Fuller noticed while reviewing available global statistics that as an area's per capita energy consumption increased, the average birth rate for that area decreased!
This fact has held true for every country of the world as it went through the process of industrialization. As the world's energy per capita goes up, the world's birth rate will go down.
One possible explanation: as the use of artificial power increases, the need to have lots of kids to do the work decreases,
I suspect you'd see a similar graph if you plotted energy use v. slavery over the centuries.