Before the government enacted the Social Security Amendments of 1983, social security income was not taxable.
The legislation’s enactment created the need to withhold federal income tax. Form W-4V, (Voluntary Withholding Request), allows benefit recipients to have the federal government withhold a portion of their social security income.
In addition to social security benefits, taxpayers can use Form W-4V to voluntarily withhold from unemployment compensation, Tier 1 railroad retirement benefits, Commodity Credit Corporation loans, certain crop disaster payments, or title II of the Disaster Assistance Act of 1988. Taxpayers can file the form again to stop withholding.
This is why today you are taxed on Social Security Income, on which you already paid tax on this income.