Between 2008 and 2012, several of the developed world's most fiscally challenged nations (including the United Kingdom, Ireland and Spain) increased top personal income tax rates by an average of 8%. In the United States, the expiration of the Bush tax cuts pushed the highest federal income tax bracket to 39.6% from 35%.

What the IMF calls "revenue-maximizing top income tax rates" may be a good indication of how much further those rates could rise: As the IMF calculates, the average revenue-maximizing rate for the main Organization of Economic Cooperation and Development countries is around 60%, way above existing levels.