"northern Europeans have been getting taller much faster than Americans. (The latter might even be getting shorter.) These could be due to a lot of causes - "
The "average American" is getting shorter due to the large influx of non-European immigrants (not that there's anything wrong with that).
I had heard (and I unfortunately can't remember the source) that immigration doesn't explain the Euro/US divergence. (The Euros, recall, have a lot of 3rd World immigrants too.)