Most claims about variances in programmer productivity cite a study that is more than fifty years old.A much better study is the one described in Peopleware, which happened about 25 years ago. It found that 10 to 1 differences are common, but also found that much of that difference is explainable by environmental factors.
...and who (if anybody) has done empirical research into deciding whether there are attributes of a language that have a direct bearing on productivity?
In Code Complete Steve McConnell cites research that indicates that programmer productivity in lines of code per day is relatively constant across a wide range of languages. Therefore the conciseness of the language is directly tied to productivity. He then provides a chart estimating the relative productivity of different languages. If I remember correctly (I don't have my books at hand) he was able to cite more research on this in Software Estimation.
In Software Estimation he also discusses the COCOMO II model. His opinion, which I think is reasonable, is that COCOMO II is a bad estimation tool because there are so many factors that people can put their estimates in that they can get out whatever answer they want. HOWEVER it is based on measuring the contribution to actual productivity of many different factors, and so the COCOMO II cost factors are a fascinating source of information on what actually contributes.
I agree with claims that automatic garbage collection, not OO, is the single most important productivity lever to come out of Universities and into industry. How do we know if this is true or false?
We probably never will know. Personally I'd like to suggest that the idea of structured programming (which started with Edsger Dijkstra's Go To Statement Considered Harmful) deserves serious consideration as well.