Net Investment Income Tax
Also called: NIIT, 3.8% surtax, Medicare surtax on investments
The net investment income tax adds 3.8% on investment income — interest, dividends, capital gains, rents — for taxpayers above a MAGI threshold. Its thresholds are not indexed to inflation, so it reaches more households every year by design. It stacks on top of capital gains rates, which is why the true top US rate on long-term gains is 23.8% rather than the headline 20%.