A pretty thorough overview and analysis of research about productivity differences is provided in two articles written by Steve McConnell:
First article (Productivity variations...) states:
...The original study that found huge variations in individual programming productivity was conducted in the late 1960s by Sackman, Erikson, and Grant (1968). They studied professional programmers with an average of 7 years’ experience and found that the ratio of initial coding time between the best and worst programmers was about 20 to 1; the ratio of debugging times over 25 to 1; of program size 5 to 1; and of program execution speed about 10 to 1. They found no relationship between a programmer’s amount of experience and code quality or productivity.
Detailed examination of Sackman, Erickson, and Grant's findings shows some flaws in their methodology... However, even after accounting for the flaws, their data still shows more than a 10-fold difference between the best programmers and the worst.
In years since the original study, the general finding that "There are order-of-magnitude differences among programmers" has been confirmed by many other studies of professional programmers (Curtis 1981, Mills 1983, DeMarco and Lister 1985, Curtis et al. 1986, Card 1987, Boehm and Papaccio 1988, Valett and McGarry 1989, Boehm et al 2000)...
This article also has an interesting side note:
This degree of variation isn't unique to software. A study by Norm Augustine found that in a variety of professions--writing, football, invention, police work, and other occupations--the top 20 percent of the people produced about 50 percent of the output, whether the output is touchdowns, patents, solved cases, or software (Augustine 1979).
Second article (...How Valid is the Underlying Research?) has been written mainly to address critical review of the first one by Laurent Bossavit:
In second article, in section A Deeper Dive Into the Research Supporting “10x” McConnell re-checks in more details the references used in the first article and concludes:
...As I reviewed these citations once again in writing this article, I concluded again that they support the general finding that there are 10x productivity differences among programmers. The studies have collectively involved hundreds of professional programmers across a spectrum of programming activities.
...the body of research that supports the 10x claim is as solid as any research that’s been done in software engineering. Studies that support the 10x claim are singularly not subject to the methodological limitation described in Figure 1, because they are studying individual variability itself (i.e., only the left side of the figure). Bossavit does not cite even one study — flawed or otherwise — that counters the 10x claim, and I haven’t seen any such studies either. The fact that no studies have produced findings that contradict the 10x claim provides even more confidence in the 10x claim. When I consider the number of studies that have been done, in aggregate I find the research to be not only suggestive, but conclusive—which is rare in software engineering research.
For the sake of completeness, list of references used in the Productivity variations... is also quoted below:
Augustine, N. R. 1979. "Augustine’s Laws and Major System Development Programs." Defense Systems Management Review: 50-76.
Boehm, Barry W., and Philip N. Papaccio. 1988. "Understanding and Controlling Software Costs." IEEE Transactions on Software Engineering SE-14, no. 10 (October): 1462-77.
Boehm, Barry, et al, 2000. Software Cost Estimation with Cocomo II, Boston, Mass.: Addison Wesley, 2000.
Boehm, Barry W., T. E. Gray, and T. Seewaldt. 1984. "Prototyping Versus Specifying: A Multiproject Experiment." IEEE Transactions on Software Engineering SE-10, no. 3 (May): 290-303. Also in Jones 1986b.
Card, David N. 1987. "A Software Technology Evaluation Program." Information and Software Technology 29, no. 6 (July/August): 291-300.
Curtis, Bill. 1981. "Substantiating Programmer Variability." Proceedings of the IEEE 69, no. 7: 846.
Curtis, Bill, et al. 1986. "Software Psychology: The Need for an Interdisciplinary Program." Proceedings of the IEEE 74, no. 8: 1092-1106.
DeMarco, Tom, and Timothy Lister. 1985. "Programmer Performance and the Effects of the Workplace." Proceedings of the 8th International Conference on Software Engineering. Washington, D.C.: IEEE Computer Society Press, 268-72.
DeMarco, Tom and Timothy Lister, 1999. Peopleware: Productive Projects and Teams, 2d Ed. New York: Dorset House, 1999.
Mills, Harlan D. 1983. Software Productivity. Boston, Mass.: Little, Brown.
Sackman, H., W.J. Erikson, and E. E. Grant. 1968. "Exploratory Experimental Studies Comparing Online and Offline Programming Performance." Communications of the ACM 11, no. 1 (January): 3-11.
Valett, J., and F. E. McGarry. 1989. "A Summary of Software Measurement Experiences in the Software Engineering Laboratory." Journal of Systems and Software 9, no. 2 (February): 137-48.
Weinberg, Gerald M., and Edward L. Schulman. 1974. "Goals and Performance in Computer Programming." Human Factors 16, no. 1 (February): 70-77.