In a recent paper, Piketty et al. suggest that
You can read Piketty et al.'s VOX piece on the taxation of the mega rich here.
"The top 1% of US earners now command a far higher share of the country's income than they did 40 years ago. This column looks at 18 OECD countries and disputes the claim that low taxes on the rich raise productivity and economic growth. It says the optimal top tax rate could be over 80% and no one but the mega rich would lose out."
You can read Piketty et al.'s VOX piece on the taxation of the mega rich here.