In his 1930 essay “Economic Possibilities for our Grandchildren,” the British economist John Maynard Keynes predicted that technological advancements would mean that future generations would work no more than 15 hours a week.

While technological progress has indeed occurred at a level and pace that Keynes himself would scarce be able to fathom, this has not led to a reduction in the working week. Indeed, many of us work more than 40 hours from Monday to Sunday, often depending on a mix of culture and public policy.

In my home country of the United States, the average workweek is 38 hours, after factoring in for part-time jobs. I know what you’re thinking: “I work way more than 38 hours a week!” Believe me, I did too when I lived in New York. The most standard U.S. workweek is 40 hours, which is fairly average compared to other countries around the world.

European countries generally work shorter weeks, with France (where I now live and work) famously defending a 35-hour week. Side note: as I have discovered since working in Paris, most French employees actually work much longer weeks, they just get extra vacation days to balance things out at the end of the month. On the other hand, Asian countries work much longer weeks.

Choosing the number of hours in a workweek is all about efficiency. Studies and common sense show that productivity is lost if an employee works too few hours but also if an employee works too many hours and “burns out.” Though many employees, myself often included, work weeks closer to 50 hours and feel we could keep going, longer weeks have repeatedly been shown to not only fail to increase efficiency but to actually decrease productivity. A 2008 study comparing the respective effects of 40- and 55-hour weeks on productivity showed that those who worked the longer hours scored worse on vocabulary and reasoning tests over a period of years.

Tom Walker, of the Work Less Institute agrees, stating, “that output does not rise or fall in direct proportion to the number of hours worked is a lesson that seemingly has to be relearned each generation.”

In the early 1990s, the famous industrialist Henry Ford decreased the lengths of working weeks from 45 to 40 hours, to the surprise and mockery of his competitors. Over the next decade, Ford’s business boomed and, in 1937, the 40-hour workweek was enshrined in Franklin D. Roosevelt’s New Deal.

As stated above, not only do longer working weeks fail to bring proportionate gains in productivity, they also engender negative consequences such as workplace accidents and the inevitable lawsuits that follow. This argument is perhaps best outlined in a 2012 Salon article entitled “Bring back the 40-hour workweek.”

However, data from other countries suggests that perhaps even a 40-hour workweek is too long and that the optimum number of hours lies in the 30s. The French, with their legally enshrined 35-hour week and lengthy holiday period (the entire country more or less shuts down every August), work the least amount of hours per year in the world.

For the hours the French do work, though, they are much more productive than workers almost anywhere else. France’s total economic output divided by the number of hours worked is among the highest in the world, even higher than in Germany. Quality over quantity seems to be the French philosophy, working less but working better.

In Germany, the average workweek is also only 35 hours, and the notoriously efficient German economy is the fourth largest in the world. Working fewer hours could also be one of the reasons that Germany has maintained such a low unemployment rate (currently sitting at 5.2%) compared to the United States (7.3%).

Of course there are obvious limits to these theories. France works few hours and has an appalling unemployment rate (11.1%), and dividing total economic output by the number of hours worked does not take into account the resources and infrastructure a country is endowed with.

Nevertheless, many still believe that the 40-hour week is too long. According to researchers at the British New Economics Foundation, the optimum number of hours in a workweek would be 30. In their book “Time On Our Side,” they argue that the 30-hour week would safeguard natural resources, reduce greenhouse gases, undercut unemployment by creating new jobs and benefit workers’ general health.

Like Pandora’s box, policy initiatives that include shortening the workweek open a whole new series of difficult questions. Should the state mandate how much or little an individual is allowed to work in a given week? Can the same number of work hours be sufficient for every kind of job, from banking to software development to babysitting? These are questions that will have to be addressed if voices demanding a shorter workweek are taken seriously by policymakers.