The authors analyze social fluidity among Swedish men and women using a series of 24 annual surveys, 1976–99 (N=63,280). A theoretical model suggests that changes in fluidity are normally driven by cohort rather than period effects. The results support this argument: changes in fluidity between the mid‐1970s and late 1990s were due to the successive replacement of older and less fluid, by younger and more fluid, cohorts. Cohorts differed in their fluidity because the effect of class origins on educational attainment declined (an equalization effect) and because greater shares of each cohort had higher levels of educational attainment, which placed them in labor markets that operate more meritocratically (a compositional effect). The article discusses the relevance of these results for other countries and for policy.