Using the latest long-term budget projections from the Congressional Budget Office, we project that individual income tax revenues under current law will increase as a share of GDP from a little over 9.5 percent in 2025 to a little less than 13.3 percent in 2090, an increase of over 3.7 percentage points. This paper describes the factors that explain this differential in growth rates and provides estimates from the Tax Policy Center’s new long-run microsimulation model of the relative importance of each of these factors over the 2025-2090 period. We find that 80 percent of the increase in revenues as a share of GDP occurs because current law does not adjust some individual income tax parameters for inflation and none of the parameters for changes in real income.