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.
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.