Steve Sailer claims, in More on Height, that Europeans have caught up to Americans in height, because they used to be significantly inbred:
There’s another reason why Europeans have caught up with Americans in height that’s genetic but not racial — the recent decline in inbreeding in Europe. Most Englishmen married somebody living 900 feet away on average, according to one study (which found that the introduction of the bicycle almost doubled that traditional radius or romance). People tended to become some kind of cousins to most of their neighbors via multiple genealogical pathways. Inbred people tend to be shorter.Americans, on the other hand, tended to be well mixed up, both by the trip across the ocean and by subsequent moves within America. It doesn’t take much to eliminate most of the deficits caused by inbreeding. So, if a Puritan man married a Puritan woman in Boston, and they were from towns 30 miles apart in England, their kids wouldn’t suffer much deficit. Repeat for another generation and it’s almost all gone.
There were some exceptions to this process. Italian immigrants tended to cluster on streets according to their home village, and they stayed quite short. But WWII shook up Italian-American society and the next generation tended to marry anybody Italian, and the generation after married anybody Catholic.
But, now, sedentary Europeans have cars and find their mates over a much larger radius, so the inbreeding depression that held them down for a long time is no more of a problem for them than it is for Americans.
Inbred Puritans — and I was just reading some H.P. Lovecraft last night…