DIFFERENTIAL FERTILITY.
Although birthrate differences have narrowed in ,recent years (see Chapter 17); there is still a good deal of truth in the saying, "the lich get richer, and the poor get babies." The combination of an expansion of higher-status jobs and a relatively low birthrate in the upper classes means that the upper classes do not replace themselves and are constantly replenished by mobiles from the lower classes. Figure 15-2 (with its explanatory note) shows how well over 15 percent of the population moves from lower into middle and upper classes each generation.
