Changes in height are a good proxy measure for R/k selection, given a well-defined population. Unsurprisingly, Americans are getting shorter. But “American” doesn’t mean what is used to, and tended to exclude 50 million 5-foot Mexicans with heavily native admixtures. (We might find upon investigation that white people in Massachusetts are getting slightly taller.)
This is only a reliable measure if the population is reasonably well-nourished. I’d describe this in terms of pure calorie intake to avoid overly complicating things, due to diminishing returns between the 2nd and 1st world lifestyles and the fact that most of this variation should vanish when we take the first derivative for height.