Since the passage of the 1983 Amendments to the Social Security Act Social Security benefits are subject to taxation. The amount of Social Security income which is taxable depends on your taxable income. Most high-income retirees will have 85% of Social Security benefits taxable. For lower-income retirees, less than 85% will be taxable, but many retirees in a 15% tax bracket will face a marginal tax rate much higher than 15%.