The author defines the "population science" as the system of relationships centering the demographic variables and puts the methodological property upon its own research objects. Because of the variety of objects, the cooperation of disciplines is indispensable for the study of population. Considering that the methodological conflict among disciplines has often prevented the development of population science as independent discipline, the author advocated "the object approach", which is practically embodied in three main fields of economic demography, social demography and biodemography. Recently, the field of economic demography has already acquired the citizenship internationally both in name and reality. The endeavor of study in economic demography has mostly been devoted to the inquiry into the economic determinants of fertility. It was since Becker's seminal paper in 1960 that the economic analysis of fertility became a matter of concern of economists. In 1966, Richard Easterlin challenged the early Becker formulation, and then the conflict between two schools following Becker and Easterlin began. The essence of this conflict is the collision of the method of economics and of sociology. However, the ending of conflict is at hand. It is nothing but the result of the fact that the scholars with different methods intensively studied the common object of the determinants of fertility. We should appreciate this success story of "the object approach" which could overcome the difficulties of methodological integration. This is the first step to a synthetic population science.
