Last month, in a speech in Boston, Federal Reserve Chair Janet Yellen provoked some grumbling from conservatives when she said, “The extent and continuing increase in inequality in the United States greatly concern me…. I think it is appropriate to ask whether this trend is compatible with values rooted in our nation's history, among them the high value Americans have traditionally placed on equality of opportunity.” But she is hardly alone in her concerns. Since at least Occupy Wall Street, income inequality has been one of the most intensely debated issues in American politics. Commentators fret that rising inequality hurts the poor, gives the rich the upper hand in politics, and will create a caste system in the United States. Proposed solutions range from the practical but weak, like raising the minimum wage, to the fanciful, like Thomas Piketty's global wealth tax.

But the most powerful force to reduce inequality worldwide has gone largely unrecognized by the West, even though their value has been proven in the Gulf nations: open migration laws that are coupled, paradoxically, with caste systems.

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