High-income taxpayers face two special taxes — a 3.8% net investment income tax (NIIT) and a 0.9% additional Medicare tax on wage and self-employment income. Here’s an overview of the taxes and what they may mean for you. 3.8% NIIT This tax applies, in addition to income tax, on your net investment income. The NIIT only affects taxpayers with adjusted gross income (AGI) exceeding $250,000 for joint filers, $200,000 for single taxpayers and heads of household, and $125,000 for married individuals filing separately. If your AGI is above the threshold that applies ($250,000, $200,000 or $125,000), the NIIT applies to the lesser of 1) your net investment income for the tax year or 2) the excess of your AGI for the tax year over your threshold amount. The “net...