I can see why workers are most concerned with their own benefit but total productivity is important to the health of a business and economy, which is important to workers as well. And no, working 32 hours is not as productive as 40 hours.
It's a non-sensical argument. If a 4 day work week is more productive than a 5 day work week it means the 5th days is producing negative work. You could literally do nothing every Friday and you would not be less productive than a 4 day work week.
Or it could mean that increasing weekly time off by 50% improves people's happiness and productivity, or that a 5 day work week results in days of work that are less productive than in a 4 day work week.
This would be an accurate statement for most work places. People are often creating negative value in the modern workplace in the form on tech debt, new bugs, unwanted features, meetings that drain the rest of the staff...
5 days isn't productive for knowledge and creative work, it leaves people in a state of perpetual burn out and greatly lowers the quality of output.
Your take is equally non-sensical. There is no magical constant unit of time that represents maximum productivity. It’s all relative and fluid. It’s based on many many variables. And even if such a time existed for one person in one company for one week, it wouldn’t be neatly packaged like “40 hours”. It would likely seem random like 34 hours 20 minutes and 25 seconds.
Arguing over such a universal time is as much a waste a time as arguing over the perfect diet. It’s relative.
It also sounds like you're not familiar with simple ideas like Parkinson’s Law. Listen to yourself, you don't think people are fucking off at work? It's business hours and you're on hacker news not doing work, case and point.
Absolute statements like this on this topic are invalid. It’s relative and fluid and not something you can prove scientifically and definitively.
The gray area is the point at which there are diminishing returns. Is that between 32 and 40 hours, or more? And how much do they diminish?
This is too simple. 32 hours of huge output may produce more than 40 hours of light output. If the machine/human output is a constant, then sure you’re correct. But that’s not true. And that’s my point. It’s relative and fluid. It’s relative to the work, the individual, the health of the individual, the weather that day, and on and on.
Clearly there is some cost to more hours that causes the maximum to be at strictly less than the maximum. Given that we have no reason to assume 40 hours is better than 32 hours.
By this logic, working 48 hours is more productive, working 54 hours even more productive, 62 hours even more productive. It can be cranked up even more - the English did and took meticulous records on it.
Dialecticaly it created a strong Labour party which took power in 1945 and nationalized coal, the bank of England, hospitals, telecommunications, national rail, iron and steel - the commanding heights of the economy.
In this meaning, when the headline says "unions work", it is being 100% honest.
My point is there exist many things that workers might want for their immediate benefit but shouldn't want because ultimately it will be bad for them via an unhealthy business and/or economy.