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. …

Of course these measures won’t return the world’s top economies to sustainable levels of debt. That could be achieved only through significant economic growth (the good way) or, as the IMF puts it, “by repudiating public debt or inflating it away” (the bad way). In October the IMF floated a bold idea that didn’t get the attention it deserved: lowering sovereign debt levels through a one-off tax on private wealth.

As applied to the euro zone, the IMF claims that a 10% levy on households’ positive net worth would bring public debt levels back to pre-financial crisis levels. Such a tax sounds crazy, but recall what happened in euro-zone country Cyprus this year: Holders of bank accounts larger than 100,000 euros had to incur losses of up to 100% on their savings above that threshold, in order to “bail-in” the bankrupt Mediterranean state.