> In order to hit peak child, every subgroup will also have to hit peak child. This is an interesting argument; thanks for sharing it. I didn't understand it r

> In order to hit peak child, every subgroup will also have to hit peak child. This is an interesting argument; thanks for sharing it. I didn't understand it right away and my intuition was that you must be incorrect. But after playing with some actual numbers I've come to understand. In an attempt to restate your claim in a way that's more intuitive to me: If a group is reproducing above the replacement ratio, then they haven't hit peak child yet. So if only one subgroup remains that hasn't hit peak child, then that subgroup is also the only one that's growing (all others are shrinking). Assuming that trend continues (read: assuming subgroups have birth rates that reflect their culture, and that culture will be preserved in the next generation), this subgroup will grow without bound. The pressure-release on a system like this (one which the article you linked points out in its conclusion) is that birthrate within a subgroup isn't fixed. The birthrate among the Amish appears to be declining slightly, perhaps influenced by economic forces in the US. In your example, while it's true that _tommorow_ there will be more Mormons and fewer Italians than today, that doesn't guarantee that eventually the world will be populated entirely by Mormons, because the birthrate in that subgroup isn't wholly 'inherited' (in the cultural sense) -- it may change in ways that we can't forecast today.