Your question is a request for external resources on a very broad and vague subject. That by definition is Not Constructive:
- There's no actual practical problem to be solved, and that's a requirement for all questions as per our FAQ.
- The question is naturally attractive to equally valid answers, you give us absolutely no parameters to help us judge whether an answer referencing one study would be a better or worse answer than another one referencing a different study.
The vagueness of the topic alone could even justify closing it as Not A Real Question.
Furthermore there's no evident prior research or effort in your question. That usually would be a reason to downvote the question, not close it, but I'm mentioning it because some times prior research helps better define the question itself, and it might have helped it escape closure.
I don't see how the question could be improved without changing its core, all three answers are opinion heavy (again, a strong hint of a not constructive question), and all of them comment, implicitly or explicitly, on the vagueness of the topic and how productivity can't be easily quantified.
There are studies out there, for example this empirical study on Programming Language Fragmentation and Developer Productivity, but if you research the subject yourself you'll notice that the studies are sharply focused on specific aspects of productivity, and probably wouldn't be good answers to your question.
Sorry, won't be casting my re-open vote on this one.