Marriage patterns can be well understood only if researchers employ measures of marriage rates that are appropriate for the question asked. In this paper, we provide evidence that the two classes of measures typically used in the literature, the number of new marriages per population and the share of individuals currently or ever married within an age range, generally lead to misleading inference when used to study the probability someone marries during his or her life or fertile life, how it evolves, and how it differs across populations. An alternative measure, the share of individuals ever married in a given cohort by a given age, is better suited for such studies. When researchers are interested in year-on-year changes in marriage probabilities of singles, age-specific marriage hazards are more reliable than population-based measures. We conclude by discussing implications of our findings for studies of the drivers and consequences of marriage formation.

Few studies have examined differences of civil status of twins and singletons and the conclusions are contradictory. In the present study, based on a linkage between the Danish Twin Register, a random 5% sample of the total Danish population, and administrative register databases, the authors compare rates of marriage and divorce in a sample of 35,975 twins and 81,803 singletons born 1940–1964. Cox-regressions are used in order to control for potential confounders. We find that compared with singletons twins have significantly lower marriage rates: (males: 15–19 years: Hazard Ratio (HR) = 0.66 (95%CI: 0.58–0.76); 20–24 years: 0.85 (0.82–0.88); 25 years or more: 0.96 (0.93–0.98) and females: 15–19 years: 0.70 (0.67–0.75); 20–24 years: 0.83 (0.80–0.85); 25 years or more: 0.94 (0.91–0.97)). There is no difference in divorce rates for males, but a significantly lower divorce rate for female twins compared with singletons (HR=0.87, 95%CI: 0.83–0.90). These differences offset each other, thus 57% of both populations remain in their first marriage until censoring. The interpretation may be that since twins have a partner from birth, they do not have the same need for marriage as singletons but have more experience in maintaining a relationship if they do marry.