Was at a conference this winter and heard a presentation by a OSU extension corn specialist (edit: specialist's name is Peter Thomison).  He showed graphs and statistics, and from I took out of the presentation:

- if you took today's hybrids and planted them at the populations we used 30 and 60 years ago, you would get about the same yields as you did 30 and 60 years ago.

- if you took hybrids from 30 and 60 years ago, and planted them at today's populations, they would "bomb" - barren stalks et al.

-clearly the real advantage to today's hybrids is to plant at increasingly higher populations and get yield.

Maybe I mis-understood his point, but that is what I got out of his presentation.