See Ezra Klein's post
here. Tax experts say the
Laffer Curve bends at 60-69%. Lefties say 70% or higher, and Righties say anywhere from 19 to 63%. The major issue seems to be long-term versus short-term: a very high income tax rate might be optimal in terms of revenue, but not in terms of growth, so in the long-term, the optimal revenue-producing rate might end up not being so optimal.