[ajzinsbwbs]

I think the long-term historical drop in the ratio of [top marginal rate on individual income] to [tax as a % of GDP] is mainly the result of these factors:

1) increased tax compliance

2) more tax deductions

Overall, you’d need to use a more comprehensive “area under the curve” analysis of every tax to predict the impact of tax policy on total tax revenue, and it would be surprising if it could be predicted based only on the top marginal rate.

A major source of short-term noise is the business cycle, where capital gains and corporate profits fluctuate much faster than GDP, and some automatic stabilizers decrease revenue.

[bhupy]

It's not just the top marginal tax rate. The full bracket has fallen across the board, not just on the top marginal income tax payer.

> A major source of short-term noise is the business cycle, where capital gains and corporate profits fluctuate much faster than GDP, and some automatic stabilizers decrease revenue.

Yes, which is why it's tough to suss anything out from the minor variations between consecutive years, and it makes a little more sense to look at a longer time horizon.