Andrew Dalke reminded me of an essay by Stephen J. Gould (discussed at:

http://www.michaelshermer.com/1996/10/bicycles-baseball-bacteria-and-bach/ and http://www.motherjones.com/commentary/columns/1997/01/outspoken.html) about the decreasing deviation in performance as a field matures. Relating it to the old studies on programming productivity, Dalke wonders:

If that applies to programming, and I expect that's a strong confounding effect, then those 1980-era studies have another problem - the field was too new. Many of the good programmers my age have been programming since high school or earlier, so about 20 years. Who in the 1980s had that background?