I think this is caused by the fact that innovative societies are that way because their more open to new ideas. But being open to new ideas means that your memetic defenses are by definition weaker.
Notice also that innovative societies generally aren't defeated until they stop innovating.
http://vimeo.com/22099396
What do people think of this, from a Bayesian perspective?
It is a talk given to the Oxford Transhumanists. Their previous speaker was Eliezer Yudkowsky. Audio version and past talks here: http://groupspaces.com/oxfordtranshumanists/pages/past-talks