A few wars take place, explains population decline. Men come back from WWII...huge surge in long term pop trend (in US). Something happened in the mid 50s to reverse long term trend in pop growth.

Polio vax introduced in 55'....late 50s onward, strong downward trend in population growth thereafter. Global population halved between 55' & 2012'. As more vaccines came out, pop. continued to decline world-wide.

Can't use the 'kids are just expensive' excuse (in US) because cost of living didn't surge until 70s onward due to dollar-gold de-indexing. And divorce rate didn't uptick until 70s onward.