For much of the last century it seemed that the slice of the total economic pie going to workers was — like the speed of light — constant. No matter what the economy’s makeup, labor could collectively depend on taking home roughly two-thirds of the country’s total output as compensation for its efforts. Workers’ unchanging share, the economist John Maynard Keynes declared in 1939, was “one of the most surprising, yet best-established, facts in the whole range of economic statistics.”

But in recent decades, that steady share — which includes everything from the chief executive’s bonuses and stock options to the parking-lot attendant’s minimum wage and tips — started to flutter. In the 2000s, it slipped significantly. Although the numbers have inched up in the last couple of years, labor’s portion has not risen above 59 percent since before the recession.

The decline has coincided with a slowdown in overall growth as well as a stark leap in inequality. “Labor is getting a shrinking slice of a pie that’s not growing very much,” David Autor, an economist at M.I.T., said. It is a development that is upending political establishments and economic policies in the United States and abroad.