Still, it's not like historical geniuses all grew up as pampered aristocrats left to pursue whatever they liked. Many of them grew up as poor commoners destined for an entirely unremarkable life, but their exceptional brightness as kids caught the attention of the local teacher, priest, or some other educated and influential person who happened to be around, and who then used his influence to open an exceptional career path for them. Thus, if the distribution of kids' general intelligence is really going up all the way, we'd expect teachers and professors to report a dramatic increase in the number of such brilliant students, but that's apparently not the case.
Moreover, many historical geniuses had to overcome far greater hurdles than having to chase grants and learn a lot before reaching competence for original work. Here I mean not just the regular life hardships, like when Tesla had to dig ditches for a living or when Ramanujan couldn't afford paper and pencil, but also the intellectual hurdles like having to become professionally proficient in the predominant language of science (whether English today or German, French, or Latin in the past), which can take at least as much intellectual effort as studying a whole subfield of science thoroughly.
So, while your hypothesis makes sense, I don't think it can fully explain the puzzle.
It could also be communications.
Many high intelligence situations involve disorders that also have as an effect anti-social behavior. Academia is highly geared against this in some cases going so far as to evaluate people's chances for success in a PhD based on their ability to form working relationships with a peer group during their MSc. Travel is easier and correspondence is far more personal.
Would the mathematicians of the past have been as interested in this model? Perhaps some of them were the type of people that were happy to correspond by mail ...
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.