The amusing thing for me is that there are certain incomes where the marginal rate actually goes down when income goes up (although absolute taxes paid always go up), due to phase-outs of deductions, various regressive taxes like FICA, local, etc.. It's actually quite complicated -- someone did a table on Quora, which I can't readily find right now, and it looks like the marginal rate peak for a single person was usually around 180-220k. From a purely efficiency standpoint, it seems like one would want the marginal rate function to be as smooth and ideally monotonic (if not strictly monotonic) as possible.