Here are the references I know of:
“An Empirical Comparison of Seven Programming Languages” by Lutz Prechlt, University of Karlsruhe. http://www.openfoundry.org/of/download/pyzope/1.0.0/article.pdf
1968, referenced in Mythical Man-Month: PL/I lines/year comparable to Assembler words/year
1971, referenced in Mythical Man-Month: Assembler, Fortran, Cobol: roughly equal
1981: “Amount of effort per source statement was highly independent of language level” (Barry W. Boehm’s “Software Engineering Economics” p. 477)
1970: High level languages 3 times as productive as Assembler
Walston-Felix, 1977 "A method of programming measurement and estimation", IBM Syst. J., 16, 1, 1977, pp. 54-73.
Nelson, 1978 "Software data collection and analysis", Rome Air Development Center, Rome, NY, September, 1978
References found in:
Chapter 8 of “The Mythical Man-Month” by Fred Brooks
Barry W. Boehm’s “Software Engineering Economics” p. 477