Population Pyramids in Action

Found a neat site where you can run an interactive population pyramid of several of the developed nations, including the US. 
As a cursor below the graph runs through the years, the pyramid shows the population distribution of males and females by age group.  You can stop it and a mouse-over will highlight the age group of each bar for males and females. 

Here is the present Graph showing that edge of the baby-boomers hitting the social security bracket, and two mild periods of reduced birth rates around 1990 and 1970 (why???... are these just the dampened oscillations of the baby-boom... born around 1950 we would have our children born around 1975-1980 and grandchildren around 2000 to 2005? or do they cue off economic boom and bust... what was the impact of Aids )

This one is for 1950, I had just moved into the second bar, and the peak of the baby boom was about to hit. In the next five years the population would grow from 159 million to 171 million. 

A little research suggested that the first population pyramid split by gender was created for the US census by General Francis Walker, who used them in the 1840, 1850, 1860 and 1870 census.  The image below is one such for the state of Nebraska which it seems is from the 1874 census data-book.  It was around this time that Heinrich Schwabe (who discovered that sunspots moved in ten-eleven year cycles) adjusted them to be more like bar graphs and they have changed little since that time. 

I also came across this interesting population "pyramid" which is in three dimensions.  It was by Luigi Perozzo and was modeled on the Swedish Census from 1750 to 1875.   The graph shows the number of births each year over the period, and then shows the population distributed by age in perspective for the census results each five year period.  The chart was presented, and apparenty well praised at the 1875 Geographic Congress and again at the 1878 Paris Exhibition.