OK, I'll try again. There are two, and only two, factors which account for human stature.

a) your genes, and
b) nutrition, over the course of several generations.

That is, given optimum nutrition (including maternal nutrition during pregnancy) different geographic groups will differ in final height outcomes.

Hence, other things being equal, Americans are likely to be shorter than Swedes or other Nordic types for the exactly the same reason that they're less likely to have blond hair: their genes.

Swedes will be taller than Italians given equal diets.

Swedes will also be taller than Englishmen, but not by as much.

The English will be slightly taller than the Welsh.

The Irish will be about the same height as the English.

Follow me so far? I'm excluding recent immigrants in all these cases, of course.

Once you've got an optimum number of calories from animal products, you achieve the maximum height your genes will allow, over the course of a couple of generations.

Then you stop. The upward curve in heights levels off. No matter what you feed people, they're not going to average 8 feet tall.

Americans achieved modern heigh outcomes earlier than other groups because they got modern standards of nutrition earlier; and at that time, they were also largely derived from a gene pool (British) which tended to produce tall individuals (though not as tall as, say, Scandinavians).

Americans have been at roughly their modern heights since the 18th century. George Washington's native-born white Southern soldiers were around the same height as their many-times-great-grandsons enlisting today.

At that time, Americans were unusually tall. As other groups got to eat better, and in particular to eat more meat, they started to catch up to Americans in height.

And of course non-British, non-North European immigrants altered the average genetic potential for maximum height downward, particularly from the 1880's on, when southern and eastern Europeans and people from the Balkans and the Middle East started entering the US in very large numbers.

Once dietary differences are no longer a significant factor, Americans can be expected to be taller than some European nationalities and shorter than others because Americans are _a mix of various European, African and other non-European groups genetically_.

OK? Native Swedes, when equally well-fed, will be taller than the American average because of their _genes_, got it?

Scandinavians have more potential height for the environmental factors (diet, nutritional stress) to unlock.

(Last time I looked, the average male Swede was about 2.5 inches taller than the average male American.)

Americans are not of exclusively North European descent. They do not on average have the _genetic potential_ to achieve average heights equal to those of equally well-nourished North Europeans.