It's conceivable that there are institutional barriers to genius expressing itself-- partly that there really is more knowledge to be assimilated before one can do original work, and partly that chasing grants just sucks up too much time and makes it less likely for people to work on unfashionable angles.
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 ...
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.