An old idea might be slowly creeping back into the economic mainstream: A four-day work week.

The latest flirtation happened in Japan, where Microsoft's local division tried giving its employees five consecutive Fridays off over the summer — and found sales per employee jumped 40 percent during the period. Meeting times were cut, the office consumed fewer resources, and nearly everyone said they were satisfied with the program.

Nor is Microsoft the first company to find that experiments with shorter work weeks actually improve the bottom line. In fact, a four-day work week could come with all sorts of benefits: In productivity, in lower stress, in happier lives, and in more economic justice.

Back in the 1930s, the economist John Maynard Keynes famously predicted that work weeks would eventually fall to 15 hours — or roughly two-day weeks — as technology advanced and economies became more productive. The logic for this is pretty simple: If a society increases the amount of wealth an hour of labor can produce, people can take the benefits of that in one of two ways: They can work more and take home more income, or they can take the same income home and work less.

Of course, it hasn't worked out that way. Advanced economies have cut the amount of hours their workers put in each year by all sorts of amounts. At the cutting edge, the Netherlands has roughly a four-day work week already. But most advanced countries fall well short of that mark, never mind two-day weeks.

And none fall shorter than America. As my colleague Ryan Cooper lays out in a new paper, America barely reduced hours for its average employee at all over the last five decades. As much as news stories paint Japan as overworked, the U.S. is even worse: In 2016, the average American put in 1,781 hours on the job — more than any other advanced nation. The amount of GDP we produce per hour is greater than just about any other developed Western economy. But the total hours we work is much higher. We haven't translated more productivity into more leisure at all.

In fact, Cooper calculated that if we had reduced our hours worked as much as other countries that produce similar amounts of GDP per hour of labor, Americans would already have the equivalent of three Fridays off every month.

What happened? Part of it, obviously, is we took some of that increased productivity as income instead. Economic wealth per person is much higher in the U.S. and other advanced countries today than it used to be. But another answer is inequality: It's been rising across the West, and much more severely in America than in most countries. That story about how people can take more income for the same hours, or the same income for fewer hours? It only works if the gains from productivity are shared equally. Rising inequality means they’re not. Thus, many working Americans aren't getting fewer hours or more income — they're working as hard as ever and their wages are stagnating.

A lot of people are working full 40-hour weeks — or much more, in a lot of instances — not because their company needs that labor time, but simply because they need to do so to bring home enough income to get by. Which implies their hours could be cut, but their pay maintained, without any loss to the bottom line — not to mention a decrease in stress and burnout.

And that's often what companies discover. Microsoft in Japan is not an isolated incident. A trust management firm in New Zealand recently made permanent a policy of 30-hour weeks — with pay equivalent to 37.5 hours — after it found significant increases in both worker productivity and reports of improved work-life balance. The Belgian nonprofit Femma is experimenting with a four-day work week this year, and the results will be followed by researchers at the University of Brussels. A Swedish city tried out six-hour work days recently, and found its officials nonetheless completed the same amount of work as before, if not more. Meanwhile, multiple research studies attest that working fewer hours can actually improve productivity and performance.

In fact, one survey of Western countries found a third of workers would prefer a four-day week, and the number was 40 percent in America. It's even more popular among younger workers. The idea is also gaining ground politically, with unions and activists pushing it in Europe and North America. "Shortening the work week is certainly one idea that we have got to look at," Democratic presidential candidate Sen. Bernie Sanders (I-Vt.) recently said.

Of course, not everyone's on board. If companies adopt four-day work weeks voluntarily and piecemeal, for example, they may run up against a competitor that doesn't, and lose out. To really work for everyone, a four-day work week would have to be a rule enforced across the whole economy. You'd need to cut the overtime threshold to four eight-hour days a week (to avoid stressed-out people working fewer days but more hours per day), and strengthen overtime law to apply it to as many workers as possible. You'd also need to raise per hour pay to make sure no one loses income by working fewer hours. That means minimum wage hikes, more powerful unions, and macroeconomic policies to achieve and sustain full employment.

As Cooper notes, there's also another way to go at this problem: Nationally-mandated paid vacation, sick leave, family leave, and more generous unemployment benefits. Even if we don't officially reduce the 40-hour work week, we can punch a lot more holes in it.

But this all gets to something the wealthy elite class might not like about a shorter work week: The loss of power of money. Some employers may actually prefer that work be the sole focus of their employees' lives. And as I pointed out, our lack of leisure time is intrinsically bound up with inequality.

Sure, there's some "slack" in business models, where people could work less and be paid the same and profits won't shrink. But if we really tried to enforce a four-day work week across the country, we'd probably use up that slack and then some. At that point, to maintain productivity, companies would have to start hiring more people. That would be great for everyday workers: more jobs means less unemployment. It would also mean more money going to labor costs, and less money left over for profits and shareholder payouts.

That seems like a small price to pay to give the vast majority of Americans happier lives, less stress, and more time with family and friends.

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