Thanks for your reply cbi1972. The question was is a 90%marginal rate a dis-incentive?. That what my statement about empirical evidence was referring to. You're welcome.
If you want to use the 1950s as a point of evidence, we can go back to before the income tax existed to see the implications of that (the nation existed without it, and the people had a healthier relationship with government).
The top marginal tax rate is less of a factor in our economy than things like slavery, industrialization, technology, etc.
Principles of economics still hold. If you want to see disincentives at work, look at what policies government implements when it wishes to shape behavior without straight up making it illegal. It taxes the hell out of things it wants less of, and gives credit for things it wants more of.
Want to reduce greenhouse gas emissions? Cap and trade.
Want to reduce smoking and alcohol consumption? Tax stamps.
And look at the enormous list of tax credits on the tax forms. Earned Income Tax Credit, Education Credits, Child and Dependent Care Credit, Adoption Credit, Saver's Credit, Small Business Health Care Credit, Plug-in Electric Vehicle Credit
A 90% marginal tax rate is absolutely a disincentive. "Empirical evidence" from the 1950s is fraught with so many confounding variables, it's not worth anything.