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.
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.