Culture. It's principally cultural not biological. It's perhaps clearer to say it is a decrease in fecundity rather than fertility. Worldwide experience has be

Culture. It's principally cultural not biological. It's perhaps clearer to say it is a decrease in fecundity rather than fertility. Worldwide experience has been that increasingly developed nations have lower average fecundity. There are various likely reasons but the correlation seems to be quite robust. In most countries it seems that the causal link was from development to fertility. It is arguable that China reversed the causation, politically limiting fecundity. This is a uniquely human dynamic. Pretty much every other species in the world drives its population to its Malthusian limit.