It is a common conception that the diversity of family formation increased due to higher standards of living and educational attainment. In this study, I investigate whether the diversity of family formation increased or decreased across birth cohorts in Sweden, and to what extent changes in diversity be attributed to compositional differences between birth cohorts. Using Swedish register data and an innovative methodological approach, combining metrics developed in sequences analysis with Oaxaca-Blinder decomposition models, I find that the diversity of family formation has decreased across birth cohorts in Sweden. Although increases in single parenthood during childhood would have increased diversity, larger increases in parental resources and educational attainment account for the decrease in family formation diversity. To solidify my results, I demonstrate data from the 1979 and 1997 National Longitudinal Surveys of Youth that increasing diversity in the United States is attributable only to increases in childless cohabitation, and that my results are valid in both Sweden and the United States.