"it used to be (up until the 1950s) that U.S. Citizens were taller than Europeans; this trend has been reversed in the last 30 years - now the Euros are taller (especially the Dutch)"
This is related to the fact that recent immigration trends into the United States have favored the shorter peoples of the world.
Other studies actually show slight overall height gains. Compare NHANES III (1988 - 94) HANES (1999 - 2000) and there is a 0.1" gain. I find that data from the New Yorker quite unbelievable, as the average height has been increasing http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5431a5.htm , so how could there be "no subgroup that is getting taller"? I haven't taken math for a while, but that doesn't make sense.
The same article also reports: "Since the nineteen-twenties, the median height of Mexican-American teen-agers has nearly reached the United States’ norm. It’s that norm, and not the immigrants, that has failed to rise." So, you add a lot of people below the mean and wonder why the mean isn't rising?
This is related to the fact that recent immigration trends into the United States have favored the shorter peoples of the world.