Agreed. Furthermore, I think that we should consider the fact that even if there was significant selection pressure for the ability to be literate (I would argue that this is most likely not the case) that there is insufficient evidence that this pressure could result in a significant social difference so quickly.

Furthermore, the converse appears to be currently true: the more educated a women is (I would assume this relates to reading comprehension, although not necessarily so) the expected number of children she will have generally decreases (http://economix.blogs.nytimes.com/2010/06/25/educated-women-opting-for-motherhood/).