The Western theory of Optimum Population came into being at the end of the nineteenth century and the beginning of the twentieth century. At first, it studied the most appropriate population quantity and population scale of a country and then studied the most appropriate population density and population quality. The idea of optimum population originated from the British economist Edwin Cannan. In his Elementary Political Economy published in 1888, he studied the problem of optimum population from the perspective of whether population growth is conducive to maximum productivity. He believes that the study of “population law” is to find the population that can obtain the greatest productiveness. Too much or too little population is not conducive to the development of a country or region. There must be the most appropriate population between the two. In any period of time, assuming that resources, capital, technology, and other conditions remain unchanged, the optimum population of a country is the population with the maximum per capita output or per capita income. The point with the maximum per capita output or income corresponding to the optimum population is called the “point of maximum returns.” The country is underpopulated before reaching the “point of maximum returns” and overpopulated after exceeding the “point of maximum returns” (see Fig. 51.1).