So are you saying its based on genetics? I always thought that it was related to economics of keeping things like supply chains for food going and other factors.
Not even genetics, just simple population numbers. It takes 2 humans to make a new human. If each female human on earth isn't making, on average, 2 new humans (plus the .1 extra to make up for infertility and mortality) then the population of the world will shrink. Simple as.