The contention that a flat tax would be simpler because it involves only a single rate is flatly wrong. The complexity of the current system has nothing to do with its multiple income brackets.
The hard step in figuring your tax bill is to compute your adjusted gross income — roughly, the amount you earn, less the myriad exemptions, deductions and various other offsets described in the 3.4-million-word code of the Internal Revenue Service. You’d also have to calculate your adjusted gross income under a flat tax. But once you’ve completed that step under either system, you consult the tax tables to see how much you owe. In the current system, the entries have multiple brackets and rates already built into them, so this step is no harder than it would be under the tables for a flat tax.
The much more serious concern is that a flat tax would reinforce the trends toward greater income inequality that have been seen over the last several decades. As documented by a recent Congressional Budget Office study, the top 1 percent of income recipients in the United States earned 275 percent more in 2007 than they did in 1979, adjusted for inflation, a period when the earnings of middle-income households grew by less than 40 percent. A flat tax would increase inequality by substantially reducing rates on the most prosperous households, while increasing them on low- and middle-income households.