This essay is the third chapter in a larger project aimed at updating my knowledge, analysis and views on global population dynamics. The analysis of fertility sits at the heart of the demographic analysis and inquiry. It relies on a myriad of disciplines, ranging from biology to economics. Several chapters are needed to offer an adequate analysis and description of the phenomenon that is fertility, and even then, significant detail has to be omitted, or presented to the reader as topics for further study. This chapter is the first of three, covering the data on the global fertility transition to date, before moving on to the theory of sexual selection, serving as an introduction to the processes driving and describing fertility and birth rates. The initial version of this text was published in January 2022. Updated versions might follow, and will be available here.