This study uses Japan as a case study and seeks to contribute to a better understanding and theorizing of the phenomenon of marriage decline in the industrial world. Guided by a theoretical framework that synthesizes the life course perspective, William Ogburn's hypothesis of cultural lag, and Emile Durkheim's concepts of anomie and egoism, this study hypothesizes cohort differences in causes of increased singlehood. Employing mixed methods, four hypotheses that predict cohort differences in marriage age norms and conceptions of gender roles are tested by original, in-depth interview research with forty never-married and married women in Japan, and by statistical analysis of the 2005 Japanese General Social Survey (n = 1,167). The research findings support three of the four hypotheses and uncover other important factors such as cohort differences in views of parents' and peer marriages. Reasons for non-marriage differ by cohort, and important causal factors include changes in employment opportunities; lag, adaptation, and erosion of gender culture; and presence of other variables that impede marriage. This study contributes by empowering women, informing better social policies with the potential to enhance the lives of individuals in Japan, and constructing a more applicable theory of non-marriage.