Why don't the lispers organize a RAD competition between lisp and all-comers, surely if lisp is that much more productive you would see the top ten places being lisp dialects.
Would you agree?
So, no, I wouldn't agree.
The best study I could think of off hand would be a randomized study of programming productivity (measured somehow) of programming naïve high schoolers. Each subject gets an incentive to learn some language over the course of X weeks and then program something. The sample size would have to be magnificent even to prove a large effect because you'd need to demonstrate control for intelligence/drive and regressing out math scores (or anything similar) would not be convincing really. At the end, you'd still not be able to talk too much about working productivity, but you might have a chance of talking about... anything at all.
Actually, this is an extraordinarily difficult study I think.
As for studies, a quick search on google came up with this: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.117...
Which concludes that "Haskell prototype took significantly less time to develop and was considerably more concise and easier to understand than the corresponding prototypes written in several different imperative languages, including Ada and C++."
I couldn't find much more in a short time. Perhaps my search terms, "functional programming imperative productivity" were inappropriate.
This paper although makes me think they compared programmers not languages, as the imperative language programmers didn't have any functional experience. It would be interesting to see a comparison between programmers of roughly equal competency.
Despite that complaint, the above is nearly impossible to do in practice. So you have to settle for smaller problems, which can be done in several months perhaps. Even at that, it would take some very difficult collaboration, and would still be a single data point.
Further complicating issues is the academic world vs. industry. Again, ideally you'd want to do the test in industry, with an appropriate team of programmers and managers. But industry doesn't want to waste time with such academic matters, nor does it want to open it's code for analysis.
In other words, it's a very difficult problem to address, despite being seemingly simple on the surface.
Also, though it's not explicit, I feel it's obvious that the researchers had a prior bias toward functional languages.
In other words, the reason why not is probably "because organizing things is hard work".
Many thousands of programmers spend hours each week in pissing matches online. Why is it none of them feel like truly putting their money where their mouth is?
Writing a "Hello, world" is going to take about the same amount of time in any language that isn't a turing tarpit. At issue is generally some sort of asymptotic advantage, not a guarantee of "language X will always be superior every time always". In fact, for any pair of one easy and one hard problem, I'm pretty confident I could find (or invent, if necessary) two languages that each beat the other for solving one of the problems.
The claimed benefits of a more expressive language generally involve things like better management of application complexity, or greater ability to modify and restructure late in the project. Hammering out a program of a scale such that the programmer can keep an accurate model of the entire thing in mind at one time, and that is only written and never modified, would be more a measure of the programmer's typing speed and the sophistication of the IDE used than anything else.