The Green Party is proposing a maximum four-day week.

It's one of the policies included in the Green Guarantee. The first thing to say is that it's not quite as radical as it sounds - the maximum number of hours worked a week would be 35, which is the same as has applied in France since 2000, although the exceptions in the French system mean that their average number of hours worked per week is actually a bit above 35, according to the OECD.

Also, we already have a number of four-day weeks each year thanks to bank holidays.

Explaining the policy on the BBC's Andrew Marr Show last month, Green Party co-leader Caroline Lucas said: "I think there's a lot of evidence that suggests that when people are exhausted their productivity goes down.

"People are working ever more hours, getting ever more stressed, getting ever more ill-health - mental health problems as well."

The manifesto also said that the 35-hour week would be phased in. In the UK, the current limit is 48 hours a week, although you can opt out of it and various jobs are excluded.

Media playback is unsupported on your device Media caption Should we work a four day week?

The average number of paid hours worked by full-time employees in the UK in 2016 was 39.2.

While it is relatively straightforward to impose a maximum number of hours worked in the week for employees, it is harder to do so for the self-employed and harder still to legislate to force people to work a four-day week, especially because not everybody could take the same days off and there would presumably need to be some flexibility about when the hours were worked.

The idea is that working fewer hours would boost productivity, which is the value created by each hour worked.

The UK has comparatively low productivity. The UK's national income per hour worked is 22.7% below that of France, which means that if we could be as productive as the French then we could work a four-day week and not lose much output.

There are various suggestions in the parties' manifestos on what to do about UK productivity.

Trying to boost productivity by reducing hours worked would not be without its costs. France has higher unemployment than the UK. It is likely that its more restrictive labour laws have meant that companies have invested more in machinery to reduce the number of people they need.

Also, a working paper from the International Monetary Fund suggested that the 35-hour week in France had reduced employment and not made workers any happier.

But it's not just about maintaining economic growth - part of the idea of the four-day week is that to create a more sustainable economy we need to stop being obsessed by growth and start thinking about having a lower impact on the environment.

The left of centre think tank, the New Economics Foundation, did some work on the idea of a more radical 21-hour working week.

It said that in addition to reducing environmental impact, a 21-hour week would distribute work more evenly across the population, reducing both the problems of over-work and unemployment, as well as evening up the amount of unpaid work done by men and women.

It identified potential problems with the introduction of such a system such as increased poverty by reducing the number of hours worked by low-paid workers and increased unemployment.

Some businesses would also be likely to resent the extra regulation. The Green Party maintains that working fewer hours would reduce stress and ill-health.

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