The second demographic transition differs significantly from the first transition both in terms of demographic predictions and in terms of underlying motivations. Because the SDT predicts generalized sub-replacement fertility in tandem with a greater plurality of living arrangements and household structures, it also points to the growing importance of international migration. Furthermore, the SDT predictions depart from the benign equilibrium outcomes of the first transition, such as a stationary population, not much need for migration, and the predominance of the stable conjugal family. By contrast, sustained subreplacement fertility will accelerate population aging and put a strain on welfare systems. Such low fertility will also stimulate replacement migration as a means of countering labor force shortages. In addition, some of the new living arrangements may be more unstable than traditional arrangements, or may be less adequate as a setting for procreation and socialization. Union dissolution will remain a major cause of low fertility as well.