Flat tax

Andrew Sullivan recently mentioned in an offhand way that he favors a flat tax, which I suppose triggered a big reaction, because he has followed it up with a longer post defending his position. He says:

So yes: a flat tax so far as possible for as many as possible and no deductions. That's my goal. How that differentially impacts the lives of citizens should not be government's primary concern.

Government's primary concern is to raise money as efficiently and as leanly and as equally as possible. I'm happy with the government then setting up programs to assist the poor, to provide better education for those at the bottom, safety-net healthcare and better policing. i.e. to gear spending toward social ends that might help the poor the most. These are measurable, practical goods. What I'm not happy with is the assumption that tax policy should really be about redistributing wealth, and engineering substantive economic outcomes. Yes, of course, at lower income levels, a 20 percent flat income tax will be more onerous proportionally than at higher incomes. So what? Why should that even concern a government that is not aiming to socially engineer more substantive equality? and the alternative - skewing taxes to target success - is an absurd set of incentives to put into a growing society.

So Sullivan wants the government to raise money "as equally as possible", and does not want the government to redistribute wealth. But why does a flat tax accomplish this any more than a progressive tax? The law of diminishing returns states that the 100,oooth dollar you earn is less valuable to you than the 10,000th dollar you earn, and that the 10,000th dollar you earn is less valuable to you than the 1,oooth. Given this, it makes sense that the government should tax at the lowest rates for the first dollars you earn and gradually raise that rate for successive dollars earned. Since the same progressive rate is applied to everyone, money is raised "as equally as possible".

Yes, of course, at higher income levels, a progressive tax structure will be nominally higher than at lower incomes. So what? Why should that even concern a government that is not aiming to socially engineer more substantive equality? And the alternative - skewing taxes to target poverty - is an absurd set of arbitrary penalties to put into a fair society.