China’s total fertility rate has been below 2.1 for nearly three decades, the necessary number of children born to every woman to maintain current population levels. Families valued having children more in ancient China than today. As a Chinese saying goes: “emperors love their eldest son most, while ordinary people love their youngest son most.”  In addition, The link between prosperity and population is prevalent in Chinese history. In ancient China, prioritizing agriculture provided incentive to have more children.  And how changes in genetics and environment affect human fertility has been an increasingly important focus of study over the past 20 years.