This essay is the fifth chapter in a larger project aimed at updating my knowledge, analysis and views on global population dynamics. This chapter describes and analyses the second of the major drivers of falling fertility during the demographic transition, the tempo effect. Fertility rates do not appear to be stabilising at the replacement level in the final stages of the demographic transition. This chapter explains the factors which work together to drive birth rates significantly below the replacement level in one country after the other, and keep them there, potentially for generations and beyond.