If current law continued, revenues would also rise considerably; by the 2020s, they would reach higher levels relative to the size of the economy than ever recorded in the nation’s history. Under current law, revenues would jump from about 15 percent of GDP now to 19 percent in 2013 as the economic recovery increased taxable income, as the tax cuts enacted since 2001 expired in 2012 and 2013 as scheduled, and as the reach of the AMT expanded greatly (because, unlike most of the tax code, the dollar amounts of its parameters do not automatically increase with inflation). In later years, revenues would continue to rise relative to GDP, for three main reasons. First, ongoing increases in real income would push taxpayers into higher tax rate brackets. Second, ongoing inflation, although it is projected to be modest, would cause more people to owe tax under the AMT. And third, the excise tax on certain high-premium health insurance plans, which is scheduled to take effect in 2018, would have a growing impact on revenues. Taken together, those factors would cause marginal tax rates to increase and federal revenues to grow faster than the economy, reaching 23 percent of GDP in 2035. By comparison, federal revenues averaged 18 percent of GDP between 1971 and 2010, peaking at 20.6 percent of GDP in 2000.
If current law regarding the AMT remained unchanged, as assumed in the extended-baseline scenario, the alternative minimum tax would ultimately affect a significant share of taxpayers. Just 3 percent of households will pay the AMT in 2011—the last year in which temporarily higher exemption amounts are in effect under current law. However, in 2012—following the expiration of AMT relief at the end of 2011 but before the expiration at the end of 2012 of the income tax cuts extended by the 2010 tax act—the AMT would affect 18 percent of households, CBO projects. In 2013, the share of households affected by the AMT is estimated to fall to 11 percent because of the expiration of the income tax cuts extended by the 2010 tax act. In subsequent years, the share of households who owed more under the AMT than under the regular tax would gradually rise. By 2035, nearly half of the nation’s households would be subject to the alternative tax.