The economist John Maynard Keynes predicted a society so prosperous that people would hardly have to work. But that isn’t exactly how things have played out.

How will we all keep busy when we only have to work 15 hours a week? That was the question that worried the economist John Maynard Keynes when he wrote his short essay ” Economic Possibilities for Our Grandchildren ” in 1930. Over the next century, he predicted, the economy would become so productive that people would barely need to work at all.

For a while, it looked like Keynes was right: In 1930 the average workweek was 47 hours. By 1970 it had fallen to slightly less than 39.

But then something changed. Instead of continuing to decline, the duration of the workweek stayed put; it’s hovered just below 40 hours for nearly five decades.

So what happened? Why are people working just as much today as in 1970?

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The Fundamental Misalignment of Work and Life

There would be no mystery in this if Keynes had been wrong about the economy’s increasing productivity, which he thought would lead to a standard of living “between four and eight times as high as it is today.” But Keynes got that right: Technology has made the economy massively more productive. According to Benjamin M. Friedman, an economist at Harvard, “the U.S. economy is right on track to reach Keynes’s eight-fold multiple” by 2029-100 years after the last data Keynes would have had. (Keynes did not specify what he meant by a “standard of life,” so Friedman uses per-capita output as a proxy.)

In a new paper, Friedman tries to figure out why that increased productivity has not translated into increased leisure time. Perhaps people just never feel materially satisfied, always wanting more money for the next new thing. “This argument is, at best, far from sufficient,” he writes. If that were the case, why did the duration of the workweek decline in the first place?