For many years, the economic rules were supported by both common sense and the data: when unemployment falls, wages rise soon after. But since the turn of the century and before, that relationship has broken down across the developed world, according to data from the OECD (scroll over the chart below for detail).

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Data: Unemployment and annual wage data from the OECD; Chart: Lazaro Gamio / Axios

Stagnant wages aren't just an American problem: Workers in the wealthier nations are facing similar headwinds, like declining union membership, increased competition from foreign workers in a global marketplace, and slow productivity growth. But no one knows precisely why economics are failing to observe the traditional supply-and-demand rules.

The question is not academic: Frustrated by stagnant income, fears for their children's future, and the deterioration of their towns and cities, ordinary people in the U.S. and across Europe are taking it out on migrants and their traditional politicians, shaking up the western-led political system.