Earlier quoted context omitted.
Look, I completely understand what you are saying, because it's something I believe as well. However, what I am trying to find out more is whether this belief is correct or not and whether anyone has done any actual studies.
This is the way it is. Anyone who's worked in the industry (or any industry?) knows from experience that this is the way it is. I'm sure that there are data available. A quick Googling for "productivity vs hours worked" led me to this: http://bleedingheartlibertarians.com/2013/10/hours-worked-an...
But what we are talking about here specifically is the software development industry, where quantity does not always mean quality. So herein lies the problem that potentially a lot of our beliefs that "more hours worked means diminished return" could just be based on bad data analysis.