In searching for new sources of funding, the United Nations this month called for a tax on billionaires to raise money for poor countries. According to the assessment in the U.N. World Economic and Social Survey, an annual tax on the world's super-rich would yield almost $400 billion a year.

That money could go toward improving conditions for the world's estimated 1.4 billion people living below the poverty line of $1.25 per day.

The math seems simple enough, though the politics far less so. The recommendation is not about gouging the super-wealthy, but about finding sources of funding.