[bhouston]

> The issue with this analysis is that culture isn't genetic.

I didn't say it is genetic or mean to imply it is.

> A steady stream of people coming from high birth rate cultures are going to be adopting these modern lower birth rate cultures.

Yes. People from high fertility cultures who move and adopt a different cultural norm, usually adopt the same fertility as well. This happens constantly to immigrant populations who move to Western nations. Western nations are sort of population sinks for the rest of the world.