Billionaires and Bernie Sanders agree on at least one thing: They see a four-day work week in America’s future. Hedge fund manager Steve Cohen is investing in golf courses because he anticipates a big increase in leisure time, and IAC founder Barry Diller is expecting people to be in the office only four days a week. The senator from Vermont, meanwhile, has proposed legislation that will set the work week to 32 hours.

Put me down as skeptical. Some big technological innovations promise to make people more productive, but a four-day work week will not be the norm anytime soon. And legislation imposing it over the next four years would harm the economy.

The first question is what a “four-day work week” means. Sometimes it means working 40 hours in four days instead of five, though this tends to be less efficient. Or it may mean working an eight-hour day four days a week, which is what Sanders has in mind. Both kinds of arrangements have become more common over the years in the US and elsewhere — though much of the increase is among people who work fewer than 40 hours.

As for the merits of the idea: There are basically two arguments for a shorter work week. One presumes that people waste so much time at work that working 20% less won’t make a difference if they use the time more efficiently. A few small studies, mainly in non-customer-facing service jobs, find that a 20% drop in hours does not result in a decline in revenue.

But whether this finding applies to more labor-intensive jobs is doubtful. The only large experiment comes from France, which imposed a 35-hour work week in 1998 on large firms, with the hope it would increase employment. Studies show it did not increase employment or happiness — and France later tried to appeal it.

The Sanders bill is in many ways worse than the French law. Like that law, it does not force people to work fewer hours. Instead, it lowers the threshold at which overtime pay kicks in. The Sanders bill also stipulates that employers can’t reduce pay if people work fewer hours as a result, so for some workers it would be akin to a 20% pay increase on an hourly basis.

That is a remarkable burden to impose on the economy (though some workers will be exempt). Perhaps some very profitable companies will be able to accommodate such a big pay increase. Advocates insist that people will be so happy and productive, companies won’t notice any difference. But a 20% productivity increase just from being happier and more engaged? And for many firms — in, say, manufacturing, or industries where face time is necessary — the productivity gains from higher morale are more limited.

The labor-time-intensive jobs also tend to be at companies that have lower margins. When Iceland reduced the work week by just a few hours for 1% of its population about a decade ago, the Icelandic government had to hire more people, increasing labor costs. A 20% wage increase will force many private firms to either close, increase prices or replace workers with technology. The gains from the Sanders bill will mostly benefit high-skill and well-paid workers in already productive companies.

Besides which, imposing fewer hours may not be necessary. The 40-hour work week has been the full-time standard since 1938 in the US, but even without big legislative changes, work hours have fallen as technology and wealth have shortened the work week for many people in the developed world. There are reasons to believe this trend will continue.

The other, somewhat related, argument for a shorter work week is that technology makes people more productive, enabling them to work less. In 1930, for example, John Maynard Keynes envisioned a 15-hour work week.

That did not pan out. But Keynes was not totally wrong: People are working fewer hours than they did in his day. Back then, low-wage workers tended to work more. Today, high earners do. Technology does not always mean people work less. What matters is how technology affects their job. In the last 50 years technology has tended to benefit high-skill workers, so working more brought them greater gains.

Now technology allows for working from home and more flexibility — for white-collar workers. This will probably enable more part-time work and fewer hours for high earners, and perhaps a convergence toward everyone working less.

Overall, however, when it comes to economic matters, I have more faith in John Maynard Keynes than in Bernie Sanders. By which I mean, we’ll all probably be working fewer work hours in the future. But it will be several decades before the 32-hour work week is standard — and moving to it too soon will make a lot of people worse off.

Allison Schrager is a senior fellow at the Manhattan Institute and a contributing editor of City Journal.