend Social Security will eventually fall into deficit by 2018, and the Treasury won't have the necessary cash on hand to redeem the bonds and pay back the fund. [...]