Cheng, Hao-ChunThis study aims to investigate changes in the variability of age at first union formation and first marriage and how the relative timing potentially affects the likelihood of experiencing depression at the age of forty in the United States. Both the destandardization of the life course and the deinstitutionalization of marriage over the past few decades may have contributed to less pronounced cultural conformity regarding the age at first union formation and first marriage. This, in turn, leads to a more diverse variability of age at first union formation and first marriage in recent cohorts among the overall and married populations. In addition, owing to the divergent paces of destandardization of the life course and the deinstitutionalization of marriage across gender, education levels, and race, changes in variability may differ among these social groups. Using multiple nationally representative datasets, the results reveal a U-shaped trend in changes in the variability of age at first union formation and first marriage. Specifically, the variability becomes less diverse in the mid-twentieth cohorts and then becomes more diverse in recent cohorts. However, the trend varies across gender, education levels, and race. While Black and highly-educated women have a more diverse variability of age at first union formation, women, non-Hispanic White, and highly educated people tend to have a less diverse variability of age at first marriage. The results indicate an insignificant effect of relative timing on the likelihood of experiencing depression. These findings shed light on changes in cultural conformity embedded within marriage as a social institution and highlight the important roles played by the destandardization of the life course and the deinstitutionalization of marriage in these changes in the United States.enDissertationSociology