Fertility is dropping very rapidly worldwide. We do not know precisely why, although changing cultural norms, increasing access to contraceptive technology, rising incomes, urbanization, and even hormone-disrupting pollution could all play a role. Look at the following table of selected countries from the prolific Twitter account Birth Gauge:First, we see that a wide range of countries are below the replacement total fertility rate (TFR) of 2.1 children per woman. Second, we find that the fertil...