Unlike the previous cross-sectional studies, this study takes a longitudinal approach to examining the causes of low fertility over the development of the industrial society. In particular, the study looked into how marriage and the functions of family have changed along with the development of the industrial society, why women’s participation in social and economic activities is on the rise in the post-industrial society, and what kind of impact this increase has on the functions of family, marriage, and birth rates. It turned out that low fertility in the post-industrial society is the result of women's rational choice. In the pre-industrial society, most women had no choice but to marry, give birth to and raise children. Women gained more freedom in decision-making in the industrial society, which has expanded much larger to the extent that they can now freely choose whether and whom to marry, whether, when, and how many babies to have. In addition, there is an institutional barrier that forces women to postpone their marriage. The division of labor by sex deeply rooted in the thoughts and institutions of the Korean society are causing women to marry at a later age and to have less children than they want. Also, there is an issue of economic inequity. In the post-industrial society, there is no social reward for having more babies. Businesses do not reward those who produce the workforce they heavily rely on. It is suggested that an institutional foundation should be laid to induce women to opt for marriage and more babies, and businesses should proactively participate in helping raise birth rates in close cooperation with the government.