Best explanation I read from Hal_AL…….
When you were working, you only paid half of your SS tax. Your employer paid the other half. At first SS benefits were tax free; then Congress decided to tax 50% of it. You know; the half your employer paid for. But all those rich retirees were still getting half that money tax free, so they settled (for now) on taxing 85% of it. Then they ran for re-election on the 15% they "saved" you.
