It sounds super-nerdy - and it is -  but it's also fascinating. Nate Silver, reacting to Yglesias, informs me that

The average top marginal tax rate since the income tax was established is 60 percent.

But even this is misleading because of the threshhold question:

In 1940, for example, the top marginal tax rate was 81.1 percent -- but this rate only kicked in once you made $5,000,000 or more in income, which is equivalent to about $75,000,000 in today's dollars.

But today, the threshold where the top tax bracket kicks in isn't $75 million, or $5 million, or even $1 million ... it's a mere $357,700. The progressivity of the tax code stops there.

Nate draws an obvious conclusion:

The question, of course, is why there isn't a millionaires tax bracket now ... or even a multi-millionaires tax bracket. I haven't run the numbers, but I'm guessing that if you established a new tax bracket at, say, 40.5 percent, that started at incomes of $1,000,000 or more, this would bring in as much revenue to the government as restoring the $250K tax bracket (which is really $360K now given indexing to inflation) to 39.6 percent, as it was under Clinton.

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