In 2009, net contributions accounted for 83 percent of total trust fund income. Net contributions consist of taxes paid by employees, employers, and the self-employed on earnings covered by Social Security. These taxes are paid on covered earnings up to a specified maximum annual amount, which was $106,800 in 2009. The tax rates scheduled under current law for 2009 and later are shown in table II.B2.
Three percent of OASDI Trust Fund income came from subjecting up to 50 percent of Social Security benefits above specified levels to Federal personal income taxation, and 15 percent of OASDI income came from interest earned on investment of OASDI Trust Fund reserves. Trust fund assets are invested in interest-bearing securities of the U.S. Government. In 2009, the combined trust fund assets earned interest at an effective annual rate of 4.9 percent. More than 98 percent of expenditures from the combined OASDI Trust Funds in 2009 were retirement, survivor, and disability benefits totaling $675.5 billion. The financial interchange with the Railroad Retirement program resulted in a net payment of $4.1 billion from the combined OASDI Trust Funds, or about 0.6 percent of total expenditures. The administrative expenses of the Social Security program were $6.2 billion, or about 0.9 percent of total expenditures.
Assets of the trust funds provide a reserve to pay benefits whenever total program cost exceeds income. Trust fund assets increased by $121.7 billion in 2009 because income to the combined funds exceeded expenditures. At the end of 2009, the combined assets of the OASI and the DI Trust Funds were 355 percent of estimated expenditures for 2010, up from an actual level of 353 percent at the end of 2008.