In the paper linked below, Mingroni states a few compelling criticisms of Flynn's explanations for the Flynn effect.

http://www.scribd.com/doc/79914286/Resolving-the-IQ-Paradox-Heterosis-as-a-Cause-of-the-Flynn-Effect

Flynn's response seemed less than thorough:

http://www.brookings.edu/~/media/research/files/articles/2001/4/01iq/0401iq_critics_dickens.pdf

It's all very interesting. I wish I had more time to read up on it, but between work and 2 young children there's not much time left for anything else.