Something interesting I learned from The Bell Curve is that in the past, the upper class had more children and married younger than the lower class, whereas now the situation is reversed. The change started around the course of the Renaissance in Europe.

If the upper class is the more creative group and is doing more to populate the lower class than vice versa, then the upper class is going to have a much easier time passing on ideas to one another and to the lower class than vice versa. It would be easier for them to maintain themselves as a political coalition and easier to get the lower class to identify with the upper class. On the other hand, the reverse would be true in modern times. So before we had cultural transmission from upper to lower, now we have it from lower to upper.

This may be understood as a mechanism by which capitalism "sows the seeds of its own destruction", though not the way that Marx envisioned it at all. Today we have big problems with ideas ...