Flynn himself thinks nutrition got better but the data are not clear about that.
I doubt Flynn thinks much of the nutrition hypothesis any more; his recent paper 'Requiem for nutrition as the cause of IQ gains' argues against nutrition as a major cause of IQ gains in developed nations. He would likely agree with you that the kinds of social changes you're thinking of had a big impact; I seem to remember him writing in his book from three years back that contemporary people make more of a habit of thinking about things abstractly, and learn more of the mental tools needed to do well on IQ tests.
When I did fieldwork in the late 1960s in backcountry Botswana I hit upon the idea of asking my sister (a dairy farmer) to send me a box of back issues of American cattle magazines. It was unbelievable: I could have made a fortune selling pictures from them, not to mention whole issues, to the local cattle people. At that time people carefully hoarded little scraps of paper to use writing messages.
In the late 1980s I brought some more such magazines with me, and no one was interested at all. The media storm had penetrated and everyone had school textbooks, magazines, radios, etc.
Edit: Q&A is now closed. Thanks to everyone for participating, and thanks very much to Harpending and Cochran for their responses.
In response to Kaj's review, Henry Harpending and Gregory Cochran, the authors of the The 10,000 Year Explosion, have agreed to a Q&A session with the Less Wrong community.
If you have any questions for either Harpending or Cochran, please reply to this post with a question addressed to one or both of them. Material for questions might be derived from their blog for the book which includes stories about hunting animals in Africa with an eye towards evolutionary implications (which rose to Jennifer's attention based on Steve Sailer's prior attention).
Please do not kibitz in this Q&A... instead go to the kibitzing area to talk about the Q&A session itself. Eventually, this post will be edited to note that the process has been closed, at which time there should be no new questions.