Here's an excerpt from an article in today's NY Times that's pretty provocative theorizing:
"Since biologists believe that 80 percent of height is determined by genetics and 20 percent by environmental conditions, height — and sometimes weight — can be an index of childhood nutrition, health care and exposure to disease. Thus smaller stature may be a sign of an impoverished upbringing.
Mr. Komlos, for example, wrote a 2007 paper with Benjamin E. Lauderdale that found that Americans were the 'tallest in the world between colonial times and the middle of the 20th century,' but have since 'become shorter (and fatter) than Western and Northern Europeans. In fact, the United States population is currently at the bottom end of the height distribution in advanced industrial countries.'
'We conjecture,' they concluded, 'that the United States health-care system, as well as the relatively weak welfare safety net, might be why human growth in the United States has not performed as well in relative terms as one would expect on the basis of income alone.'"
[PS: And I might add something I've observed just living through the last half of the 20th Century and noticing the spread between taller "Americans" and shorter immigrants in general reversing.]